A migration to add unique constraint to a combination of columns

Question

What I need is a migration to apply unique constraint to a combination of columns. i.e. for a people table, a combination of first_name, last

Answer 1

You may want to add a constraint without an index. This will depend on what database you're using. Below is sample migration code for Postgres. (tracking_number, carrier) is a list of the columns you want to use for the constraint.

class AddUniqeConstraintToShipments < ActiveRecord::Migration
  def up
    execute <<-SQL
      alter table shipments
        add constraint shipment_tracking_number unique (tracking_number, carrier);
    SQL
  end

  def down
    execute <<-SQL
      alter table shipments
        drop constraint if exists shipment_tracking_number;
    SQL
  end
end

There are different constraints you can add. Read the docs