Change exists? so that it doesn't instantiate records [#6127 state:resolved]

activerecord/lib/active_record/relation/finder_methods.rb

@@ -183,7 +183,9 @@ def all(*args)
     def exists?(id = nil)
       id = id.id if ActiveRecord::Base === id
 
-      relation = select("1").limit(1)
+      join_dependency = construct_join_dependency_for_association_find
+      relation = construct_relation_for_association_find(join_dependency)
+      relation = relation.except(:select).select("1").limit(1)
 
       case id
       when Array, Hash
@@ -192,14 +194,13 @@ def exists?(id = nil)
         relation = relation.where(table[primary_key].eq(id)) if id
       end
 
-      relation.first ? true : false
+      connection.select_value(relation.to_sql) ? true : false
     end
 
     protected
 
     def find_with_associations
-      including = (@eager_load_values + @includes_values).uniq
-      join_dependency = ActiveRecord::Associations::JoinDependency.new(@klass, including, [])
+      join_dependency = construct_join_dependency_for_association_find
       relation = construct_relation_for_association_find(join_dependency)
       rows = connection.select_all(relation.to_sql, 'SQL', relation.bind_values)
       join_dependency.instantiate(rows)
@@ -207,6 +208,11 @@ def find_with_associations
       []
     end
 
+    def construct_join_dependency_for_association_find
+      including = (@eager_load_values + @includes_values).uniq
+      ActiveRecord::Associations::JoinDependency.new(@klass, including, [])
+    end
+
     def construct_relation_for_association_calculations
       including = (@eager_load_values + @includes_values).uniq
       join_dependency = ActiveRecord::Associations::JoinDependency.new(@klass, including, arel.froms.first)

activerecord/test/cases/finder_test.rb

@@ -74,6 +74,11 @@ def test_exists_with_scoped_include
     end
   end
 
+  def test_exists_does_not_instantiate_records
+    Developer.expects(:instantiate).never
+    Developer.exists?
+  end
+
   def test_find_by_array_of_one_id
     assert_kind_of(Array, Topic.find([ 1 ]))
     assert_equal(1, Topic.find([ 1 ]).length)