Only merge scopes with zero arity in has_many through

with dynamic conditions.

Fixes #16128

This bug was introduced in https://github.com/rails/rails/commit/c35e438620f2d56562251571377995359546393d
so it's present from 4.1.2-rc1 and after.

https://github.com/rails/rails/commit/c35e438620f2d56562251571377995359546393d
merges any relation scopes passed as proc objects to the relation,
but does *not* take into account the arity of the lambda.

To reproduce: https://gist.github.com/Agis-/5f1f0d664d2cd08dfb9b

1 files changed, 13 insertions, 0 deletions

diff --git a/activerecord/test/cases/relation/merging_test.rb b/activerecord/test/cases/relation/merging_test.rb
index 2b5c2fd5a4..54a581e416 100644
--- a/activerecord/test/cases/relation/merging_test.rb
+++ b/activerecord/test/cases/relation/merging_test.rb
@@ -4,6 +4,7 @@ require 'models/comment'
 require 'models/developer'
 require 'models/post'
 require 'models/project'
+require 'models/rating'
 
 class RelationMergingTest < ActiveRecord::TestCase
   fixtures :developers, :comments, :authors, :posts
@@ -144,4 +145,16 @@ class MergingDifferentRelationsTest < ActiveRecord::TestCase
 
     assert_equal ["Mary", "Mary", "Mary", "David"], posts_by_author_name
   end
+
+  test "relation merging (using a proc  argument)" do
+    dev = Developer.where(name: "Jamis").first
+
+    comment_1 = dev.comments.create!(body: "I'm Jamis", post: Post.first)
+    rating_1 = comment_1.ratings.create!
+
+    comment_2 = dev.comments.create!(body: "I'm John", post: Post.first)
+    rating_2 = comment_2.ratings.create!
+
+    assert_equal dev.ratings, [rating_1]
+  end
 end