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:
Storage of user-defined or extensible sets of attributes
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:
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:
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.
Objects are instances of developer-defined object types. Object types allow developers to customize the data that is stored with each object.
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 particular programming idioms that we wish to generalize.
This design document should be read along with the design documents for the new groups system, subsites and the permissions system
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 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
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
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
user_group_map table as a
way to identify data associated with a single membership
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
identifiers are a good example of metadata in the new system. Each
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
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
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
the name of the group type.
The user/groups data model also provided the
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
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
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 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 the Ecommerce data model, the
ec_custom_product_fieldstable defines attributes for catalog products, and the
ec_custom_product_field_valuestable stores values for those attributes.
In the Photo DB data model, the
ph_custom_photo_fieldstable 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.
which stores preferences for registered users in columns such as
"standard" way for an OpenACS 3.x-based application to
add to the list of user preferences is to add a column to the
(exactly the kind of data model change that has historically
complicated the process of upgrading to a more recent OpenACS
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
that kept track of which users belonged to what groups. In
addition, as we discussed in the previous section, the system used
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
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 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:
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.
Like most data models, the OpenACS Core data model has two levels:
The knowledge level (i.e. the metadata model)
The operational level (i.e. the concrete data model)
You can browse the data models themselves from here:
(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.
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
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:
For every type, we store metadata for how to display this type in certain contexts (
If the type is a subtype, then its parent type is stored in the column
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 (
type_extension_tableis 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 (
data_typecolumn 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.
sort_ordercolumn 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_nameof 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.
min_n_valuescolumns encode information about the number of values an attribute may hold. Attributes can be defined to hold 0 or more total values.
static_pflag 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
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:
The main part of this table records the fact that the relation is between instances of
object_type_oneand instances of
object_type_two. Therefore, each instance of this relation type will be a pair of objects of the appropriate types.
rolecolumns 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
min_n_rels_onecolumn, 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_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
stores the context hierarchy.
Other tables in the core data model store additional information
related to objects. The table
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_relsis 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_idis 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_typeis 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_attribute_values generalize the old
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.
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.
The object system provides an API for creating new object types
and then attaching attributes to them. The procedures
drop_type are used to create and delete
The two calls show up in the package
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' );
indicates whether dropping a type should also remove all its
subtypes from the system.
We define a similar interface for defining attributes in the
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.
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_objectstable, then your new type must be a subtype of
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
must reference the
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.
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
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
function is used if you don't want to define your own name
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
table. In addition, all such objects must be instances of some
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
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
The relations system defines two packages:
acs_rel_type for creating and managing
relation types, and
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' );
provides an API that you use to create and destroy instances of a
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
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.
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.