belongs_to gets the database column 🔗

For a one-to-one or one-to-many relationship, the side with the belongs_to is the one where the database column (other_model_id) goes.

Add the migration on an existing table like this:

def change
  add_reference :things, :other_thing
end

add_reference is aliased add_belongs_to. It will add an index by default.

has_one and has_many don't get a database column 🔗

Because it's on the belongs_to, get it?