Adding timestamps to join tables in Rails

Rails doesn’t include timestamps in join tables by default but sometimes it’s nice to see when a relationship was formed. Knowing when these relationships are formed can be helpful for auditing purposes and may be useful to expose within your application.

To create a join table write a migration like the one below in your terminal.

rails generate migration CreateJoinTableCatsOwners cats owners

Which will result in a migration which looks like this:

class CreateJoinTableCatsOwners < ActiveRecord::Migration[7.0]
  def change
    create_join_table :cats, :owners do |t|
      t.index [:cat_id, :owner_id]
      t.index [:owner_id, :cat_id]
    end
  end
end

So you can see that by default timestamps aren’t included in this migration so let’s go ahead and amend the above migration and add some timestamps:

class CreateJoinTableCatsOwners < ActiveRecord::Migration[7.0]
  def change
    create_join_table :cats, :owners do |t|
      t.index [:cat_id, :owner_id]
      t.index [:owner_id, :cat_id]

      t.timestamps
    end
  end
end

This will add the timestamps to your join table and allow you to use the created_at and updated_at fields.

What if you want to add timestamps to a join table which already exists? You’ll need to create another migration to do this for you:

rails generate migration AddTimestampsToCatsOwners

This will create an empty migration which you can edit to include timestamps:

class AddTimestampsToCatsOwners < ActiveRecord::Migration[7.0]
  def change
    add_timestamps :cats_owners, default: Time.zone.now
    change_column_default :cats_owners, :created_at, nil
    change_column_default :cats_owners, :updated_at, nil
  end
end

Setting the default when adding the timestamps will ensure that if you have any existing records that they will have a timestamp applied albeit not an accurate one.