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.

1 files changed, 2 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/associations/has_one_association.rb b/activerecord/lib/active_record/associations/has_one_association.rb
index 98bd010f70..920038a543 100644
--- a/activerecord/lib/active_record/associations/has_one_association.rb
+++ b/activerecord/lib/active_record/associations/has_one_association.rb
@@ -25,9 +25,8 @@ module ActiveRecord
         raise_on_type_mismatch!(record) if record
         load_target
 
-        # If target and record are nil, or target is equal to record,
-        # we don't need to have transaction.
-        if (target || record) && target != record
+        return self.target if !(target || record)
+        if (target != record) || record.changed?
           transaction_if(save) do
             remove_target!(options[:dependent]) if target && !target.destroyed?