don't cache invalid subsets when preloading hmt associations.

Backport #9252.

Conflicts:

	activerecord/CHANGELOG.md
	activerecord/test/cases/associations/eager_test.rb

The preloader code on 3-2-stable is not based on relations but on option hashes.
I had to modify the original patch and comparing the option hashes could be more
fuzzy than comparing the relations. All the tests passed though.

3 files changed, 31 insertions, 1 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index febead3067..cd105b8355 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,28 @@
 ## unreleased ##
 
+*   Preloading `has_many :through` associations with conditions won't
+    cache the `:through` association. This will prevent invalid
+    subsets to be cached.
+    Fixes #8423.
+    Backport #9252.
+
+    Example:
+
+        class User
+          has_many :posts
+          has_many :recent_comments, -> { where('created_at > ?', 1.week.ago) }, :through => :posts
+        end
+
+        a_user = User.includes(:recent_comments).first
+
+        # this is preloaded
+        a_user.recent_comments
+
+        # fetching the recent_comments through the posts association won't preload it.
+        a_user.posts
+
+    *Yves Senn*
+
 *   Fix handling of dirty time zone aware attributes
 
     Previously, when `time_zone_aware_attributes` were enabled, after
diff --git a/activerecord/lib/active_record/associations/preloader/through_association.rb b/activerecord/lib/active_record/associations/preloader/through_association.rb
index ad6374d09a..4cb7b56b57 100644
--- a/activerecord/lib/active_record/associations/preloader/through_association.rb
+++ b/activerecord/lib/active_record/associations/preloader/through_association.rb
@@ -37,7 +37,8 @@ module ActiveRecord
             through_records = Array.wrap(owner.send(through_reflection.name))
 
             # Dont cache the association - we would only be caching a subset
-            if reflection.options[:source_type] && through_reflection.collection?
+            if (preload_options != through_options) ||
+               (reflection.options[:source_type] && through_reflection.collection?)
               owner.association(through_reflection.name).reset
             end
 
diff --git a/activerecord/test/cases/associations/eager_test.rb b/activerecord/test/cases/associations/eager_test.rb
index 20c6b691fc..5d07ffa241 100644
--- a/activerecord/test/cases/associations/eager_test.rb
+++ b/activerecord/test/cases/associations/eager_test.rb
@@ -1112,4 +1112,10 @@ class EagerAssociationTest < ActiveRecord::TestCase
       Post.includes(:comments).order(nil).where(:comments => {:body => "Thank you for the welcome"}).first
     end
   end
+
+  test "preloading does not cache has many association subset when preloaded with a through association" do
+    author = Author.includes(:comments_with_order_and_conditions, :posts).first
+    assert_no_queries { assert_equal 2, author.comments_with_order_and_conditions.size }
+    assert_no_queries { assert_equal 5, author.posts.size, "should not cache a subset of the association" }
+  end
 end