Migration with constraint to foreign_key.

migration.rb

class CreateComments < ActiveRecord::Migration
  def change
    create_table :comments do |t|
      t.references :post, null: false
      t.references :user, null: false      
      t.text :description

      t.timestamps
    end

    reversible do |dir|
      dir.up do
        execute <<-SQL
          ALTER TABLE comments
            ADD CONSTRAINT comments_post_id_fk
            FOREIGN KEY (post_id)
            REFERENCES posts(id);

          ALTER TABLE comments
            ADD CONSTRAINT comments_user_id_fk
            FOREIGN KEY (user_id)
            REFERENCES users(id);

        SQL
      end

      dir.down do
        execute <<-SQL
          ALTER TABLE comments
            DROP CONSTRAINT comments_post_id_fk;

          ALTER TABLE comments
            DROP CONSTRAINT comments_user_id_fk;
        SQL
      end
    end
  end
end