Optimize none? and one? relation query methods to use LIMIT and COUNT.

Use SQL COUNT and LIMIT 1 queries for none? and one? methods if no block or limit is given,
instead of loading the entire collection to memory. The any? and many? methods already
follow this behavior.

[Eugene Gilburg & Rafael Mendonça França]

activerecord/CHANGELOG.md

+*   Use SQL COUNT and LIMIT 1 queries for `none?` and `one?` methods if no block or limit is given,
+    instead of loading the entire collection to memory.
+    This applies to relations (e.g. `User.all`) as well as associations (e.g. `account.users`)
+
+        # Before:
+
+        users.none?
+        # SELECT "users".* FROM "users"
+
+        users.one?
+        # SELECT "users".* FROM "users"
+
+        # After:
+
+        users.none?
+        # SELECT 1 AS one FROM "users" LIMIT 1
+
+        users.one?
+        # SELECT COUNT(*) FROM "users"
+
+    *Eugene Gilburg*
+
 *   Allow `:precision` option for time type columns.
 
     *Ryuta Kamizono*

activerecord/lib/active_record/associations/collection_association.rb

@@ -319,7 +319,8 @@ def empty?
       end
 
       # Returns true if the collections is not empty.
-      # Equivalent to +!collection.empty?+.
+      # If block given, loads all records and checks for one or more matches.
+      # Otherwise, equivalent to +!collection.empty?+.
       def any?
         if block_given?
           load_target.any? { |*block_args| yield(*block_args) }
@@ -329,7 +330,8 @@ def any?
       end
 
       # Returns true if the collection has more than 1 record.
-      # Equivalent to +collection.size > 1+.
+      # If block given, loads all records and checks for two or more matches.
+      # Otherwise, equivalent to +collection.size > 1+.
       def many?
         if block_given?
           load_target.many? { |*block_args| yield(*block_args) }

activerecord/lib/active_record/null_relation.rb

@@ -30,10 +30,18 @@ def empty?
       true
     end
 
+    def none?
+      true
+    end
+
     def any?
       false
     end
 
+    def one?
+      false
+    end
+
     def many?
       false
     end

activerecord/lib/active_record/relation.rb

@@ -271,6 +271,15 @@ def empty?
       end
     end
 
+    # Returns true if there are no records.
+    def none?
+      if block_given?
+        to_a.none? { |*block_args| yield(*block_args) }
+      else
+        empty?
+      end
+    end
+
     # Returns true if there are any records.
     def any?
       if block_given?
@@ -280,6 +289,15 @@ def any?
       end
     end
 
+    # Returns true if there is exactly one record.
+    def one?
+      if block_given?
+        to_a.one? { |*block_args| yield(*block_args) }
+      else
+        limit_value ? to_a.one? : size == 1
+      end
+    end
+
     # Returns true if there is more than one record.
     def many?
       if block_given?

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -1678,6 +1678,82 @@ def test_calling_many_should_return_true_if_more_than_one
     assert_equal 3, firm.clients.size
   end
 
+  def test_calling_none_should_count_instead_of_loading_association
+    firm = companies(:first_firm)
+    assert_queries(1) do
+      firm.clients.none?  # use count query
+    end
+    assert !firm.clients.loaded?
+  end
+
+  def test_calling_none_on_loaded_association_should_not_use_query
+    firm = companies(:first_firm)
+    firm.clients.collect  # force load
+    assert_no_queries { assert ! firm.clients.none? }
+  end
+
+  def test_calling_none_should_defer_to_collection_if_using_a_block
+    firm = companies(:first_firm)
+    assert_queries(1) do
+      firm.clients.expects(:size).never
+      firm.clients.none? { true }
+    end
+    assert firm.clients.loaded?
+  end
+
+  def test_calling_none_should_return_true_if_none
+    firm = companies(:another_firm)
+    assert firm.clients_like_ms.none?
+    assert_equal 0, firm.clients_like_ms.size
+  end
+
+  def test_calling_none_should_return_false_if_any
+    firm = companies(:first_firm)
+    assert !firm.limited_clients.none?
+    assert_equal 1, firm.limited_clients.size
+  end
+
+  def test_calling_one_should_count_instead_of_loading_association
+    firm = companies(:first_firm)
+    assert_queries(1) do
+      firm.clients.one?  # use count query
+    end
+    assert !firm.clients.loaded?
+  end
+
+  def test_calling_one_on_loaded_association_should_not_use_query
+    firm = companies(:first_firm)
+    firm.clients.collect  # force load
+    assert_no_queries { assert ! firm.clients.one? }
+  end
+
+  def test_calling_one_should_defer_to_collection_if_using_a_block
+    firm = companies(:first_firm)
+    assert_queries(1) do
+      firm.clients.expects(:size).never
+      firm.clients.one? { true }
+    end
+    assert firm.clients.loaded?
+  end
+
+  def test_calling_one_should_return_false_if_zero
+    firm = companies(:another_firm)
+    assert ! firm.clients_like_ms.one?
+    assert_equal 0, firm.clients_like_ms.size
+  end
+
+  def test_calling_one_should_return_true_if_one
+    firm = companies(:first_firm)
+    assert firm.limited_clients.one?
+    assert_equal 1, firm.limited_clients.size
+  end
+
+  def test_calling_one_should_return_false_if_more_than_one
+    firm = companies(:first_firm)
+    assert ! firm.clients.one?
+    assert_equal 3, firm.clients.size
+  end
+
   def test_joins_with_namespaced_model_should_use_correct_type
     old = ActiveRecord::Base.store_full_sti_class
     ActiveRecord::Base.store_full_sti_class = true

activerecord/test/cases/relations_test.rb

@@ -342,7 +342,9 @@ def test_null_relation_content_size_methods
       assert_equal 0,     Developer.none.size
       assert_equal 0,     Developer.none.count
       assert_equal true,  Developer.none.empty?
+      assert_equal true, Developer.none.none?
       assert_equal false, Developer.none.any?
+      assert_equal false, Developer.none.one?
       assert_equal false, Developer.none.many?
     end
   end
@@ -1102,6 +1104,38 @@ def test_many_with_limits
     assert ! posts.limit(1).many?
   end
 
+  def test_none?
+    posts = Post.all
+    assert_queries(1) do
+      assert ! posts.none? # Uses COUNT()
+    end
+
+    assert ! posts.loaded?
+
+    assert_queries(1) do
+      assert posts.none? {|p| p.id < 0 }
+      assert ! posts.none? {|p| p.id == 1 }
+    end
+
+    assert posts.loaded?
+  end
+
+  def test_one
+    posts = Post.all
+    assert_queries(1) do
+      assert ! posts.one? # Uses COUNT()
+    end
+
+    assert ! posts.loaded?
+
+    assert_queries(1) do
+      assert ! posts.one? {|p| p.id < 3 }
+      assert posts.one? {|p| p.id == 1 }
+    end
+
+    assert posts.loaded?
+  end
+
   def test_build
     posts = Post.all