activerecord/lib/active_record/associations.rb states:

    # [association=(associate)]
    #   Assigns the associate object, extracts the primary key, sets it as the foreign key,
    #   and saves the associate object.

Since commit 42dd5d9f to fix #7191, this
is no longer the case if the associate has changed, but is the same
object. For example:

    # Pirate has_one :ship
    pirate = Pirate.create!(catchphrase: "A Pirate")
    ship = pirate.build_ship(name: 'old name')
    ship.save!

    ship.name = 'new name'
    pirate.ship = ship

That last line should trigger a save. Although we are not changing the
association, the associate (ship) has changed.