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diff --git a/activerecord/test/cases/relation/merging_test.rb b/activerecord/test/cases/relation/merging_test.rb
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+++ b/activerecord/test/cases/relation/merging_test.rb
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+require 'cases/helper'
+require 'models/author'
+require 'models/comment'
+require 'models/developer'
+require 'models/computer'
+require 'models/post'
+require 'models/project'
+require 'models/rating'
+
+class RelationMergingTest < ActiveRecord::TestCase
+  fixtures :developers, :comments, :authors, :posts
+
+  def test_relation_merging
+    devs = Developer.where("salary >= 80000").merge(Developer.limit(2)).merge(Developer.order('id ASC').where("id < 3"))
+    assert_equal [developers(:david), developers(:jamis)], devs.to_a
+
+    dev_with_count = Developer.limit(1).merge(Developer.order('id DESC')).merge(Developer.select('developers.*'))
+    assert_equal [developers(:poor_jamis)], dev_with_count.to_a
+  end
+
+  def test_relation_to_sql
+    post = Post.first
+    sql = post.comments.to_sql
+    assert_match(/.?post_id.? = #{post.id}\Z/i, sql)
+  end
+
+  def test_relation_merging_with_arel_equalities_keeps_last_equality
+    devs = Developer.where(Developer.arel_table[:salary].eq(80000)).merge(
+      Developer.where(Developer.arel_table[:salary].eq(9000))
+    )
+    assert_equal [developers(:poor_jamis)], devs.to_a
+  end
+
+  def test_relation_merging_with_arel_equalities_keeps_last_equality_with_non_attribute_left_hand
+    salary_attr = Developer.arel_table[:salary]
+    devs = Developer.where(
+      Arel::Nodes::NamedFunction.new('abs', [salary_attr]).eq(80000)
+    ).merge(
+      Developer.where(
+        Arel::Nodes::NamedFunction.new('abs', [salary_attr]).eq(9000)
+      )
+    )
+    assert_equal [developers(:poor_jamis)], devs.to_a
+  end
+
+  def test_relation_merging_with_eager_load
+    relations = []
+    relations << Post.order('comments.id DESC').merge(Post.eager_load(:last_comment)).merge(Post.all)
+    relations << Post.eager_load(:last_comment).merge(Post.order('comments.id DESC')).merge(Post.all)
+
+    relations.each do |posts|
+      post = posts.find { |p| p.id == 1 }
+      assert_equal Post.find(1).last_comment, post.last_comment
+    end
+  end
+
+  def test_relation_merging_with_locks
+    devs = Developer.lock.where("salary >= 80000").order("id DESC").merge(Developer.limit(2))
+    assert devs.locked.present?
+  end
+
+  def test_relation_merging_with_preload
+    [Post.all.merge(Post.preload(:author)), Post.preload(:author).merge(Post.all)].each do |posts|
+      assert_queries(2) { assert posts.first.author }
+    end
+  end
+
+  def test_relation_merging_with_joins
+    comments = Comment.joins(:post).where(:body => 'Thank you for the welcome').merge(Post.where(:body => 'Such a lovely day'))
+    assert_equal 1, comments.count
+  end
+
+  def test_relation_merging_with_association
+    assert_queries(2) do  # one for loading post, and another one merged query
+      post = Post.where(:body => 'Such a lovely day').first
+      comments = Comment.where(:body => 'Thank you for the welcome').merge(post.comments)
+      assert_equal 1, comments.count
+    end
+  end
+
+  test "merge collapses wheres from the LHS only" do
+    left  = Post.where(title: "omg").where(comments_count: 1)
+    right = Post.where(title: "wtf").where(title: "bbq")
+
+    expected = [left.bind_values[1]] + right.bind_values
+    merged   = left.merge(right)
+
+    assert_equal expected, merged.bind_values
+    assert !merged.to_sql.include?("omg")
+    assert merged.to_sql.include?("wtf")
+    assert merged.to_sql.include?("bbq")
+  end
+
+  def test_merging_keeps_lhs_bind_parameters
+    column = Post.columns_hash['id']
+    binds = [[column, 20]]
+
+    right  = Post.where(id: 20)
+    left   = Post.where(id: 10)
+
+    merged = left.merge(right)
+    assert_equal binds, merged.bind_values
+  end
+
+  def test_merging_reorders_bind_params
+    post  = Post.first
+    right = Post.where(id: 1)
+    left  = Post.where(title: post.title)
+
+    merged = left.merge(right)
+    assert_equal post, merged.first
+  end
+
+  def test_merging_compares_symbols_and_strings_as_equal
+    post = PostThatLoadsCommentsInAnAfterSaveHook.create!(title: "First Post", body: "Blah blah blah.")
+    assert_equal "First comment!", post.comments.where(body: "First comment!").first_or_create.body
+  end
+end
+
+class MergingDifferentRelationsTest < ActiveRecord::TestCase
+  fixtures :posts, :authors, :developers
+
+  test "merging where relations" do
+    hello_by_bob = Post.where(body: "hello").joins(:author).
+      merge(Author.where(name: "Bob")).order("posts.id").pluck("posts.id")
+
+    assert_equal [posts(:misc_by_bob).id,
+                  posts(:other_by_bob).id], hello_by_bob
+  end
+
+  test "merging order relations" do
+    posts_by_author_name = Post.limit(3).joins(:author).
+      merge(Author.order(:name)).pluck("authors.name")
+
+    assert_equal ["Bob", "Bob", "David"], posts_by_author_name
+
+    posts_by_author_name = Post.limit(3).joins(:author).
+      merge(Author.order("name")).pluck("authors.name")
+
+    assert_equal ["Bob", "Bob", "David"], posts_by_author_name
+  end
+
+  test "merging order relations (using a hash argument)" do
+    posts_by_author_name = Post.limit(4).joins(:author).
+      merge(Author.order(name: :desc)).pluck("authors.name")
+
+    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)
+    comment_2.ratings.create!
+
+    assert_equal dev.ratings, [rating_1]
+  end
+end