Make `relation.exists?` more performant when using eager loading

`relation.exists?` just wants to know if there is a result or not, does
not need the exact records matched. Therefore, an intermediate SELECT
query for eager loading is not necessary.

2 files changed, 16 insertions, 13 deletions

diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 50d0f14b98..12ac89f5ad 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -313,7 +313,7 @@ module ActiveRecord
       return false if !conditions || limit_value == 0
 
       if eager_loading?
-        relation = apply_join_dependency(construct_join_dependency(eager_loading: false))
+        relation = apply_join_dependency(eager_loading: false)
         return relation.exists?(conditions)
       end
 
@@ -396,10 +396,11 @@ module ActiveRecord
         )
       end
 
-      def apply_join_dependency(join_dependency = construct_join_dependency)
+      def apply_join_dependency(join_dependency = nil, eager_loading: true)
+        join_dependency ||= construct_join_dependency(eager_loading: eager_loading)
         relation = except(:includes, :eager_load, :preload).joins!(join_dependency)
 
-        if using_limitable_reflections?(join_dependency.reflections)
+        if !eager_loading || using_limitable_reflections?(join_dependency.reflections)
           relation
         else
           if has_limit_or_offset?
diff --git a/activerecord/test/cases/finder_test.rb b/activerecord/test/cases/finder_test.rb
index 95bd987551..4769ffd64d 100644
--- a/activerecord/test/cases/finder_test.rb
+++ b/activerecord/test/cases/finder_test.rb
@@ -270,25 +270,27 @@ class FinderTest < ActiveRecord::TestCase
   end
 
   def test_exists_with_includes_limit_and_empty_result
-    assert_equal false, Topic.includes(:replies).limit(0).exists?
-    assert_equal false, Topic.includes(:replies).limit(1).where("0 = 1").exists?
+    assert_no_queries { assert_equal false, Topic.includes(:replies).limit(0).exists? }
+    assert_queries(1) { assert_equal false, Topic.includes(:replies).limit(1).where("0 = 1").exists? }
   end
 
   def test_exists_with_distinct_association_includes_and_limit
     author = Author.first
-    assert_equal false, author.unique_categorized_posts.includes(:special_comments).limit(0).exists?
-    assert_equal true, author.unique_categorized_posts.includes(:special_comments).limit(1).exists?
+    unique_categorized_posts = author.unique_categorized_posts.includes(:special_comments)
+    assert_no_queries { assert_equal false, unique_categorized_posts.limit(0).exists? }
+    assert_queries(1) { assert_equal true, unique_categorized_posts.limit(1).exists? }
   end
 
   def test_exists_with_distinct_association_includes_limit_and_order
     author = Author.first
-    assert_equal false, author.unique_categorized_posts.includes(:special_comments).order("comments.tags_count DESC").limit(0).exists?
-    assert_equal true, author.unique_categorized_posts.includes(:special_comments).order("comments.tags_count DESC").limit(1).exists?
+    unique_categorized_posts = author.unique_categorized_posts.includes(:special_comments).order("comments.tags_count DESC")
+    assert_no_queries { assert_equal false, unique_categorized_posts.limit(0).exists? }
+    assert_queries(1) { assert_equal true, unique_categorized_posts.limit(1).exists? }
   end
 
   def test_exists_should_reference_correct_aliases_while_joining_tables_of_has_many_through_association
-    developer = developers(:david)
-    assert_not_predicate developer.ratings.includes(comment: :post).where(posts: { id: 1 }), :exists?
+    ratings = developers(:david).ratings.includes(comment: :post).where(posts: { id: 1 })
+    assert_queries(1) { assert_not_predicate ratings.limit(1), :exists? }
   end
 
   def test_exists_with_empty_table_and_no_args_given
@@ -1315,12 +1317,12 @@ class FinderTest < ActiveRecord::TestCase
 
   test "#skip_query_cache! for #exists? with a limited eager load" do
     Topic.cache do
-      assert_queries(2) do
+      assert_queries(1) do
         Topic.eager_load(:replies).limit(1).exists?
         Topic.eager_load(:replies).limit(1).exists?
       end
 
-      assert_queries(4) do
+      assert_queries(2) do
         Topic.eager_load(:replies).limit(1).skip_query_cache!.exists?
         Topic.eager_load(:replies).limit(1).skip_query_cache!.exists?
       end