All the tables that can be included in a lock object must be linked with foreign keys. There are a number of restrictions to the valid relationships.

1. The foreign key relationships of the tables of the lock object must form a tree. The tables are the nodes of the tree. The links of the tree mean is the check table of.
2. The foreign key fields must be key fields of the foreign key table.
3. The foreign key relationships defined between the base tables of the lock objects may not have any field that is checked against more than one other field. A field therefore may not occur twice as foreign key field in a relationship and may not be checked against two different fields in two different foreign key relationships.
4. You must keep one restriction in mind for multi-structured foreign keys. If a field is assigned to a field that is outside the check table, the table containing this field must be in a sub-tree that contains the check table of this foreign key relationship as a root.

Conditions 2, 3 and 4 are meaningless if the particular foreign key field was excluded from the assignment to the key fields of the check table by marking it as generic or setting it to a constant. This is also true for multi-structured foreign keys if the foreign key field refers to a table that is not contained in the lock object.