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, 17 insertions, 3 deletions

diff --git a/activerecord/test/cases/associations/eager_test.rb b/activerecord/test/cases/associations/eager_test.rb
index 19c69ed7bc..2a9351507a 100644
--- a/activerecord/test/cases/associations/eager_test.rb
+++ b/activerecord/test/cases/associations/eager_test.rb
@@ -902,11 +902,25 @@ class EagerAssociationTest < ActiveRecord::TestCase
     end
   end
 
-  def test_preloading_empty_polymorphic_parent
+  def test_preloading_empty_belongs_to
+    c = Client.create!(:name => 'Foo', :client_of => Company.maximum(:id) + 1)
+
+    client = assert_queries(2) { Client.preload(:firm).find(c.id) }
+    assert_no_queries { assert_nil client.firm }
+  end
+
+  def test_preloading_empty_belongs_to_polymorphic
     t = Tagging.create!(:taggable_type => 'Post', :taggable_id => Post.maximum(:id) + 1, :tag => tags(:general))
 
-    assert_queries(2) { @tagging = Tagging.preload(:taggable).find(t.id) }
-    assert_no_queries { assert ! @tagging.taggable }
+    tagging = assert_queries(2) { Tagging.preload(:taggable).find(t.id) }
+    assert_no_queries { assert_nil tagging.taggable }
+  end
+
+  def test_preloading_through_empty_belongs_to
+    c = Client.create!(:name => 'Foo', :client_of => Company.maximum(:id) + 1)
+
+    client = assert_queries(2) { Client.preload(:accounts).find(c.id) }
+    assert_no_queries { assert client.accounts.empty? }
   end
 
   def test_preloading_has_many_through_with_uniq