public class SharedJoinTableUniqueKeyBinder
extends java.lang.Object

Gives every collection with a join table a uniqueness constraint of its own over its key column and the element, so that a join table several collections of one owner share stays unique per collection.

Two unidirectional hasMany to the same class without a joinTable mapping share one join table, with a key column for each of them and one element column. Hibernate creates the table's implicit unique key when it creates the keys of the first collection, over that collection's key column and the element column, and creates none for the others, as the table has a unique key already (Set.createPrimaryKey). This binder runs after the keys of each collection are created and completes what is missing, in the shape the database needs (SharedJoinTableUniqueness). A collection whose table is its own already has its constraint and is left as Hibernate bound it, and so is a collection without a join table, whose key the other side owns, or whose element is a large object, which cannot be indexed and which Hibernate leaves without a key too. It also restricts a collection that shares a key column with another on that table to its own rows, which Hibernate's removal of all the rows of a collection by its key would otherwise not (collections with separate key columns delete only their own rows by key and need no restriction).

Hibernate applies the restriction to the statements that load, count, join and delete the rows of a collection, but loses it in the test of emptiness (is empty, is not empty); GORM translates that test as a size test for a restricted many-to-many collection instead, through RestrictedCollectionSqmTranslatorFactory (apache/grails-core#16748), except in an update or delete of an entity whose hierarchy spans several tables.

A set cannot share a join table with a list or a map: the index column of a list or a map is not null, and the rows of the set leave it null, so saving the set fails on the first insert.

Since:

8.0.1