Rails Composite Keys

When to Use

• Use composite primary keys when uniqueness must span multiple columns
• Avoid them unless your data model or legacy database demands it

Migrations

• Use primary_key: %i[key1 key2] when creating the table
• Do not use t.primary_key :id — Rails will create a single-column ID instead
• Omit id: false when using the primary_key: option directly on create_table

Model Declarations

• Rails 7.1+ ships native composite keys — no gem required
• Use self.primary_key = [:user_id, :group_id] on the model
• For non-PK composite identity (e.g. tenant-scoped records with surrogate id), use query_constraints :tenant_id, :id so all generated SQL scopes by the full tuple