Summary

Support entity associations and joins involving an entity whose primary key is a composite, declared with @EmbeddedId on a plain @Embeddable and stored as an _id sub-document (the model introduced by HIBERNATE-207).

Scope

1. Composite foreign keys: a @ManyToOne (or other FK-bearing mapping) whose target entity has a composite key. Boot currently rejects an association as an id component and has no layout for a composite FK. Open design question: where the FK lives in the document, per-component columns or a sub-document, in a canonical component order (the _id uniqueness semantics are field-order-sensitive, per HIBERNATE-207).
2. HQL joins over composite keys: mapped association joins (@ManyToOne / @OneToMany) and explicit entity joins with ON conditions, where either side may have a composite key.

Out of scope, tracked separately: @IdClass (HIBERNATE-235), non-scalar id component (HIBERNATE-236), @MapsId / derived identity (HIBERNATE-237).

Current state

Verified by probe against a composite-keyed Book and Review sharing component names (publisherId, bookNo):

SELECT b.id, r.id FROM Book b JOIN Review r ON b.id = r.id

fails with FeatureNotSupportedException "TODO-HIBERNATE-168" from AbstractMqlTranslator.buildJoinStages. The joined entity's table group carries a nested TableGroupJoin, which Hibernate materializes for the embeddable id, and the HIBERNATE-168 guard rejects all nested table group joins (its own scope is association navigation inside ON, a different trigger). Adding per-component conjuncts (ON b.id = r.id AND b.title = r.comment) fails identically; the guard runs before any predicate translation.

Notes on what already exists:

• Compound ON conditions between scalar-key entities work (HIBERNATE-164, $lookup with $expr). HIBERNATE-164 covers only scalar keys on both sides of each conjunct; composite keys are not covered.
• Comparisons between whole composite ids already decompose into per-component equalities (the SqlTuple row-value path from HIBERNATE-210 / HIBERNATE-207), and inside a $lookup sub-pipeline the ON predicate reuses the general visitors with outer columns bound to let variables. That machinery is untested for composite ids only because translation never reaches it.

Work items

1. Boot: define the composite-FK document layout, and lift the association-component rejection for this case.
2. Translator: let buildJoinStages accept the embeddable-id nested table group, and confirm the emitted $lookup decomposes a whole-composite ON predicate into per-component $expr conjuncts.
3. Tests: the probe (Book / Review entity join, whole-id ON, and ON with an added conjunct) becomes the first test; extend to mapped @ManyToOne storage and association joins as boot support lands.