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=Problem=
Today, IdAS defines a set of special interfaces for accessing a context's model elements.  There are a number of problems with these interfaces:
# They are 'fixed' in nature.  The only way to represent extra information about the model of context elements is to revise these interfaces.  For example, say there's a need to associate an image with the attribute called <nowiki>http://example.com/some/name/space#telephoneNumber</nowiki> (this image might be used by a management UI).  To make that association we need a new method like IAttributeModel.getManagementImage().  
## Even if we felt like it's ok to revise the current I*Model interfaces, we do not know what kinds of information will need to be associated with the models of elements in a context.  It may be the case that some people will have needs that we think are unworthy of revising the interfaces for.  For example, say someone wants to associate an astrological symbol to each different attribute model.  Does that mean we should add a new IAttributeModel.getAstrologicalSymbol()?
# There is no way to update a context's model.  We have a need to be able to add models for new entity types, attribute types and so-on. We also need to be able to change existing models (such as changing the cardinality of an attribute).  Also, we may need to remove models.
# We cannot look up a model with partial information.  For example, say I know there's an attribute with the word "telephoneNumber" in its ID, but I don't know the entire ID.  It would be nice to be able to have a way to find what I need without enumerating through the entire set of attribute models.

=Proposal=
It may be better to move away from the existing special interfaces for the purpose of defining the model of elements, but instead simply re-use the existing interfaces that are used to access normal information within a Context (entities and their attributes).

Following is a proposal which shows how these model entities might look.  To illustrate the proposal, we start by showing how an instance of a person looks (nothing new) and then examining the model entities that govern instances of entities and attributes:

==A person==
Example of an instance of a person entity.
* Java type: some impl of IEntity. The context provider implements this class
* getEntityID() returns: the contextually unique ID for this person
* getAttributes() returns: typical attributes about this person (things like a full name and email address)
* getType() returns: an identifier for this person's entity class.  assume it's called <nowiki>http://example.com/class#Person</nowiki>
** Note that getType is only a convenience for getTypeEntity().getEntityID()
* getTypeEntity() returns: some impl of IEntity, assume it returns a PersonClass.class

==A person's entity class==
Now, say we look at the “<nowiki>http://example.com/class#Person</nowiki>” entity we saw above (we got this either by calling IEntity.getTypeEntity(), or IEntity.getType() followed by IContext.getEntity(EntityID)), we see (NOTE: this entity represents the class definition of a person):
* Java type: some impl of IEntity – This is implemented by the context provider.  We can assume it's a PersonClass.class.
* getEntityID() returns: the contextually unique ID for this entity.  This is the same value we got when calling IEntity.getType() on the person instance above, so in this case it would be <nowiki>http://example.com/class#Person</nowiki>.
* getAttributes() returns: the following and other attributes:
** <nowiki>http://www.eclipse.org/higgins/ontologies/2008/6/higgins#validAttributes</nowiki>:  This attribute has complex values, each of which represent a valid attribute for this type of entity.  Each value has the following literal elements:
*** <nowiki>http://www.w3.org/2000/01/rdf-schema#Property</nowiki> = <attribute type uri like <nowiki>http://example.com/prop#fullname</nowiki>>
*** <nowiki>http://www.w3.org/2002/07/owl#minCardinality</nowiki> = <minimum cardinality for values of this attribute as it applies to the person class.>
*** <nowiki>http://www.w3.org/2002/07/owl#maxCardinality</nowiki> = <maximum cardinality for values of this attribute as it applies to the person class.>
*** Note that the full list of valid attributes for this type of entity must be discovered by looking at this list as well as the list of all this entity class's supertypes (see the <nowiki>http://www.w3.org/2000/01/rdf-schema#subClassOf</nowiki> attribute).  We can talk about possible shortcuts to get them all at once.
** <nowiki>http://www.w3.org/2000/01/rdf-schema#subClassOf</nowiki>:  This has a single, literal value which names the supertype of person.  If person is derived from something like <nowiki>http://example.com/class#Mammal</nowiki> it would name that.  If person is at the top of it's entity type hierarchy in this context, this attribute would name the well-known entity class entity that defines IEntitys in general (that would be called <nowiki>http://www.eclipse.org/higgins/ontologies/2008/6/higgins#Entity</nowiki>)
* getType() returns: the well-known identifier <nowiki>http://www.w3.org/2002/07/owl#Class</nowiki>
* getTypeEntity() returns: an implementation of IEntity which represents <nowiki>http://www.w3.org/2002/07/owl#Class</nowiki>.  A default object would be implemented by Higgins, but could be overridden by context providers which wish to add additional capabilities.

==The "top" entity class==
Ok, now let's look at the instance of the entity called <nowiki>http://www.eclipse.org/higgins/ontologies/2008/6/higgins#Entity</nowiki>.  This is simply the top-level class definition for all entity class hierarchies.  Thus it contains class definitions for things common to all entities:
* Java type: an impl of IEntity (maybe we'll call it Entity.class).  This is implemented by Higgins and is always the same
* getEntityID() returns: the unique ID for this entity.  This is <nowiki>http://www.eclipse.org/higgins/ontologies/2008/6/higgins#Entity</nowiki>
* getAttributes() returns: the following attributes:
** <nowiki>http://www.eclipse.org/higgins/ontologies/2008/6/higgins#validAttributes</nowiki>:  In the base higgins definition of the top entity class, only one value exists at this point, and that is:
*** <nowiki>http://www.w3.org/2000/01/rdf-schema#Property</nowiki> = <nowiki>http://www.eclipse.org/higgins/ontologies/2006/higgins#EntityID</nowiki>
*** <nowiki>http://www.w3.org/2002/07/owl#minCardinality</nowiki> = 1
*** <nowiki>http://www.w3.org/2002/07/owl#maxCardinality</nowiki> = 1
* getTypeEntity() returns: an implementation of IEntity which represents <nowiki>http://www.w3.org/2002/07/owl#Class</nowiki>.  A default object would be implemented by Higgins, but could be overridden by context providers which wish to add additional capabilities.

==The definition for entity classes==
What defines an entity class entity?  This does:
* Java type: an impl of IEntity.  Higgins will provide a default class called EntityClass.class, but a context provider is free to return its own (which may or may not be a subclass of the Higgins EntityClass.class), as long as it contains the <nowiki>http://www.eclipse.org/higgins/ontologies/2008/6/higgins#validAttributes</nowiki> and its values mentioned below.  A context provider is free to override and add more values here.
* getEntityID() returns: the unique ID for this entity.  This is <nowiki>http://www.w3.org/2002/07/owl#Class</nowiki>
* getAttributes() returns: the following attributes:
** <nowiki>http://www.eclipse.org/higgins/ontologies/2008/6/higgins#validAttributes</nowiki>: Names the attributes (along with their min/max cardinality) valid for entity classes.  These values exists here (note that a context provider may add to this set of values):
*** {<nowiki>http://www.w3.org/2000/01/rdf-schema#Property</nowiki> = <nowiki>http://www.eclipse.org/higgins/ontologies/2006/higgins#validAttributes</nowiki>, minCard = 0, maxCard = 0}
*** {<nowiki>http://www.w3.org/2000/01/rdf-schema#Property</nowiki> = <nowiki>http://www.w3.org/2000/01/rdf-schema#subClassOf</nowiki>, minCard = 0, maxCard = 1} 
* getType() returns: the well-known identifier <nowiki>http://www.w3.org/2002/07/owl#Class</nowiki>, or maybe null
* getTypeEntity() returns this object, or maybe it should be null

==An attribute definition==
Now for fun, let's look at what defines an attribute, and follow that.  We'll take the example of <nowiki>http://example.com/prop/fullname</nowiki>:
* Java type: an impl of IEntity. This is created by the context provider.
* getEntityID() returns: the unique ID for this entity.  In this case it's <nowiki>http://example.com/prop/fullname</nowiki>
* getAttributes() returns: the following attributes:
** <nowiki>http://www.w3.org/2000/01/rdf-schema#range</nowiki>:  Each value would be the identifier of an entity which represents a permitted data type for the value(s) of attribute instances that this attribute definition describes.  An example might be <nowiki>http://www.w3.org/2001/XMLSchema#string</nowiki>, but could also point at any other value data type (for simple types) or entity class (for complex types).
** <nowiki>http://www.w3.org/2000/01/rdf-schema#subPropertyOf</nowiki>:  This attribute would point at another attribute definition entity.  If fullname is derived from something like <nowiki>http://example.com/prop#name</nowiki> it would point at that.  If fullname is at the top of it's attribute type hierarchy in this context, this attribute would be absent.
** <nowiki>http://www.eclipse.org/higgins/ontologies/2008/6/higgins#validAttributes</nowiki>:  This is the list of (metadata) attributes valid for this attribute.  If no attributes are valid, this attr is not populated.
* getType() returns: the well-known identifier <nowiki>http://www.w3.org/2000/01/rdf-schema#Property</nowiki>
* getTypeEntity() returns: an implementation of IEntity representing <nowiki>http://www.w3.org/2000/01/rdf-schema#Property</nowiki>.  This object would be implemented by Higgins and thus would be the same for all context providers.

==The definition for attribute definitions==
And here's that the entity identified by <nowiki>http://www.w3.org/2000/01/rdf-schema#Property</nowiki> looks like:
* Java type: an impl of IEntity.  Higgins will provide a default class called AttributeDefinition.class, but a context provider is free to return its own (which may or may not be a subclass of the Higgins AttributeDefinition.class), as long as it contains the <nowiki>http://www.eclipse.org/higgins/ontologies/2008/6/higgins#validAttributes</nowiki> and its values mentioned below.  A context provider is free to add more values to the <nowiki>http://www.eclipse.org/higgins/ontologies/2008/6/higgins#validAttributes</nowiki> attribute.
* getEntityID() returns: the unique ID for this entity.  This is <nowiki>http://www.w3.org/2000/01/rdf-schema#Property</nowiki>
* getAttributes() returns: the following attributes:
** <nowiki>http://www.eclipse.org/higgins/ontologies/2008/6/higgins#validAttributes</nowiki>:  These values represent the types of attributes allowed on an attribute definition entity (note that a context provider may add to this set of values):
*** <nowiki>http://www.w3.org/2000/01/rdf-schema#range</nowiki>
*** <nowiki>http://www.w3.org/2000/01/rdf-schema#subPropertyOf</nowiki>
*** <nowiki>http://www.eclipse.org/higgins/ontologies/2008/6/higgins#validAttributes</nowiki>
* getType() returns: the well-known identifier <nowiki>http://www.w3.org/2002/07/owl#Class</nowiki>
* getTypeEntity() returns: an implementation of IEntity representing <nowiki>http://www.w3.org/2002/07/owl#Class</nowiki>.  This object would be implemented by Higgins and thus would be the same for all context providers.

Then of course we have data value definitions.  Again from the bottom-up, here's an entity that describes a value type called <TODO: finish this>

= TODO =
* Need model element(s) for data types.  These should follow what is seen at http://www.w3.org/TR/xmlschema-2

[[Category:Higgins IdAS]]