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## Parties in OpenACS

By Rafael H. Schloming

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

### Introduction

While many applications must deal with individuals and many applications must deal with groups, most applications must deal with individuals or groups. It is often the case with such applications that in many respects both individuals and groups are treated in an identical manner. It is this latter class of application that makes it extremely useful to model individuals and groups as specializations of one supertype. This concept is so commonly used throughout human language and thought that we don't even need to invent for it some bizarre and specialized terminology. This supertype is called a "party".

A classic example use of the "party" supertype is evident in a common address book. A typical person's address book might contain the address of his doctor, and his cable company. So what do we label the first field in an entry in his address book? It isn't a person, and it isn't a company. It is a "party".

### The Data Model

Most developers who do significant work with the OpenACS will become intimately familiar with the parties data model, and probably extend it on many occasions. For this reason the parties developer guide will begin with an introduction to the data model.

Parties

The central table in the parties data model is the parties table itself. Every party has exactly one row in this table. Every party has an optional unique email address and an optional url. A party is an acs object, so permissions may granted and revoked on parties and auditing information is stored in the acs objects table.

    create table parties (
        party_id    not null
                constraint parties_party_id_fk references
                acs_objects (object_id)
                constraint parties_pk primary key,
        email       varchar(100)
                constraint parties_email_un unique,
        url     varchar(200)
    );

There are two tables that extend the parties table. One is the persons table and one is the groups table. A row in the persons table represents the most basic form of individual that is modeled by the parties data model. A row in the groups table represents the most basic form of an aggregation of individuals that is represented.

Persons

If a party is an individual then there will be a row in the persons table containing first_names and last_name for that individual. Note that the primary key of the persons table (person_id) references the primary key of the parties table (party_id). This guarantees that if there is a row in the persons table then there must be a corresponding row in the parties table. Also note that an individual need not be known to the system as a user. A user is in fact a further specialized form of an individual.

    create table persons (
        person_id   not null
                constraint persons_person_id_fk
                references parties (party_id)
                constraint persons_pk primary key,
        first_names varchar(100) not null,
        last_name   varchar(100) not null
    );

Users

The users table is an even more specialized form of an individual. A row in the users table represents an individual that has login access to the system. Note that the primary key of the users table references the primary key of the persons table. Once again this guarantees that if there is a row in the users table then there must be a row in the persons table and a row in the parties table.

One of the interesting benefits of decomposing all the information associated with a user into the four tables (acs_objects, parties, persons, users) is that it is now possible to "nuke" a user from a live system by removing his entry from the users table, but leaving the rest of his information present (i.e. turning him from a user into a person). This is because wherever possible the OpenACS data model references the persons or parties table, not the users table. If this feature is desired when extending the system, then the developers should be careful to only references the users table in situations where it is clear that the references is to a user and not to an individual.

    create table users (
        user_id         not null
                    constraint users_user_id_fk
                    references persons (person_id)
                    constraint users_pk primary key,
        password        varchar(100),
        salt            varchar(100),
        screen_name     varchar(100)
                    constraint users_screen_name_un
                    unique,
        priv_name       integer default 0 not null,
        priv_email      integer default 5 not null,
        email_verified_p    char(1) default 't'
                    constraint users_email_verified_p_ck
                    check (email_verified_p in ('t', 'f')),
        email_bouncing_p    char(1) default 'f' not null
                    constraint users_email_bouncing_p_ck
                    check (email_bouncing_p in ('t','f')),
        no_alerts_until     date,
        last_visit      date,
        second_to_last_visit    date,
        n_sessions      integer default 1 not null,
        password_question   varchar(1000),
        password_answer     varchar(1000)
    );

Groups

The final piece of the parties data model is the groups data model. A group is a specialization of a party that represents an aggregation of other parties. The only extra information directly associated with a group (beyond that in the parties table) is the name of the group. As you might guess there is another piece to the groups data model that records relations between parties and groups.

    create table groups (
        group_id    not null
                constraint groups_group_id_fk
                references parties (party_id)
                constraint groups_pk primary key,
        group_name  varchar(100) not null
    );

Group Relations

One surprise here is that there are actually two relations involved. One is the normal membership relation between parties and groups. A party may be a "member" of a group. The other relation is a composition relation between two groups. To fully understand why two relations are necessary, and the situations in which each is appropriate, let's consider an example. Greenpeace is an organization that can have as members both individuals and organizations. Hence the membership relation between groups and parties. But just because you are a member of an organization that is a member of Greenpeace, that doesn't make you a member of Greenpeace, i.e., membership is not transitive with respect to itself. Now let's consider a multinational corporation. This corporation might have a U.S. division and a European division. A member of either the U.S. or European division is automatically a member of the company. In this situation the U.S. and European divisions are "components" of the company, i.e., membership is transitive with respect to composition. Having a membership relation between groups and parties, and having a composition relation between groups and groups allows us to easily model the full range of sophisticated group structures that exist in the real world.

Group Membership

Group memberships can be created and manipulated using the membership_rel package. Note that you can only create one membership object for a given group, party pair. Because of composition, it is possible in some circumstances to make someone a member of a group of which they are already a member. This is because there is a distinction between direct membership and indirect membership (membership via some component or sub component).

    create or replace package membership_rel
    as

      function new (
        rel_id      in membership_rels.rel_id%TYPE default null,
        rel_type        in acs_rels.rel_type%TYPE default 'membership_rel',
        object_id_one   in acs_rels.object_id_one%TYPE,
        object_id_two   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;

      procedure ban (
        rel_id  in membership_rels.rel_id%TYPE
      );

      procedure approve (
        rel_id  in membership_rels.rel_id%TYPE
      );

      procedure reject (
        rel_id  in membership_rels.rel_id%TYPE
      );

      procedure unapprove (
        rel_id  in membership_rels.rel_id%TYPE
      );

      procedure deleted (
        rel_id  in membership_rels.rel_id%TYPE
      );

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

    end membership_rel;
    /
    show errors

Group Composition

Composition relations can be created or destroyed using the composition_rel package. The only restriction on compositions is that there cannot be a cycle, i.e., a group cannot be a component of itself either directly or indirectly. This constraint is maintained for you by the API, but if you don't want users seeing some random PL/SQL error message, it is a good idea to not give them the option to create a composition relation that would result in a cycle.

    create or replace package composition_rel
    as

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

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

    end composition_rel;
    /
    show errors

### Views

The data model described above does a reasonable job of representing many of the situations one is likely to encounter when modeling organizational structures, but we still need to be able to efficiently answer questions like "what members are in this group and all of its components?", and "of what groups is this party a member either directly or indirectly?". Composition relations allow you to describe an arbitrary Directed Acyclic Graph (DAG) between a group and its components. For these reasons the party system provides a bunch of views that take advantage of the internal representation of group relations to answer questions like these very quickly.

This view returns all the subcomponents of a group including components of sub components and so forth. The container_id column is the group_id of the group in which component_id is directly contained. This allows you to easily distinguish whether a component is a direct component or an indirect component. (If a component is a direct component then group_id will be equal to container_id.) You can think of this view as having a primary key of group_id, component_id, and container_id. The rel_id column points to the row in acs_rels that contains the relation object that relates component_id to container_id. The rel_id might be useful for retrieving or updating standard auditing info for the relation.

    create or replace view group_component_map
    as select group_id, component_id, container_id, rel_id
    ...

This is similar to group_component_map except for membership relations. Note that this view will return all membership relations regardless of membership state.

    create or replace view group_member_map
    as select group_id, member_id, container_id, rel_id
    ...

The group_approved_member_map is the same as the group_member_map except it only returns entries that relate to approved members.

    create or replace view group_approved_member_map
    as select group_id, member_id, container_id, rel_id
    ...

This view is useful if you don't care about the distinction between direct membership and indirect membership. It simply returns all members of a group including members of components. (It is the transitive closure.)

    create or replace view group_distinct_member_map
    as select group_id, member_id
    ...

This view is the same as group_distinct_member_map, except it includes the identity mapping. In other words it maps from a party to the fully expanded list of parties represented by that party including the party itself. So if a party is an individual this view will have exactly one mapping that is from that party to itself. If a view is a group containing three individuals, this view will have four rows for that party, one for each member, and one from the party to itself.

    create or replace view party_member_map
    as select party_id, member_id
    ...

This view is the same as above except that when it expands groups, it only pays attention to approved members.

    create or replace view party_approved_member_map
    as select party_id, member_id
    ...

### Extending The Parties Data Model

As is, the parties data model can represent some fairly sophisticated real world situations, and a lot of work has been put into making this efficient, but it is foolish to assume that this data model is sufficient for every application. It therefore seems likely that developers will want to extend the parties data model in a number of ways. This section will describe some of the more common ways.

Specializing Users

It is conceivable that some applications will want to collect more detailed information for people using the system. If it is the case that there can be only one such piece of information per user, then it might make sense to create another type of individual that is a further specialization of a user. For example a MENSA community web site might want to record IQs for each user. In a situation like this it would be appropriate to create a subtype of users, say mensa_users. This child table of the users table would have a primary key that references the users table, thereby guaranteeing that each row in the mensa_users table has a corresponding row in each of the users, persons, parties, and acs_objects tables. This child table could then store any extra information relevant to the MENSA community.

Specializing Groups

If one were to build an intranet application on top of the party system, it is likely that one would want to take advantage of the systems efficient representation of sophisticated organizational structures, but there would be much more specialized information associated with each group. In this case it would make sense to specialize the group party type into a company party type in the same manner as above.

Specializing Membership Relations

The final portion of the parties data model that is designed to be extended is the membership relationship. Consider the intranet example again. It is likely that a membership in a company would have more information associated with it than a membership in an ordinary group. An obvious example of this would be a salary. It is exactly this need to be able to extend membership relations with mutable pieces of state that drove us to include a single integer primary key in what could be thought of as a pure relation. Because a membership relation is an ordinary acs object with object identity, it is as easy to extend the membership relation to store extra information as it is to extend the users table or the groups table.

($Id$)

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