Adds a new check constraint to the table. expression is a String representation of verifiable boolean condition. add_check_constraint :products, "price > 0", name: "price_check" generates: ALTER TABLE "products" ADD CONSTRAINT price_check CHECK (price > 0) The options hash can include the following keys: :name The constraint name. Defaults to chk_rails_<identifier>. :if_not_exists Silently ignore