Properly append preload / includes args on Merger

Couldn't find other way to get the association name from a given class
other than looping through `reflect_on_all_associations` reflections ..

Noticed this one while looking at this example:

```ruby
class Product < ActiveRecord::Base
  has_many :variants
  has_many :translations
end

class Translation < ActiveRecord::Base
  belongs_to :product
end

class Variant < ActiveRecord::Base
  belongs_to :product
end

class BugTest < Minitest::Test
  def test_merge_stuff
    product = Product.create! name: 'huhu'
    variant = Variant.create! product_id: product.id
    Translation.create! locale: 'en', product_id: product.id

    product_relation = Product.all
                              .preload(:translations)
                              .joins(:translations)
                              .merge(Translation.where(locale: 'en'))
                              .where(name: 'huhu')

    assert_equal variant, Variant.joins(:product).merge(product_relation).first
  end
end
```

2 files changed, 50 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
index c3054f1fe9..01c05462b2 100644
--- a/activerecord/lib/active_record/relation/merger.rb
+++ b/activerecord/lib/active_record/relation/merger.rb
@@ -50,7 +50,7 @@ module ActiveRecord
 
       NORMAL_VALUES = Relation::VALUE_METHODS -
                       Relation::CLAUSE_METHODS -
-                      [:joins, :order, :reverse_order, :lock, :create_with, :reordering] # :nodoc:
+                      [:includes, :preload, :joins, :order, :reverse_order, :lock, :create_with, :reordering] # :nodoc:
 
       def normal_values
         NORMAL_VALUES
@@ -75,6 +75,7 @@ module ActiveRecord
         merge_multi_values
         merge_single_values
         merge_clauses
+        merge_preloads
         merge_joins
 
         relation
@@ -82,6 +83,27 @@ module ActiveRecord
 
       private
 
+      def merge_preloads
+        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.includes! other.includes_values unless other.includes_values.empty?
+        else
+          reflection = relation.klass.reflect_on_all_associations.find do |reflection|
+            reflection.class_name == other.klass.name
+          end || return
+
+          unless other.preload_values.empty?
+            relation.preload! reflection.name => other.preload_values
+          end
+
+          unless other.includes_values.empty?
+            relation.includes! reflection.name => other.includes_values
+          end
+        end
+      end
+
       def merge_joins
         return if other.joins_values.blank?
 
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index b8e2041b6d..53dd8a39fe 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -17,7 +17,7 @@ require 'models/tyre'
 require 'models/minivan'
 require 'models/aircraft'
 require "models/possession"
-
+require "models/reader"
 
 class RelationTest < ActiveRecord::TestCase
   fixtures :authors, :topics, :entrants, :developers, :companies, :developers_projects, :accounts, :categories, :categorizations, :posts, :comments,
@@ -621,6 +621,32 @@ class RelationTest < ActiveRecord::TestCase
     assert_equal 1, query.to_a.size
   end
 
+  def test_preloading_with_associations_and_merges
+    post = Post.create! title: 'Uhuu', body: 'body'
+    reader = Reader.create! post_id: post.id, person_id: 1
+    comment = Comment.create! post_id: post.id, body: 'body'
+
+    assert !comment.respond_to?(:readers)
+
+    post_rel = Post.preload(:readers).joins(:readers).where(title: 'Uhuu')
+    result_comment = Comment.joins(:post).merge(post_rel).to_a.first
+    assert_equal comment, result_comment
+
+    assert_no_queries do
+      assert_equal post, result_comment.post
+      assert_equal [reader], result_comment.post.readers.to_a
+    end
+
+    post_rel = Post.includes(:readers).where(title: 'Uhuu')
+    result_comment = Comment.joins(:post).merge(post_rel).first
+    assert_equal comment, result_comment
+
+    assert_no_queries do
+      assert_equal post, result_comment.post
+      assert_equal [reader], result_comment.post.readers.to_a
+    end
+  end
+
   def test_loading_with_one_association
     posts = Post.preload(:comments)
     post = posts.find { |p| p.id == 1 }