[aitorlb]

Ticket: CASE-4447

Draft. Related: toptal/career-services-backend#2195 adds the composite keys that hit this, and merges only after this fix is released.

Problem

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 PostgresqlAdapter.foreign_key_error_column returned that whole list as a single column name, so no validator matched and the ActiveRecord::InvalidForeignKey was raised instead.

In career-services-backend (CASE-4447, toptal/career-services-backend#2195), 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 becomes an exception.

Fix

columns = error.message[/Key \(([^)]+)\)/, 1]
columns ? columns.split(', ') : []

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.

Tests

• spec/adapters/foreign_key_error_column_spec.rb: the adapter extracts every column of a composite key.
• spec/validations/db_belongs_to_spec.rb: 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, next to [CASE-4454] Unique violation without key detail is handled on PostgreSQL #82's entry; one release ships both.

Locally on PostgreSQL 16 both specs pass (26 examples in the postgresql group, 0 failures, plus the adapter specs). RuboCop is clean on the changed lines (the one offense in db_belongs_to_spec.rb:145 is already on master).

Not run

MySQL and SQLite: there was no MySQL server, and the SQLite examples stop at setup with Gem::LoadError because this environment's bundle has no sqlite3. The change touches only the PostgreSQL adapter; CI covers the others.

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