How do I add a check constraint in a Rails migration?

Question

I need to add a new integer column to an existing table in my Rails app. The column can only have values 1, 2, 3, so I'd like to add a check constraint to the table/column. How do I specify this constraint within a Rails migration?

Answer 1

Rails migration does not provide any way to add Constraints, but you can still do it via migration but by passing actual SQL to execute()

Create Migration file:

ruby script/generate Migration AddConstraint 

Now, in the migration file:

class AddConstraint < ActiveRecord::Migration   def self.up     execute "ALTER TABLE table_name ADD CONSTRAINT check_constraint_name CHECK (check_column_name IN (1, 2, 3) )"   end    def self.down     execute "ALTER TABLE table_name DROP CONSTRAINT check_constraint_name"   end end 

Answer 2

Rails 6.1+ Check Constraints

Rails 6.1 added basic support for check constraints to database migrations.

So now, a migration for adding a check constraint which restricts integer column values only to 1, 2, and 3 can be written as follows:

class AddConstraint < ActiveRecord::Migration   def up     add_check_constraint :table_name, 'check_column_name IN (1, 2, 3)', name: 'check_constraint_name'   end    def down     remove_check_constraint :table_name, name: 'check_constraint_name'   end end 

Here is a link to the relative PR where you can find more details about add_check_constraint and remove_check_constraint.