Support composite foreign keys via migration helpers

[fatkodima]

Closes #47593.

# Assuming "carts" table has "(shop_id, id)" as a primary key.

add_foreign_key(:orders, :carts, primary_key: [:shop_id, :id])
# or
add_foreign_key(:orders, :carts, column: [:cart_shop_id, :cart_id])

remove_foreign_key(:orders, :carts, primary_key: [:shop_id, :id])
foreign_key_exists?(:orders, :carts, primary_key: [:shop_id, :id])

We discussed 3 possible ways of using add_foreign_key.

The first one (add_foreign_key :brochures, :cars) (no :column and :primary_key options) turned out to be complex to implement, as it introduced a lot of kinda ugly changes in many places because we needed to properly generate these column names (consider configured table name prefixes and suffixes, pass a connection around to be able to query a primary key for the to_table. Some classes, like ForeignKeyDefinition does not have access to the connection, and so we needed to calculate these columns at several places differently and pass there).

And the biggest problem is when we need to add a foreign key for a self referencing tables, like

rails/activerecord/test/cases/migration/references_foreign_key_test.rb

Lines 221 to 225 in 1df6ad0

	@connection.create_table :testings do |t|
	t.references :parent1, foreign_key: { to_table: :testing_parents }
	t.references :parent2, foreign_key: { to_table: :testing_parents }
	t.references :self_join, foreign_key: { to_table: :testings }
	end

That would produce very not elegant code.

So I made that it is required to pass at least one of :column or :primary_key options, as it is easy to infer one from the other.

cc @eileencodes @nvasilevski

[eileencodes]

Hey @fatkodima thanks for working on this! I've left some comments.

[eileencodes]

Suggested change

	# # Assuming "carts" table has "(shop_id, id)" as a primary key.
	# Assuming "carts" table has "(shop_id, id)" as a primary key.

[eileencodes]

I think we are actually going to discourage using id as one of the ids in the composite key when we write the docs so can we change this to something like (shop_id, oid)?

[eileencodes]

I'm not super clear on why we need to support both, can we support just primary_key and simplify the code a bit?

[fatkodima]

@eileencodes Thanks for the review, addressed the feedback.

I initially added support for both :primary_key and :column to be able to specify either one and the other will be inferred. Small convenience. But yeah, changed to require passing a :primary_key manually, if the :column is passed as an array.