[aitorlb]

Description

Draft. On PostgreSQL, db_belongs_to turns a foreign-key violation into the association's must exist error only when the key has one column. A composite key reports every column, Key (tenant_id, company_id)=(2, 1) is not present, and the adapter returned that whole list as a single column name, so no validator matched and the ActiveRecord::InvalidForeignKey was raised instead.

The adapter now returns each column as a candidate, as the SQLite adapter already does for a multi-column insert. The rescuer picks the candidate with a db_belongs_to, and when there are several it already checks which referenced row is missing (foreign_key_invalid?), so nothing else changes.

Why it matters: in career-services-backend (CASE-4447), every foreign key between two brand-owned tables gets a composite partner over (brand_id, <column>) next to the single-column key. Postgres checks a row's keys in the order of their internal trigger names, so for a parent id that does not exist either key can report it; when it is the composite one, a must exist validation failure became an exception.

• PostgresqlAdapter.foreign_key_error_column splits the key's columns.
• Spec: the adapter extracts every column of a composite key; and on PostgreSQL, a db_belongs_to :company record pointing at a company of another tenant (single-column key satisfied, composite key violated) gets errors[:company] instead of an exception. Without the fix, that example raises ActiveRecord::InvalidForeignKey.
• CHANGELOG under Unreleased.

How to test

Locally on PostgreSQL 16: spec/adapters/foreign_key_error_column_spec.rb and spec/validations/db_belongs_to_spec.rb pass for PostgreSQL (26 examples in the postgresql group, 0 failures) and for the adapter specs. The SQLite and MySQL groups could not run in this environment (no MySQL server, and the SQLite examples stop at setup with Gem::LoadError: this environment's bundle has no sqlite3); CI covers them. RuboCop is clean on the changed lines (the one offense in db_belongs_to_spec.rb:145 is already on master).

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