Adding a Model Reference to existing Rails model

After you set belongs_to in Rabbit, and has_many in Clown, you can do a migration with:

add_column :rabbit, :clown_id, :integer

EDIT: See Paulo's answer below for a more updated answer (Rails 4+)

If you are using edge rails (4.0) you can use:

rails generate migration AddAddressRefToContacts address:references

As you can see by the docs.