Not yet linked.

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.

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:

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 two separate documents: one for the permissions system and another for the party groups system. The context system is also used to implement subsites.

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:

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 use this mechanism without worrying about this bookkeeping themselves, and we avoid having applications pollute the core data model with their specific information.

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 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 ACS Object 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.

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.

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 nonportable 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).

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 too 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 run time 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.

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(1000) not null primary key,
        supertype            references acs_object_types (object_type),
        abstract_p           char(1) default 'f' not null
        pretty_name          varchar(1000) not null unique,
        pretty_plural        varchar(1000) 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:

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:

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(1000) 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:

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.

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:

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.

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.

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:

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.

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:

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.

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;

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.