# 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 (
Add the migration on an existing table like this:
add_reference :things, :other_thing
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?