Rails 7 adds support for deferrable foreign key constraints in PostgreSQL


In the database, the Foreign Key constraint enforces referential integrity. This ensures the parent record has to be present before creating a child record and these constraints are immediately enforced(checked after each statement). But, this behavior can be changed within a transaction by changing a constraints deferrable characteristics, which means instead of checking the constraints immediately, these checks will be deferred until the transaction is committed and PostgreSQL supports this feature in which constraints can be deferred

So, before Rails 7, we had to write a raw SQL in migrations to use the deferrable constraints in Postgres.

Before

# /db/migrate/20220322071034_add_foreign_key_to_reviews.rb

add_foreign_key :reviews, :products
Product.transaction do
  review = Review.create!(product_id: 11, comment: "amazing product")
  product = Product.create!(id: 11, title: "Hand Sanitizer", description: "new sanitizer in town")
end

It will result in the below error

TRANSACTION (1.0ms)  ROLLBACK
/Users/murtazabagwala/.rvm/gems/ruby-3.0.2/gems/activerecord-7.0.2.3/lib/active_record/connection_adapters/postgresql_adapter.rb:768:in `exec_params': PG::ForeignKeyViolation: ERROR:  insert or update on table "reviews" violates foreign key constraint "fk_rails_bedd9094d4" (ActiveRecord::InvalidForeignKey)
DETAIL:  Key (product_id)=(11) is not present in table "products".

After

But, in Rails 7, while passing the :deferrable option to the add_foreign_key statement in migrations, it is possible to defer this check.

We can specify whether or not the foreign key should be deferrable. Valid values are booleans or :deferred or :immediate.

# /db/migrate/20220322071050_add_foreign_key_to_reviews.rb

add_foreign_key :reviews, :products, deferrable: :deferred
Product.transaction do
  review = Review.create!(product_id: 11, comment: "amazing product")
  product = Product.create!(id: 11, title: "Hand Sanitizer", description: "new sanitizer in town")
end

Records will be created successfully

TRANSACTION (1.2ms)  BEGIN
Review Create (1.7ms)  INSERT INTO "reviews" ("comment", "created_at", "updated_at", "product_id") VALUES ($1, $2, $3, $4) RETURNING "id"  [["comment", "amazing product"], ["created_at", "2022-03-22 07:44:17.296514"], ["updated_at", "2022-03-22 07:44:17.296514"], ["product_id", 11]]
Product Create (1.5ms)  INSERT INTO "products" ("id", "title", "description", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id"  [["id", 11], ["title", "Hand Sanitizer"], ["description", "new sanitizer in town"], ["created_at", "2022-03-22 07:44:17.306941"], ["updated_at", "2022-03-22 07:44:17.306941"]]
TRANSACTION (2.4ms)  COMMIT

Check out this pull request to know about the change.

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