Read already loaded association records from association.target

For performance, Avoid instantiate CollectionProxy.
Fixes #12537

2 files changed, 13 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/associations/preloader/through_association.rb b/activerecord/lib/active_record/associations/preloader/through_association.rb
index 56aa23b173..d9cd90db1a 100644
--- a/activerecord/lib/active_record/associations/preloader/through_association.rb
+++ b/activerecord/lib/active_record/associations/preloader/through_association.rb
@@ -18,7 +18,8 @@ module ActiveRecord
           through_records = owners.map do |owner|
             association = owner.association through_reflection.name
 
-            [owner, Array(association.reader)]
+            center = target_records_from_association(association)
+            [owner, Array(center)]
           end
 
           reset_association owners, through_reflection.name
@@ -49,7 +50,7 @@ module ActiveRecord
               rhs_records = middles.flat_map { |r|
                 association = r.association source_reflection.name
 
-                association.reader
+                target_records_from_association(association)
               }.compact
 
               rhs_records.sort_by { |rhs| record_offset[rhs] }
@@ -89,6 +90,10 @@ module ActiveRecord
 
           scope
         end
+
+        def target_records_from_association(association)
+          association.loaded? ? association.target : association.reader
+        end
       end
     end
   end
diff --git a/activerecord/test/cases/associations/eager_test.rb b/activerecord/test/cases/associations/eager_test.rb
index 7d8b933992..2c6f199cf1 100644
--- a/activerecord/test/cases/associations/eager_test.rb
+++ b/activerecord/test/cases/associations/eager_test.rb
@@ -1349,4 +1349,10 @@ class EagerAssociationTest < ActiveRecord::TestCase
     post = Post.eager_load(:tags).where('tags.name = ?', 'General').first
     assert_equal posts(:welcome), post
   end
+
+  # CollectionProxy#reader is expensive, so the preloader avoids calling it.
+  test "preloading has_many_through association avoids calling association.reader" do
+    ActiveRecord::Associations::HasManyAssociation.any_instance.expects(:reader).never
+    Author.preload(:readonly_comments).first!
+  end
 end