Adding timestamps to join tables in Rails

How to go about adding timestamps to your Rails join tables

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.


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