Allow assignment on has_one :through where the owner is a new record [#5137 state:resolved]

This required changing the code to keep the association proxy for a belongs_to around, despite its target being nil. Which in turn required various changes to the way that stale target checking is handled, in order to support various edge cases (loaded target is nil then foreign key added, foreign key is changed and then changed back, etc). A side effect is that the code is nicer and more succinct.

Note that I am removing test_no_unexpected_aliasing since that is basically checking that the proxy for a belongs_to *does* change, which is the exact opposite of the intention of this commit. Also adding various tests for various edge cases and related things.

Phew, long commit message!

1 files changed, 4 insertions, 13 deletions

diff --git a/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb b/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
index 4608ffad67..4f67b02d00 100644
--- a/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
+++ b/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
@@ -2,19 +2,6 @@ module ActiveRecord
   # = Active Record Belongs To Polymorphic Association
   module Associations
     class BelongsToPolymorphicAssociation < BelongsToAssociation #:nodoc:
-      def stale_target?
-        if @target && @target.persisted?
-          target_id    = @target.send(@reflection.association_primary_key).to_s
-          foreign_key  = @owner.send(@reflection.foreign_key).to_s
-          target_type  = @target.class.base_class.name
-          foreign_type = @owner.send(@reflection.foreign_type).to_s
-
-          target_id != foreign_key || target_type != foreign_type
-        else
-          false
-        end
-      end
-
       private
 
         def replace_keys(record)
@@ -38,6 +25,10 @@ module ActiveRecord
         def raise_on_type_mismatch(record)
           # A polymorphic association cannot have a type mismatch, by definition
         end
+
+        def stale_state
+          [super, @owner[@reflection.foreign_type].to_s]
+        end
     end
   end
 end