From http://edgeguides.rubyonrails.org/active_record_migrations.html#types-of-schema-dumps:

There is however a trade-off: db/schema.rb cannot express database specific items such as triggers, stored procedures or check constraints. While in a migration you can execute custom SQL statements, the schema dumper cannot reconstitute those statements from the database. If you are using features like this, then you should set the schema format to :sql.

No longer is this the case. You can now use the default schema format (:ruby) and still preserve your check constraints.

At this time, this only supports check constraints for the postgresql ActiveRecord database adapter.

Add check constraints to a table in a migration:

create_table :people do |t|
  t.string :title
  t.check_constraint title: ['Mr.', 'Mrs.', 'Dr.']
end

OR

add_check_constraint :people, title: ['Mr.', 'Mrs.', 'Dr.']

# If you don't need it to be reversible:
remove_check_constraint :people

# If you need it to be reversible (Recommended):
remove_check_constraint :people, title: ['Mr.', 'Mrs.', 'Dr.']

Add this line to your application's Gemfile:

gem 'active_record-postgres-constraints'

And then execute:

$ bundle

$ (cd spec/dummy && bin/rake db:create RAILS_ENV=test) # One time before running tests
$ bundle exec rspec # To run tests as often as you'd like

If you're interested in building support for other database adapters, we welcome your contribution!

The gem is available as open source under the terms of the MIT License.