ObjAttribute Mapping Specification.

This document describes new principles of mapping ObjAttributes
to DbAttributes and related operations on DataObjects.

1) Each ObjAttribute in an ObjEntity is mapped to a DB_PATH
comprised of 0..n DbRelationships and 1 DbAttribute.

2) If there are 0 DbRelationships in the DB_PATH
DbAttribute must belong to the DbEntity "MasterTable" 
this ObjEntity refers to (defined by the attribute 
dbEntityName in element obj-entity, see data-map.dtd).

3) If there are 1..n DbRelationships ("r_1",.."r_i",.."r_n")in the DB_PATH
the following rules apply:
    a) Each "r_i" is "to-one" DbRelationship defined in this DataMap.
    b) "r_1" originates from "MasterTable".
    c) Target DbEntity of each "r_i" coincides with Source of "r_i+1".
    d) DbAttribute belongs to the target DbEntity of "r_n".
    
4) The element obj-attribute in data-map.dtd will be modified
to support the mapping. Namely:

<!ATTLIST obj-attribute
         name CDATA #REQUIRED
         db-attribute-path CDATA #REQUIRED
         type CDATA #REQUIRED
>

Where the attribute db-attribute-path will accept values like:
DB_ATTR_NAME - name of one of the DbAttributes belonging to "MasterTable".
dbRelationshipName1.DB_ATTR_NAME,
dbRelationshipName1.dbRelationshipName2.dbRelationshipName3.DB_ATTR_NAME,
etc.

5) An ObjEntity is allowed to be "read-write" only if for each of
its ObjAttributes one of the following statements is true:
    a) ObjAttribute is mapped to a DbAttribute of the DbEntity
    this ObjEntity refers to (defined by attribute dbEntityName in
    element obj-entity, see data-map.dtd).

    b) ObjAttribute is mapped to a dbRelationship1.dbAttribute
    where dbRelationship1 is a "one-to-one" DbRelationship with
    the target column defined as the primary key of the target
    DbEntity. That is, if DbEntity of this ObjEntity is a master
    table and target DbEntity is a dependent table for which
    the primary key is exactly that of the master table.

6) For the sake of simplicity any ObjEntity that does not satisfy 5)
must be "read-only" until and if, someday, "read-only" ObjAttributes
are introduced and allow fine-grained tuning.

7) Specifics of the operations INSERT, DELETE, UPDATE for DataObjects
defined by ObjEntities having ObjAttributes that comply with 5.b):
    INSERT) New record is inserted into the dependent table along with
    the one inserted into the master table. Its primary key value is 
    the same as the primary key value for the master table record.
    DELETE) The matching record is deleted from the dependent table
    along with the record in the master table.
    UPDATE) Values of the corresponding ObjAttributes are used to update
    the matching record in the dependent table when the record in the
    master table is updated.
    
8) Specifics of SELECT for "read-write" and "read-only" ObjEntities:
DbRelationships indicated in the DB_PATH provide the sufficient information
to construct additional inner joins to retrieve the attribute
values from the referred tables. If the matching record in the referred 
table does not exist such a SELECT query will not return the corresponding 
record in the master table in the result set which is considered to be 
correct behavior (!!?).