rails/rails — pull request #9601 by cliochris (2013-03-07 21:48:39+00:00)
Title: Fix #8856 Ensure has_one association=(associate) triggers save.

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 42dd5d9f2976677a4bf22347f2dde1a8135dfbb4 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.