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## Object Model Design

By Pete Su, Michael Yoon, Richard Li, Rafael Schloming

OpenACS docs are written by the named authors, and may be edited by OpenACS documentation staff.

### Essentials

#### Data Model

- acs-metadata-create.sql

- acs-objects-create.sql

- acs-relationships-create.sql

#### Tcl Files

Not yet linked.

#### Requirements

- Object Model Requirements

- Groups Requirements

- Permissions Requirements

### Introduction

Before OpenACS 4, software developers writing OpenACS applications or modules would develop each data model separately. However, many applications built on OpenACS share certain characteristics or require certain common services. Examples of such services include:

- User comments

- Storage of user-defined or extensible sets of attributes

- Access control

- General auditing and bookkeeping (e.g. creation date, IP addresses, and so forth)

- Presentation tools (e.g. how to display a field in a form or on a page)

All of these services involve relating additional service-related information to application data objects. Examples of application objects include:

- forum messages

- A user home page

- A ticket in the ticket tracker

In the past, developers had to use ad-hoc and inconsistent schemes to interface to various "general" services. OpenACS 4 defines a central data model that keeps track of the application objects that we wish to manage, and serves as a primary store of metadata. By metadata, we mean data stored on behalf of an application outside of the application's data model in order to enable certain central services. The OpenACS 4 Object Model (or object system) manages several different kinds of data and metadata to allow us to provide general services to applications:

- Object Identification

  Every application object is given a unique identifier in the system. This identifier can be used to find all data related to a particular object.

- Object Context and Access Control

  Every object is created in a particular security context, so the system can provide centralized access control.

- Object Types and Attributes

  Objects are instances of developer-defined object types. Object types allow developers to customize the data that is stored with each object.

- Relation Types

  Relation types provide a general mechanism for mapping instances of one object type (e.g. users) to instances of another object type (e.g. groups).

The next section will explore these facilities in the context of the the particular programming idioms that we wish to generalize.

Related Links

This design document should be read along with the design documents for the new groups system, subsites and the permissions system

### History

The motivation for most of the facilities in the OpenACS 4 Object Model can be understood in the context of the 3.x code base and the kinds of programming idioms that evolved there. These are listed and discussed below.

#### Object Identification

Object identification is a central mechanism in OpenACS 4. Every application object in OpenACS 4 has a unique ID which is mapped to a row in a central table called `acs_objects`. Developers that wish to use OpenACS 4 services need only take a few simple steps to make sure that their application objects appear in this table. The fact that every object has a known unique identifier means that the core can deal with all objects in a generic way. In other words, we use object identifiers to enable centralized services in a global and uniform manner.

Implicit Object Identifiers in OpenACS 3.x

The motivation for implementing general object identifiers comes from several observations of data models in OpenACS 3.x. Many modules use a `(user_id, group_id, scope)` column-triple for the purpose of recording ownership information on objects, for access control. User/groups also uses `(user_id, group_id)` pairs in its `user_group_map` table as a way to identify data associated with a single membership relation.

Also, in OpenACS 3.x many utility modules exist that do nothing more than attach some extra attributes to existing application data. For example, general comments maintains a table that maps application "page" data (static or dynamic pages on the website) to one or more user comments on that page. It does so by constructing a unique identifier for each page, usually a combination of the table in which the data is stored, and the value of the primary key value for the particular page. This idiom is referred to as the "(on_which_table + on_what_id)" method for identifying application data. In particular, general comments stores its map from pages to comments using a "(on_which_table + on_what_id)" key plus the ID of the comment itself.

All of these composite key constructions are implicit object identifiers - they build a unique ID out of other pieces of the data model. The problem is that their definition and use is ad-hoc and inconsistent, making the construction of generic application-independent services unnecessarily difficult.

Object Identifiers in OpenACS 4

The OpenACS 4 Object Model defines a single mechanism that applications use to attach unique identifiers to application data. This identifier is the primary key of the `acs_objects` table. This table forms the core of what we need to provide generic services like access control, general attribute storage, general presentation and forms tools, and generalized administrative interfaces. In addition, the object system provides an API that makes it easy to create new objects when creating application data. All an application must do to take advantage of general services in OpenACS 4 is to use the new API to make sure every object the system is to manage is associated with a row in `acs_objects`. More importantly, if they do this, new services like general comments can be created without requiring existing applications to "hook into" them via new metadata.

Note: Object identifiers are a good example of metadata in the new system. Each row in `acs_objects` stores information about the application object, but not the application object itself. This becomes more clear if you skip ahead and look at the SQL schema code that defines this table.

#### Object Context and Access Control

Until the implementation of the general permissions system, every OpenACS application had to manage access control to its data separately. Later on, a notion of "scoping" was introduced into the core data model.

"Scope" is a term best explained by example. Consider some hypothetical rows in the `address_book` table:

... | `scope` | `user_id` | `group_id` | ...
... | `user` | `123` |  | ...
... | `group` |  | `456` | ...
... | `public` |  |  | ...

The first row represents an entry in User 123's personal address book, the second row represents an entry in User Group 456's shared address book, and the third row represents an entry in the site's public address book.

In this way, the scoping columns identify the security context in which a given object belongs, where each context is either a person or a group of people or the general public (itself a group of people).

In OpenACS 4, rather than breaking the world into a limited set of scopes, every object lives in a single context. A context is just an abstract name for the default security domain to which the object belongs. Each context has a unique identifier, and all the contexts in a system form a tree. Often this tree will reflect an observed hierarchy in a site, e.g. a forum message would probably list a forum topic as its context, and a forum topic might list a subsite as its context. Thus, contexts make it easier to break the site up into security domains according to its natural structure. An object's context is stored in the `context_id` column of the `acs_objects` table.

We use an object's context to provide a default answer to questions regarding access control. Whenever we ask a question of the form "can user X perform action Y on object Z", the OpenACS security model will defer to an object's context if there is no information about user X's permission to perform action Y on object Z.

The context system forms the basis for the rest of the OpenACS access control system, which is described in in two separate documents: one for the permissions system and another for the party groups system. The context system is also used to implement subsites.

#### Object Types

As mentioned above, many OpenACS modules provide extensible data models, and need to use application specific mechanisms to keep track of user defined attributes and to map application data to these attributes. In the past, modules either used user/groups or their own ad hoc data model to provide this functionality.

User/Groups in OpenACS 3.x

The user/group system allowed developers to define group types along with attributes to be stored with each instance of a group type. Each group type could define a helper table that stored attributes on each instance of the group type. This table was called the "`_info`" table because the name was generated by appending `_info` to the name of the group type.

The user/groups data model also provided the `user_group_type_member_fields` and `user_group_member_fields` tables to define attributes for members of groups of a specific type and for members of a specific group, respectively. The `user_group_member_field_map` table stored values for both categories of attributes in its `field_value` column. These tables allowed developers and users to define custom sets of attributes to store on groups and group members without changing the data model at the code level.

Many applications in OpenACS 3.x and earlier used the group type mechanism in ways that were only tangentially related to groups of users, just to obtain access to this group types mechanism. Thus the motivation for generalizing the group types mechanism in OpenACS 4.

Object Types and Subtypes

In OpenACS 4 object types generalize the OpenACS 3.x notion of group types. Each object type can define one or more attributes to be attached to instances of the type. This allows developers to define new types without being artificially tied to a particular module (i.e. user/groups).

In addition, the OpenACS 4 object model provides mechanism for defining subtypes of existing types. A subtype of a parent type inherits all the attributes defined in the parent type, and can define some of its own. The motivation for subtypes comes from the need for OpenACS to be more extensible. In OpenACS 3.x, many applications extended the core data models by directly adding more columns, in order to provide convenient access to new information. This resulted in core data tables that were too "fat", containing a hodge podge of unrelated information that should have been normalized away. The canonical example of this is the explosion of the `users` table in OpenACS 3.x. In addition to being sloppy technically, these fat tables have a couple of other problems:

- They degrade performance.

- Denormalization can make it hard to maintain consistency constraints on the data.

Object subtypes provide a way to factor the data model while still keeping track of the fact that each member of a subtype (i.e. for each row in the subtype's table), is also a member of the parent type (i.e. there is a corresponding row in the parent type table). Therefore, applications an use this mechanism without worrying about this bookkeeping themselves, and we avoid having applications pollute the core data model with their specific information.

#### Object Attributes, Skinny Tables

As we described above, the OpenACS 3.x user/groups system stored object attributes in two ways. The first was to use columns in the helper table. The second consisted of two tables, one describing attributes and one storing values, to provide a flexible means for attaching attributes to metadata objects. This style of attribute storage is used in several other parts of OpenACS 3.x, and we will refer to it as "skinny tables". For example:

- In the Ecommerce data model, the `ec_custom_product_fields` table defines attributes for catalog products, and the `ec_custom_product_field_values` table stores values for those attributes.

- In the Photo DB data model, the `ph_custom_photo_fields` table defines attributes for the photographs owned by a specific user, and tables named according to the convention "`ph_user_<user_id>_custom_info`" are used to store values for those attributes.

In addition, there are some instances where we are not using this model but should, e.g. the `users_preferences` table, which stores preferences for registered users in columns such as `prefer_text_only_p` and `dont_spam_me_p`. The "standard" way for an OpenACS 3.x-based application to add to the list of user preferences is to add a column to the `users_preferences` table (exactly the kind of data model change that has historically complicated the process of upgrading to a more recent OpenACS version).

The Objet Model generalizes the scheme used in the old OpenACS 3.x user/groups system. It defines a table called `acs_attributes` that record what attributes belong to which object types, and how the attributes are stored. As before, attributes can either be stored in helper tables, or in a single central skinny table. The developer makes this choice on a case by case basis. For the most part, attribute data is stored in helper tables so that they can take full advantage of relational data modeling and because they will generally be more efficient. Occasionally, a data model will use skinny tables because doing so allows developers and users to dynamically update the set of attributes stored on an object without updating the data model at the code level. The bottom line: Helper tables are more functional and more efficient, skinny tables are more flexible but limited.

#### Relation Types

Many OpenACS 3.x modules use mapping tables to model relationships between application objects. Again, the 3.x user/groups system provides the canonical example of this design style. In that system, there was a single table called `user_group_map` that kept track of which users belonged to what groups. In addition, as we discussed in the previous section, the system used the `user_group_member_fields` and `user_group_member_fields_map` tables to allow developers to attach custom attributes to group members. In fact, these attributes were not really attached to the users, but to the fact that a user was a member of a particular group - a subtle but important distinction.

In OpenACS 4, relation types generalize this mechanism. Relation types allow developers to define general mappings from objects of a given type T, to other objects of a given type R. Each relation type is a subtype of `acs_object`, extended with extra attributes that store constraints on the relation, and the types of objects the relation actually maps. In turn, each instance of a relation type is an object that represents a single fact of the form "the object t of type T is related to the object r of type R." That is, each instance of a relation type is essentially just a pair of objects.

Relation types generalize mapping tables. For example, the 3.x user/groups data model can be largely duplicated using a single relation type describing the "group membership" relation. Group types would then be subtypes of this membership relation type. Group type attributes would be attached to the relation type itself. Group member attributes would be attached to instances of the membership relation. Finally, the mapping table would be replaced by a central skinny table that the relation type system defines.

Relation types should be used when you want to be able to attach data to the "fact" that object X and object Y are related to each other. On the face of it, they seem like a redundant mechanism however, since one could easily create a mapping table to do the same thing. The advantage of registering this table as a relation type is that in principle the OpenACS 4 object system could use the meta data in the types table to do useful things in a generic way on all relation types. But this mechanism doesn't really exist yet.

Relation types are a somewhat abstract idea. To get a better feel for them, you should just skip to the data model.

### Summary and Design Considerations

The OpenACS 4 Object Model is designed to generalize and unify the following mechanisms that are repeatedly implemented in OpenACS-based systems to manage generic and application specific metadata:

#### Why not Object Databases?

The presence of a framework for subtyping and inheritance always brings up the question of why we don't just use an object database. The main reason is that all of the major object database vendors ship products that are effectively tied to some set of object oriented programming languages. Their idea is to provide tight language-level integration to lower the "impedance mismatch" between the database and the language. Therefore, database objects and types are generally directly modeled on language level objects and types. Of course, this makes it nearly impossible to interact with the database from a language that does not have this tight coupling, and it limits the data models that we can write to ideas that are expressible in the host language. In particular, we lose many of the best features of the relational database model. This is a disaster from an ease of use standpoint.

The "Object relational" systems provide an interesting alternative. Here, some notion of subtyping is embedded into an existing SQL or SQL-like database engine. Examples of systems like this include the new Informix, PostgreSQL 7, and Oracle has something like this too. The main problem with these systems: each one implements their own non-portable extensions to SQL to implement subtyping. Thus, making OpenACS data models portable would become even more difficult. In addition, each of these object systems have strange limitations that make using inheritance difficult in practice. Finally, object databases are not as widely used as traditional relational systems. They have not been tested as extensively and their scalability to very large databases is not proven (though some will disagree with this statement).

#### Oracle

The conclusion: the best design is to add a limited notion of subtyping to our existing relational data model. By doing this, we retain all the power of the relational data model while gaining the object oriented features we need most.

In the context of OpenACS 4, this means using the object model to make our data models more flexible, so that new modules can easily gain access to generic features. However, while the API itself doesn't enforce the idea that applications only use the object model for metadata, it is also the case that the data model is not designed to scale to large type hierarchies. In the more limited domain of the metadata model, this is acceptable since the type hierarchy is fairly small. But the object system data model is not designed to support, for example, a huge type tree like the Java runtime libraries might define.

This last point cannot be over-stressed: the object model is not meant to be used for large scale application data storage. It is meant to represent and store metadata, not application data.

### Data Model

Like most data models, the OpenACS Core data model has two levels:

1. The knowledge level (i.e. the metadata model)

2. The operational level (i.e. the concrete data model)

You can browse the data models themselves from here:

- acs-metadata-create.sql

- acs-objects-create.sql

- acs-relationships-create.sql

(Note that we have subdivided the operational level into the latter two files.)

The operational level depends on the knowledge level, so we discuss the knowledge level first. In the text below, we include abbreviated versions of the SQL definitions of many tables. Generally, these match the actual definitions in the existing data model but they are meant to reflect design information, not implementation. Some less relevant columns may be left out, and things like constraint names are not included.

#### Knowledge-Level Model

The knowledge level data model for OpenACS objects centers around three tables that keep track of object types, attributes, and relation types. The first table is `acs_object_types`, shown here in an abbreviated form:

    create table acs_object_types (
            object_type          varchar(100) not null primary key,
            supertype            references acs_object_types (object_type),
            abstract_p           char(1) default 'f' not null
            pretty_name          varchar(100) not null unique,
            pretty_plural        varchar(100) not null unique,
            table_name           varchar(30) not null unique,
            id_column            varchar(30) not null,
            name_method          varchar(30),
            type_extension_table varchar(30)
    );

This table contains one row for every object type in the system. The key things to note about this table are:

- For every type, we store metadata for how to display this type in certain contexts (`pretty_name` and `pretty_plural`).

- If the type is a subtype, then its parent type is stored in the column `supertype`.

- We support a notion of "abstract" types that contain no instances (as of 9/2000 this is not actually used). These types exist only to be subtyped. An example might be a type representing "shapes" that contains common characteristics of all shapes, but which is only used to create subtypes that represent real, concrete shapes like circles, squares, and so on.

- Every type defines a table in which one can find one row for every instance of this type (`table_name`, `id_column`).

- `type_extension_table` is for naming a table that stores extra generic attributes.

The second table we use to describe types is `acs_attributes`. Each row in this table represents a single attribute on a specific object type (e.g. the "password" attribute of the "user" type). Again, here is an abbreviated version of what this table looks like. The actual table used in the implementation is somewhat different and is discussed in a separate document.

    create table acs_attributes (
            attribute_id    integer not null primary key
            object_type     not null references acs_object_types (object_type),
            attribute_name  varchar(100) not null,
            pretty_name     varchar(100) not null,
            pretty_plural   varchar(100),
            sort_order      integer not null,
            datatype        not null,
            default_value   varchar(4000),
            storage         varchar(13) default 'type_specific'
                            check (storage in ('type_specific',
                                               'generic')),
            min_n_values    integer default 1 not null,
            max_n_values    integer default 1 not null,
            static_p        varchar(1)
    );

The following points are important about this table:

- Every attribute has a unique identifier.

- Every attribute is associated with an object type.

- We store various things about each attribute for presentation (`pretty_name`, `sort_order`).

- The `data_type` column stores type information on this attribute. This is not the SQL type of the attribute; it is just a human readable name for the type of data we think the attribute holds (e.g. "String", or "Money"). This might be used later to generate a user interface.

- The `sort_order` column stores information about how to sort the attribute values.

- Attributes can either be stored explicitly in a table ("type specific storage") or in a skinny table ("generic storage"). In most cases, an attribute maps directly to a column in the table identified by the `table_name` of the corresponding object type, although, as mentioned above, we sometimes store attribute values as key-value pairs in a "skinny" table. However, when you ask the question "What are the attributes of this type of object?", you don't really care about how the values for each attribute are stored (in a column or as key-value pairs); you expect to receive the complete list of all attributes.

- The `max_n_values` and `min_n_values` columns encode information about the number of values an attribute may hold. Attributes can be defined to hold 0 or more total values.

- The `static_p` flag indicates whether this attribute value is shard by all instances of a type, as with static member fields in C++. Static attribute are like group level attributes in OpenACS 3.x.

The final part of the knowledge level model keeps track of relationship types. We said above that object relationships are used to generalize the 3.x notion of group member fields. These were fields that a developer could store on each member of a group, but which were contextualized to the membership relation. That is, they were really "attached" to the fact that a user was a member of a particular group, and not really attached to the user. This is a subtle but important distinction, because it allowed the 3.x system to store multiple sets of attributes on a given user, one set for each group membership relation in which they participated.

In OpenACS 4, this sort of data can be stored as a relationship type, in `acs_rel_types`. The key parts of this table look like this:

    create table acs_rel_types (
            rel_type        varchar(100) not null
                            references acs_object_types(object_type),
            object_type_one not null
                            references acs_object_types (object_type),
            role_one        references acs_rel_roles (role),
            object_type_two not null
                            references acs_object_types (object_type),
            role_two        references acs_rel_roles (role)
            min_n_rels_one  integer default 0 not null,
            max_n_rels_one  integer,
            min_n_rels_two  integer default 0 not null,
            max_n_rels_two  integer
    );

Things to note about this table:

- The main part of this table records the fact that the relation is between instances of `object_type_one` and instances of `object_type_two`. Therefore, each instance of this relation type will be a pair of objects of the appropriate types.

- The `role` columns store human readable names for the roles played by each object in the relation (e.g. "employee" and "employer"). Each role must appear in the `acs_rel_roles`.

- The `min_n_rels_one` column, and its three friends allow the programmer to specify constraints on how many objects any given object can be related to on either side of the relation.

This table is easier to understand if you also know how the `acs_rels` table works.

To summarize, the `acs_object_types` and `acs_attributes` tables store metadata that describes every object type and attribute in the system. These tables generalize the group types data model in OpenACS 3.x. The `acs_rel_types` table stores information about relation types.

This part of the data model is somewhat analogous to the data dictionary in Oracle. The information stored here is primarily metadata that describes the data stored in the operational level of the data model, which is discussed next.

#### Operational-level Data Model

The operational level data model centers around the `acs_objects` table. This table contains a single row for every instance of the type `acs_object`. The table contains the object's unique identifier, a reference to its type, security information, and generic auditing information. Here is what the table looks like:

    create table acs_objects (
            object_id               integer not null,
            object_type             not null
                                    references acs_object_types (object_type),
            context_id              references acs_objects(object_id),
            security_inherit_p      char(1) default 't' not null,
                                    check (security_inherit_p in ('t', 'f')),
            creation_user           integer,
            creation_date           date default sysdate not null,
            creation_ip             varchar(50),
            last_modified           date default sysdate not null,
            modifying_user          integer,
            modifying_ip            varchar(50)
    );

As we said in Section III, security contexts are hierarchical and also modeled as objects. There is another table called `acs_object_context_index` that stores the context hierarchy.

Other tables in the core data model store additional information related to objects. The table `acs_attribute_values` and `acs_static_attr_values` are used to store attribute values that are not stored in a helper table associated with the object's type. The former is used for instance attributes while the latter is used for class-wide "static" values. These tables have the same basic form, so we'll only show the first:

    create table acs_attribute_values (
            object_id       not null
                            references acs_objects (object_id) on delete cascade,
            attribute_id    not null
                            references acs_attributes (attribute_id),
            attr_value      varchar(4000),
            primary key     (object_id, attribute_id)
    );

Finally, the table `acs_rels` is used to store object pairs that are instances of a relation type.

    create table acs_rels (
            rel_id          not null
                            references acs_objects (object_id)
                            primary key
            rel_type        not null
                            references acs_rel_types (rel_type),
            object_id_one   not null
                            references acs_objects (object_id),
            object_id_two   not null
                            references acs_objects (object_id),
            unique (rel_type, object_id_one, object_id_two)
    );

This table is somewhat subtle:

- `rel_id` is the ID of an instance of some relation type. We do this so we can store all the mapping tables in this one table.

- `rel_type` is the ID of the relation type to which this object belongs.

- The next two object IDs are the IDs of the objects being mapped.

All this table does is store one row for every pair of objects that we'd like to attach with a relation. Any additional attributes that we'd like to attach to this pair of objects is specified in the attributes of the relation type, and could be stored in any number of places. As in the 3.x user/groups system, these places include helper tables or generic skinny tables.

This table, along with `acs_attributes` and `acs_attribute_values` generalize the old user/group tables `user_group_map`, `user_group_member_fields_map` and `user_group_member_fields`.

#### Summary and Discussion

The core tables in the OpenACS 4 data model store information about instances of object types and relation types. The `acs_object` table provides the central location that contains a single row for every object in the system. Services can use this table along with the metadata in stored in the knowledge level data model to create, manage, query and manipulate objects in a uniform manner. The `acs_rels` table has an analogous role in storing information on relations.

These are all the tables that we'll discuss in this document. The rest of the Kernel data model is described in the documents for subsites, the permissions system and for the groups system.

Some examples of how these tables are used in the system can be found in the discussion of the API, which comes next.

### API

Now we'll examine each piece of the API in detail. Bear in mind that the Object Model API is defined primarily through PL/SQL packages.

#### Object Types and Attributes

The object system provides an API for creating new object types and then attaching attributes to them. The procedures `create_type` and `drop_type` are used to create and delete type definitions.

The two calls show up in the package `acs_object_type`.

      procedure create_type (
        object_type         in acs_object_types.object_type%TYPE,
        pretty_name         in acs_object_types.pretty_name%TYPE,
        pretty_plural       in acs_object_types.pretty_plural%TYPE,
        supertype           in acs_object_types.supertype%TYPE
                               default 'acs_object',
        table_name          in acs_object_types.table_name%TYPE default null,
        id_column           in acs_object_types.id_column%TYPE default 'XXX',
        abstract_p          in acs_object_types.abstract_p%TYPE default 'f',
        type_extension_table in acs_object_types.type_extension_table%TYPE
                                default null,
        name_method         in acs_object_types.name_method%TYPE default null
      );

      -- delete an object type definition
      procedure drop_type (
        object_type         in acs_object_types.object_type%TYPE,
        cascade_p           in char default 'f'
      );

Here the `cascade_p` argument indicates whether dropping a type should also remove all its subtypes from the system.

We define a similar interface for defining attributes in the package `acs_attribute`:

      function create_attribute (
        object_type         in acs_attributes.object_type%TYPE,
        attribute_name      in acs_attributes.attribute_name%TYPE,
        datatype            in acs_attributes.datatype%TYPE,
        pretty_name         in acs_attributes.pretty_name%TYPE,
        pretty_plural       in acs_attributes.pretty_plural%TYPE default null,
        table_name          in acs_attributes.table_name%TYPE default null,
        column_name         in acs_attributes.column_name%TYPE default null,
        default_value       in acs_attributes.default_value%TYPE default null,
        min_n_values        in acs_attributes.min_n_values%TYPE default 1,
        max_n_values        in acs_attributes.max_n_values%TYPE default 1,
        sort_order          in acs_attributes.sort_order%TYPE default null,
        storage             in acs_attributes.storage%TYPE default 'type_specific',
        static_p            in acs_attributes.static_p%TYPE default 'f'
      ) return acs_attributes.attribute_id%TYPE;

      procedure drop_attribute (
        object_type in varchar,
        attribute_name in varchar
      );

In addition, the following two calls are available for attaching extra annotations onto attributes:

      procedure add_description (
        object_type         in acs_attribute_descriptions.object_type%TYPE,
        attribute_name      in acs_attribute_descriptions.attribute_name%TYPE,
        description_key     in acs_attribute_descriptions.description_key%TYPE,
        description         in acs_attribute_descriptions.description%TYPE
      );

      procedure drop_description (
        object_type         in acs_attribute_descriptions.object_type%TYPE,
        attribute_name      in acs_attribute_descriptions.attribute_name%TYPE,
        description_key     in acs_attribute_descriptions.description_key%TYPE
      );

At this point, what you must do to hook into the object system from your own data model becomes clear:

- Create a table that will store the instances of the new type.

- Call `acs_object_type.create_type()` to fill in the metadata table on this new type. If you want your objects to appear in the `acs_objects` table, then your new type must be a subtype of `acs_object`.

- Call `acs_attribute.create_attribute()` to fill in information on the attributes that this type defines.

So, suppose we are writing a new version of the ticket tracker for 4.0. We probably define a table to store tickets in, and each ticket might have an ID and a description. If we want each ticket to be an object, then `ticket_id` must reference the `object_id` column in `acs_objects`:

    create table tickets (
        ticket_id references acs_objects (object_id),
        description varchar(512),
        ...
    ) ;

In addition to defining the table, we need this extra PL/SQL code to hook into the object type tables:

    declare
     attr_id acs_attributes.attribute_id%TYPE;
    begin
     acs_object_type.create_type (
       supertype => 'acs_object',
       object_type => 'ticket',
       pretty_name => 'Ticket',
       pretty_plural => 'Tickets',
       table_name => 'tickets',
       id_column => 'ticket_id',
       name_method => 'acs_object.default_name'
     );

     attr_id := acs_attribute.create_attribute (
       object_type => 'ticket',
       attribute_name => 'description',
       datatype => 'string',
       pretty_name => 'Description',
       pretty_plural => 'Descriptions'
     );

     ... more attributes ...

    commit;
    end;

Thus, with a small amount of extra code, the new ticket tracker will now automatically be hooked into every generic object service that exists. Better still, this code need not be changed as new services are added. As an aside, the most important service that requires you to subtype `acs_object` is permissions.

#### Objects

The next important piece of the API is defined in the `acs_object` package, and is concerned with creating and managing objects. This part of the API is designed to take care of the mundane bookkeeping needed to create objects and query their attributes. Realistically however, limitations in PL/SQL and Oracle will make it hard to build generic procedures for doing large scale queries in the object system, so developers who need to do this will probably have to be fairly familiar with the data model at a lower level.

The function `acs_object.new()` makes a new object for you. The function `acs_object.del()` deletes an object. As before, this is an abbreviated interface with all the long type specs removed. See the data model or developer's guide for the full interface.

     function new (
      object_id     in acs_objects.object_id%TYPE default null,
      object_type   in acs_objects.object_type%TYPE
                               default 'acs_object',
      creation_date in acs_objects.creation_date%TYPE
                               default sysdate,
      creation_user in acs_objects.creation_user%TYPE
                               default null,
      creation_ip   in acs_objects.creation_ip%TYPE default null,
      context_id    in acs_objects.context_id%TYPE default null
     ) return acs_objects.object_id%TYPE;

     procedure delete (
      object_id     in acs_objects.object_id%TYPE
     );

Next, we define some generic functions to manipulate attributes. Again, these interfaces are useful to an extent, but for large scale queries, it's likely that developers would have to query the data model directly, and then encapsulate their queries in procedures.

For names, the `default_name` function is used if you don't want to define your own name function.

     function name (
      object_id     in acs_objects.object_id%TYPE
     ) return varchar;

     function default_name (
      object_id     in acs_objects.object_id%TYPE
     ) return varchar;

The following functions tell you where attributes are stored, and fetch single attributes for you.

     procedure get_attribute_storage (
       object_id_in      in  acs_objects.object_id%TYPE,
       attribute_name_in in  acs_attributes.attribute_name%TYPE,
       v_column          out varchar2,
       v_table_name      out varchar2,
       v_key_sql         out varchar2
     );

     function get_attribute (
       object_id_in      in  acs_objects.object_id%TYPE,
       attribute_name_in in  acs_attributes.attribute_name%TYPE
     ) return varchar2;

     procedure set_attribute (
       object_id_in      in  acs_objects.object_id%TYPE,
       attribute_name_in in  acs_attributes.attribute_name%TYPE,
       value_in          in  varchar2
     );

The main use of the `acs_object` package is to create application objects and make them available for services via the `acs_objects` table. To do this, you just have to make sure you call `acs_object.new()` on objects that you wish to appear in the `acs_objects` table. In addition, all such objects must be instances of some subtype of `acs_object`.

Continuing the ticket example, we might define the following sort of procedure for creating a new ticket:

     function new_ticket (
      package_id        in tickets.ticket_id%TYPE
                default null,
      description       in tickets.description%TYPE default '',
         ...
      ) return tickets.ticket_id%TYPE
      is
       v_ticket_id tickets
      begin
       v_ticket_id := acs_object.new(
          object_id => ticket_id,
          object_type => 'ticket',
            ...
         );
        insert into tickets
        (ticket_id, description)
        values
        (v_ticket_id, description);
        return v_ticket_id;
      end new_ticket;

This function will typically be defined in the context of a PL/SQL package, but we've left it stand-alone here for simplicity.

To summarize: in order to take advantage of OpenACS 4 services, a new application need only do three things:

- Define a data model to describe application objects. This can just be a normal SQL table.

- Create an object type, using code like in the example from the previous section.

- Make sure application objects are created using `acs_object.new()` in addition to whatever SQL code is needed to insert a new row into the application data model.

One of the design goals of OpenACS 4 was to provide a straightforward and consistent mechanism to provide applications with general services. What we have seen here is that three simple steps and minimal changes in the application data model are sufficient to make sure that application objects are represented in the `acs_objects` table. Subsequently, all of the general services in OpenACS 4 (i.e. permissions, general comments, and so on) are written to work with any object that appears in `acs_objects`. Therefore, in general these three steps are sufficient to make OpenACS 4 services available to your application.

#### Relation Types

The relations system defines two packages: `acs_rel_type` for creating and managing relation types, and `acs_rel` for relating objects.

These two procedures just insert and remove roles from the `acs_rel_roles` table. This table stores the legal relationship "roles" that can be used when creating relation types. Examples of roles are, say, "member", or "employer".

     procedure create_role (
        role        in acs_rel_roles.role%TYPE
      );

      procedure drop_role (
        role        in acs_rel_roles.role%TYPE
      );

The main functions in the `acs_rel_type` package are used to create and drop relation types.

      procedure create_type (
        rel_type            in acs_rel_types.rel_type%TYPE,
        pretty_name         in acs_object_types.pretty_name%TYPE,
        pretty_plural       in acs_object_types.pretty_plural%TYPE,
        supertype           in acs_object_types.supertype%TYPE
                               default 'relationship',
        table_name          in acs_object_types.table_name%TYPE,
        id_column           in acs_object_types.id_column%TYPE,
        abstract_p          in acs_object_types.abstract_p%TYPE default 'f',
        type_extension_table in acs_object_types.type_extension_table%TYPE
                                default null,
        name_method         in acs_object_types.name_method%TYPE default null,
        object_type_one     in acs_rel_types.object_type_one%TYPE,
        role_one            in acs_rel_types.role_one%TYPE default null,
        min_n_rels_one      in acs_rel_types.min_n_rels_one%TYPE,
        max_n_rels_one      in acs_rel_types.max_n_rels_one%TYPE,
        object_type_two     in acs_rel_types.object_type_two%TYPE,
        role_two            in acs_rel_types.role_two%TYPE default null,
        min_n_rels_two      in acs_rel_types.min_n_rels_two%TYPE,
        max_n_rels_two      in acs_rel_types.max_n_rels_two%TYPE
      );

      procedure drop_type (
        rel_type            in acs_rel_types.rel_type%TYPE,
        cascade_p           in char default 'f'
      );

Finally, the `acs_rel` package provides an API that you use to create and destroy instances of a relation type:

      function new (
        rel_id              in acs_rels.rel_id%TYPE default null,
        rel_type            in acs_rels.rel_type%TYPE default 'relationship',
        object_id_one       in acs_rels.object_id_one%TYPE,
        object_id_two       in acs_rels.object_id_two%TYPE,
        context_id          in acs_objects.context_id%TYPE default null,
        creation_user       in acs_objects.creation_user%TYPE default null,
        creation_ip         in acs_objects.creation_ip%TYPE default null
      ) return acs_rels.rel_id%TYPE;

      procedure delete (
        rel_id      in acs_rels.rel_id%TYPE
      );

A good example of how to use relation types appears in the OpenACS 4 data model for groups. As in 3.x, group membership is modeled using a mapping table, but now we create this mapping using relation types instead of explicitly creating a table. First, we create a helper table to store state on each membership fact:

    create table membership_rels (
            rel_id          constraint membership_rel_rel_id_fk
                            references acs_rels (rel_id)
                            constraint membership_rel_rel_id_pk
                            primary key,
            -- null means waiting for admin approval
            member_state    varchar(20) constraint membership_rel_mem_ck
                            check (member_state in ('approved', 'banned',
                                                    'rejected', 'deleted'))
    );

Then, we create a new object type to describe groups.

     acs_object_type.create_type (
       object_type => 'group',
       pretty_name => 'Group',
       pretty_plural => 'Groups',
       table_name => 'groups',
       id_column => 'group_id',
       type_extension_table => 'group_types',
       name_method => 'acs_group.name'
     );

In this example, we've made groups a subtype of `acs_object` to make the code simpler. The actual data model is somewhat different. Also, we've assumed that there is a helper table called `groups` to store information on groups, and that there is a helper table called `group_types` that has been defined to store extra attributes on groups.

Now, assuming we have another object type called `person` to represent objects that can be group members, we define the following relationship type for group membership:

     acs_rel_type.create_role ('member');

     acs_rel_type.create_type (
       rel_type => 'membership_rel',
       pretty_name => 'Membership Relation',
       pretty_plural => 'Membership Relationships',
       table_name => 'membership_rels',
       id_column => 'rel_id',
       object_type_one => 'group',
       min_n_rels_one => 0, max_n_rels_one => null,
       object_type_two => 'person', role_two => 'member',
       min_n_rels_two => 0, max_n_rels_two => null
     );

Now we can define the following procedure to add a new member to a group. All this function does is create a new instance of the membership relation type and then insert the membership state into the helper table that we define above. In the actual implementation, this function is implemented in the `membership_rel` package. Here we just define an independent function:

    function member_add (
        rel_id              in membership_rels.rel_id%TYPE default null,
        rel_type            in acs_rels.rel_type%TYPE default 'membership_rel',
        group               in acs_rels.object_id_one%TYPE,
        member              in acs_rels.object_id_two%TYPE,
        member_state        in membership_rels.member_state%TYPE default null,
        creation_user       in acs_objects.creation_user%TYPE default null,
        creation_ip         in acs_objects.creation_ip%TYPE default null
      ) return membership_rels.rel_id%TYPE
      is
        v_rel_id integer;
      begin
          v_rel_id := acs_rel.new (
          rel_id => rel_id,
          rel_type => rel_type,
          object_id_one => group,
          object_id_two => person,
          context_id => object_id_one,
          creation_user => creation_user,
          creation_ip => creation_ip
        );

        insert into membership_rels
         (rel_id, member_state)
        value
         (v_rel_id, new.member_state);
      end;

Another simple function can be defined to remove a member from a group:

      procedure member_delete (
        rel_id  in membership_rels.rel_id%TYPE
      )
      is
      begin
        delete from membership_rels
        where rel_id = membership_rel.delete.rel_id;

        acs_rel.del(rel_id);
      end;

#### Summary and Discussion

The Object Model's API and data model provides a small set of simple procedures that allow applications to create object types, object instances, and object relations. Most of the data model is straightforward; the relation type mechanism is a bit more complex, but in return it provides functionality on par with the old user/groups system in a more general way.

### Future Improvements/Areas of Likely Change

Nothing here yet.

### Authors

Pete Su generated this document from material culled from other documents by Michael Yoon, Richard Li and Rafael Schloming. But, any remaining lies are his and his alone.

### Revision History

Document Revision # | Action Taken, Notes | When? | By Whom?
0.1 | Creation | 9/09/2000 | Pete Su
0.2 | Edited for ACS 4 Beta | 9/30/2000 | Kai Wu
0.3 | Edited for ACS 4.0.1, fixed some mistakes, removed use of term "OM" | 11/07/2000 | Pete Su

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