Link items define a specific relationship between two object types. Link instances relate one object to one or more different objects.
There are two kinds of link item declarations: abstract links, and concrete links. Abstract links are defined on the module level and are not tied to any particular object type. Typically this is done to set some annotations, define link properties, or setup constraints. Concrete links are defined on specific object types.
Links are directional and have a source and one or more targets.
The number of targets is specified by the keywords required, single,
and multi. It is also possible to
restrict how many source objects can link to the same target via the
Links also have a policy of handling link target deletion. There are 4 possible actions that can be taken when this happens:
RESTRICT- any attempt to delete the target object immediately raises an exception;
DELETE SOURCE- when the target of a link is deleted, the source is also deleted;
ALLOW- the target object is deleted and is removed from the set of the link targets;
DEFERRED RESTRICT- any attempt to delete the target object raises an exception at the end of the transaction, unless by that time this object is no longer in the set of link targets.
This section covers the syntax of how to set these policies in more detail.