Linked attributes are pairs of attributes in which the system calculates the values of one attribute (the back link) based on the values set on the other attribute (the forward link) throughout the forest. A back-link value on any object instance consists of the DNs of all the objects that have the object's DN set in the corresponding forward link. For example, "Manager" and "Reports" are a pair of linked attributes, where Manager is the forward link and Reports is the back link. Now suppose Bill is Joe's manager. If you store the DN of Bill's user object in the "Manager" attribute of Joe's user object, then the DN of Joe's user object will show up in the "Reports" attribute of Bill's user object.
A forward link/back link pair is identified by the linkID values of two attributeSchema definitions. The linkID of the forward link is an even, positive, nonzero value, and the linkID of the associated back link is the forward linkID plus one. For example, the linkID for "Manager" is 42 and the linkID for "Reports" is 43.
The following is a list of guidelines for defining a new pair of linked attributes:
- The linkID values must be unique among all attributeSchema objects. To avoid conflicts, you should auto-generate the linkID by following the instructions in the topic Obtaining a Link ID.
- A back link must have a corresponding forward link, that is, the forward link must exist before a corresponding back link attribute can be created.
- A back link is always a multi-valued attribute. A forward link can be single-valued or multi-valued. Use a multi-valued forward link when there is a many-to-many relationship.
- The attributeSchema value of a forward link must be 126.96.36.199, 188.8.131.52, or 184.108.40.206. These values correspond to syntaxes that contain a distinguished name, such as the Object(DS-DN) syntax.
- The attributeSchema value of a back link must be 220.127.116.11, which is the Object(DS-DN) syntax.
- By convention, back link attributes are added to the mayContain value of the top abstract class. This enables the back link attribute to be read from objects of any class because they are not actually stored with the object, but are calculated based on the forward link values.