Fix regression caused by a01d164b

When preload is used in a default scope the preload_values were
returning nested arrays and causing the preloader to fail because it
doesn't know how to deal with nested arrays. So before calling preload!
we need to splat the arguments.

This is not needed to includes because it flatten its arguments.

activerecord/lib/active_record/relation/merger.rb

@@ -87,8 +87,8 @@ module ActiveRecord
         return if other.preload_values.empty? && other.includes_values.empty?
 
         if other.klass == relation.klass
-          relation.preload! other.preload_values unless other.preload_values.empty?
+          relation.preload!(*other.preload_values) unless other.preload_values.empty?
-          relation.includes! other.includes_values unless other.includes_values.empty?
+          relation.includes!(other.includes_values) unless other.includes_values.empty?
         else
           reflection = relation.klass.reflect_on_all_associations.find do |r|
             r.class_name == other.klass.name

activerecord/test/cases/relations_test.rb

@@ -647,6 +647,25 @@ class RelationTest < ActiveRecord::TestCase
     end
   end
 
+  def test_preloading_with_associations_default_scopes_and_merges
+    post = Post.create! title: 'Uhuu', body: 'body'
+    reader = Reader.create! post_id: post.id, person_id: 1
+
+    post_rel = PostWithPreloadDefaultScope.preload(:readers).joins(:readers).where(title: 'Uhuu')
+    result_post = PostWithPreloadDefaultScope.all.merge(post_rel).to_a.first
+
+    assert_no_queries do
+      assert_equal [reader], result_post.readers.to_a
+    end
+
+    post_rel = PostWithIncludesDefaultScope.includes(:readers).where(title: 'Uhuu')
+    result_post = PostWithIncludesDefaultScope.all.merge(post_rel).to_a.first
+
+    assert_no_queries do
+      assert_equal [reader], result_post.readers.to_a
+    end
+  end
+
   def test_loading_with_one_association
     posts = Post.preload(:comments)
     post = posts.find { |p| p.id == 1 }

activerecord/test/models/post.rb

@@ -208,6 +208,22 @@ class PostWithDefaultScope < ActiveRecord::Base
   default_scope { order(:title) }
 end
 
+class PostWithPreloadDefaultScope < ActiveRecord::Base
+  self.table_name = 'posts'
+
+  has_many :readers, foreign_key: 'post_id'
+
+  default_scope { preload(:readers) }
+end
+
+class PostWithIncludesDefaultScope < ActiveRecord::Base
+  self.table_name = 'posts'
+
+  has_many :readers, foreign_key: 'post_id'
+
+  default_scope { includes(:readers) }
+end
+
 class SpecialPostWithDefaultScope < ActiveRecord::Base
   self.table_name = 'posts'
   default_scope { where(:id => [1, 5,6]) }