Improve performance of #one? and #many?

#one? and #many? generate a query like `SELECT COUNT(*) FROM posts` under the hood, and then compare if the result is equal (or greater) to 1. That count operation can be really slow for large tables or complex conditions, but there's no need to count all the records in these cases. It's much faster just by adding a limit, like `SELECT COUNT(*) FROM posts LIMIT 2`

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Improve performance of `one?` and `many?` by limiting the generated count query to 2 results.
+
+    *Gonzalo Riestra*
+
 *   Don't check type when using `if_not_exists` on `add_column`.
 
     Previously, if a migration called `add_column` with the `if_not_exists` option set to true

activerecord/lib/active_record/relation.rb

@@ -33,6 +33,7 @@ module ActiveRecord
       @delegate_to_klass = false
       @future_result = nil
       @records = nil
+      @limited_count = nil
     end
 
     def initialize_copy(other)
@@ -294,13 +295,15 @@ module ActiveRecord
     # Returns true if there is exactly one record.
     def one?
       return super if block_given?
-      limit_value ? records.one? : size == 1
+      return records.one? if limit_value || loaded?
+      limited_count == 1
     end
 
     # Returns true if there is more than one record.
     def many?
       return super if block_given?
-      limit_value ? records.many? : size > 1
+      return records.many? if limit_value || loaded?
+      limited_count > 1
     end
 
     # Returns a stable cache key that can be used to identify this query.
@@ -690,6 +693,7 @@ module ActiveRecord
       @offsets = @take = nil
       @cache_keys = nil
       @records = nil
+      @limited_count = nil
       self
     end
 
@@ -964,5 +968,9 @@ module ActiveRecord
         # ignore raw_sql_ that is used by Oracle adapter as alias for limit/offset subqueries
         string.scan(/[a-zA-Z_][.\w]+(?=.?\.)/).map!(&:downcase) - ["raw_sql_"]
       end
+
+      def limited_count
+        @limited_count ||= limit(2).count
+      end
   end
 end

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -2278,6 +2278,14 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     assert_no_queries { assert firm.clients.many? }
   end
 
+  def test_subsequent_calls_to_many_should_not_use_query
+    firm = companies(:first_firm)
+    assert_queries(1) do
+      firm.clients.many?
+      firm.clients.many?
+    end
+  end
+
   def test_calling_many_should_defer_to_collection_if_using_a_block
     firm = companies(:first_firm)
     assert_queries(1) do
@@ -2354,6 +2362,14 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     assert_no_queries { assert_not firm.clients.one? }
   end
 
+  def test_subsequent_calls_to_one_should_not_use_query
+    firm = companies(:first_firm)
+    assert_queries(1) do
+      firm.clients.one?
+      firm.clients.one?
+    end
+  end
+
   def test_calling_one_should_defer_to_collection_if_using_a_block
     firm = companies(:first_firm)
     assert_queries(1) do