Return rows affected from batched update_all and delete_all

The previous return value of nil was undocumented and inconsistent with
the non-batched versions of these methods.

Also lean on `each` to create the batches, and add API documentation for
`update_all`, `delete_all`, and `destroy_all` on `BatchEnumerator`.

activerecord/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   `BatchEnumerator#update_all` and `BatchEnumerator#delete_all` now return the
+    total number of rows affected, just like their non-batched counterparts.
+
+    ```ruby
+    Person.in_batches.update_all("first_name = 'Eugene'") # => 42
+    Person.in_batches.delete_all # => 42
+    ```
+
+    Fixes #40287.
+
+    *Eugene Kenny*
+
 *   Add support for PostgreSQL `interval` data type with conversion to
     `ActiveSupport::Duration` when loading records from database and
     serialization to ISO 8601 formatted duration string on save.

activerecord/lib/active_record/relation/batches/batch_enumerator.rb

@@ -41,19 +41,35 @@ module ActiveRecord
         end
       end
 
-      # Delegates #delete_all, #update_all, #destroy_all methods to each batch.
+      # Deletes records in batches. Returns the total number of rows affected.
       #
-      #   People.in_batches.delete_all
-      #   People.where('age < 10').in_batches.destroy_all
-      #   People.in_batches.update_all('age = age + 1')
-      [:delete_all, :update_all, :destroy_all].each do |method|
-        define_method(method) do |*args, &block|
-          @relation.to_enum(:in_batches, of: @of, start: @start, finish: @finish, load: false).each do |relation|
-            relation.public_send(method, *args, &block)
-          end
+      #   Person.in_batches.delete_all
+      #
+      # See Relation#delete_all for details of how each batch is deleted.
+      def delete_all
+        sum(&:delete_all)
+      end
+
+      # Updates records in batches. Returns the total number of rows affected.
+      #
+      #   Person.in_batches.update_all("age = age + 1")
+      #
+      # See Relation#update_all for details of how each batch is updated.
+      def update_all(updates)
+        sum do |relation|
+          relation.update_all(updates)
         end
       end
 
+      # Destroys records in batches.
+      #
+      #   Person.where("age < 10").in_batches.destroy_all
+      #
+      # See Relation#destroy_all for details of how each batch is destroyed.
+      def destroy_all
+        each(&:destroy_all)
+      end
+
       # Yields an ActiveRecord::Relation object for each batch of records.
       #
       #   Person.in_batches.each do |relation|

activerecord/test/cases/batches_test.rb

@@ -356,6 +356,14 @@ class EachTest < ActiveRecord::TestCase
     assert_equal Post.all.pluck(:title), ["updated-title"] * Post.count
   end
 
+  def test_in_batches_update_all_returns_rows_affected
+    assert_equal 11, Post.in_batches(of: 2).update_all(title: "updated-title")
+  end
+
+  def test_in_batches_update_all_returns_zero_when_no_batches
+    assert_equal 0, Post.where("1=0").in_batches(of: 2).update_all(title: "updated-title")
+  end
+
   def test_in_batches_delete_all_should_not_delete_records_in_other_batches
     not_deleted_count = Post.where("id <= 2").count
     Post.where("id > 2").in_batches(of: 2).delete_all
@@ -363,6 +371,14 @@ class EachTest < ActiveRecord::TestCase
     assert_equal not_deleted_count, Post.count
   end
 
+  def test_in_batches_delete_all_returns_rows_affected
+    assert_equal 11, Post.in_batches(of: 2).delete_all
+  end
+
+  def test_in_batches_delete_all_returns_zero_when_no_batches
+    assert_equal 0, Post.where("1=0").in_batches(of: 2).delete_all
+  end
+
   def test_in_batches_should_not_be_loaded
     Post.in_batches(of: 1) do |relation|
       assert_not_predicate relation, :loaded?