Gets the relation, if this field represents a container-managed relation field in its parent entity bean.
the Relation object, or null if this field doesn't represent a relation.
Determine if the field can be used with a sequence.
For this the following must apply:
- the field needs to be a primary key
- the autogenerate primary key checkbox should be on.
- the database should be oracle or postgresql with a Serial field.
true if sequence is allowed