Backport a super-simplified version of #6792, fixing

that #exists? and others can produce invalid SQL: "SELECT DISTINCT DISTINCT"

The combination of a :uniq => true association and the #distinct call
in #construct_limited_ids_condition combine to create invalid SQL, because
we're explicitly selecting DISTINCT, and also sending #distinct on to AREL,
via the relation#distinct_value.

Where #6792 was the forever fix, this is the minimal fix. Instead of
properly indicating the distinctness of the query through #uniq_value alone,
we use a literal select statement and set #uniq_value to always be falsey

1 files changed, 12 insertions, 0 deletions

diff --git a/activerecord/test/cases/finder_test.rb b/activerecord/test/cases/finder_test.rb
index 21bd7b51d5..2efafe5c24 100644
--- a/activerecord/test/cases/finder_test.rb
+++ b/activerecord/test/cases/finder_test.rb
@@ -93,6 +93,18 @@ class FinderTest < ActiveRecord::TestCase
     assert !Topic.includes(:replies).limit(1).where('0 = 1').exists?
   end
 
+  def test_exists_with_distinct_association_includes_and_limit
+    author = Author.first
+    assert !author.unique_categorized_posts.includes(:special_comments).limit(0).exists?
+    assert author.unique_categorized_posts.includes(:special_comments).limit(1).exists?
+  end
+
+  def test_exists_with_distinct_association_includes_limit_and_order
+    author = Author.first
+    assert !author.unique_categorized_posts.includes(:special_comments).order('comments.taggings_count DESC').limit(0).exists?
+    assert author.unique_categorized_posts.includes(:special_comments).order('comments.taggings_count DESC').limit(1).exists?
+  end
+
   def test_exists_with_empty_table_and_no_args_given
     Topic.delete_all
     assert !Topic.exists?