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 42dd5d9f29 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.
ebd7cc6f45