Merge pull request rails#51987 from fatkodima/exists-and-loaded2

Optimize `ActiveRecord::Relation#exists?` with no conditions for loaded relations

Author: byroot

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Optimize `Relation#exists?` when records are loaded and the relation has no conditions.
+
+    This can avoid queries in some cases.
+
+    *fatkodima*
+
 *   Add a `filter` option to `in_order_of` to prioritize certain values in the sorting without filtering the results
     by these values.
 

activerecord/lib/active_record/relation/finder_methods.rb

@@ -365,6 +365,7 @@ def exists?(conditions = :none)
       end
 
       return false if !conditions || limit_value == 0
+      return records.any?(&:persisted?) if conditions == :none && loaded?
 
       if eager_loading?
         relation = apply_join_dependency(eager_loading: false)

activerecord/test/cases/finder_test.rb

@@ -289,6 +289,30 @@ def test_exists_returns_false_with_false_arg
     assert_equal false, Topic.exists?(false)
   end
 
+  def test_exists_with_loaded_relation
+    topics = Topic.all.load
+    assert_no_queries do
+      assert_predicate topics, :exists?
+    end
+  end
+
+  def test_exists_with_empty_loaded_relation
+    Topic.delete_all
+    topics = Topic.all.load
+    assert_no_queries do
+      assert_not_predicate topics, :exists?
+    end
+  end
+
+  def test_exists_with_loaded_relation_having_unsaved_records
+    author = authors(:david)
+    posts = author.posts.load
+    assert_not_empty posts
+    posts.each(&:destroy)
+
+    assert_not_predicate posts, :exists?
+  end
+
   # exists? should handle nil for id's that come from URLs and always return false
   # (example: Topic.exists?(params[:id])) where params[:id] is nil
   def test_exists_with_nil_arg