Fixes the Relation#exists? to work with polymorphic associations.

Fixes #15821.

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Fixed the `Relation#exists?` to work with polymorphic associations.
+
+    Fixes #15821.
+
+    *Kassio Borges*
+
 *   Currently, Active Record will rescue any errors raised within
     after_rollback/after_create callbacks and print them to the logs. Next versions of rails
     will not rescue those errors anymore, and just bubble them up, as the other callbacks.

activerecord/lib/active_record/relation/finder_methods.rb

@@ -304,7 +304,7 @@ module ActiveRecord
         end
       end
 
-      connection.select_value(relation, "#{name} Exists", relation.bind_values) ? true : false
+      connection.select_value(relation, "#{name} Exists", relation.arel.bind_values + relation.bind_values) ? true : false
     end
 
     # This method is called whenever no records are found with either a single

activerecord/test/cases/finder_test.rb

@@ -4,6 +4,7 @@ require 'models/author'
 require 'models/categorization'
 require 'models/comment'
 require 'models/company'
+require 'models/tagging'
 require 'models/topic'
 require 'models/reply'
 require 'models/entrant'
@@ -78,6 +79,19 @@ class FinderTest < ActiveRecord::TestCase
     assert_raise(NoMethodError) { Topic.exists?([1,2]) }
   end
 
+  def test_exists_with_polymorphic_relation
+    post = Post.create!(title: 'Post', body: 'default', taggings: [Tagging.new(comment: 'tagging comment')])
+    relation = Post.tagged_with_comment('tagging comment')
+
+    assert_equal true, relation.exists?(title: ['Post'])
+    assert_equal true, relation.exists?(['title LIKE ?', 'Post%'])
+    assert_equal true, relation.exists?
+    assert_equal true, relation.exists?(post.id)
+    assert_equal true, relation.exists?(post.id.to_s)
+
+    assert_equal false, relation.exists?(false)
+  end
+
   def test_exists_passing_active_record_object_is_deprecated
     assert_deprecated do
       Topic.exists?(Topic.new)

activerecord/test/models/post.rb

@@ -41,6 +41,7 @@ class Post < ActiveRecord::Base
   scope :with_tags, -> { preload(:taggings) }
 
   scope :tagged_with, ->(id) { joins(:taggings).where(taggings: { tag_id: id }) }
+  scope :tagged_with_comment, ->(comment) { joins(:taggings).where(taggings: { comment: comment }) }
 
   has_many   :comments do
     def find_most_recent