Fix the calling `merge` method at first in a scope

Changing the order of method chaining `merge` and other query
method such as `joins` should produce the same result.

```ruby
class Topic < ApplicationRecord
  scope :safe_chaininig,   -> { joins(:comments).merge(Comment.newest) }
  scope :unsafe_chaininig, -> { merge(Comment.newest).joins(:comments) } #=> NoMethodError
end
```

1 files changed, 7 insertions, 0 deletions

diff --git a/activerecord/test/cases/scoping/named_scoping_test.rb b/activerecord/test/cases/scoping/named_scoping_test.rb
index 0e277ed235..f0dac07acc 100644
--- a/activerecord/test/cases/scoping/named_scoping_test.rb
+++ b/activerecord/test/cases/scoping/named_scoping_test.rb
@@ -46,6 +46,13 @@ class NamedScopingTest < ActiveRecord::TestCase
     assert_equal Topic.average(:replies_count), Topic.base.average(:replies_count)
   end
 
+  def test_calling_merge_at_first_in_scope
+    Topic.class_eval do
+      scope :calling_merge_at_first_in_scope, Proc.new { merge(Topic.replied) }
+    end
+    assert_equal Topic.calling_merge_at_first_in_scope.to_a, Topic.replied.to_a
+  end
+
   def test_method_missing_priority_when_delegating
     klazz = Class.new(ActiveRecord::Base) do
       self.table_name = "topics"