Cache the association proxy object

Some third party modules expects that association returns same proxy
object each time (e.g. for stubbing collection methods:
https://github.com/rspec/rspec-rails/issues/1817).
So I decided that cache the proxy object and reset scope in the proxy
object each time.

Related context: https://github.com/rails/rails/commit/c86a32d7451c5d901620ac58630460915292f88b#commitcomment-2784312

activerecord/lib/active_record/associations/collection_association.rb

@@ -30,7 +30,8 @@ def reader
           reload
         end
 
-        CollectionProxy.create(klass, self)
+        @proxy ||= CollectionProxy.create(klass, self)
+        @proxy.reset_scope
       end
 
       # Implements the writer method, e.g. foo.items= for Foo.has_many :items

activerecord/lib/active_record/associations/collection_proxy.rb

@@ -1087,9 +1087,8 @@ def clear
       #   person.pets(true)  # fetches pets from the database
       #   # => [#<Pet id: 1, name: "Snoop", group: "dogs", person_id: 1>]
       def reload
-        @scope = nil
         proxy_association.reload
-        self
+        reset_scope
       end
 
       # Unloads the association. Returns +self+.
@@ -1109,9 +1108,13 @@ def reload
       #   person.pets  # fetches pets from the database
       #   # => [#<Pet id: 1, name: "Snoop", group: "dogs", person_id: 1>]
       def reset
-        @scope = nil
         proxy_association.reset
         proxy_association.reset_scope
+        reset_scope
+      end
+
+      def reset_scope # :nodoc:
+        @scope = nil
         self
       end
 

activerecord/test/cases/associations_test.rb

@@ -220,6 +220,11 @@ def test_scoped_allows_conditions
     assert_equal david.projects, david.projects.scope
   end
 
+  test "proxy object is cached" do
+    david = developers(:david)
+    assert_same david.projects, david.projects
+  end
+
   test "inverses get set of subsets of the association" do
     man = Man.create
     man.interests.create