5 files changed, 49 insertions, 0 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 1c0e453fd8..53d5ec07a3 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   `validates_size_of` / `validates_length_of` do not count records,
+    which are `marked_for_destruction?`.
+
+    Fixes #7247.
+
+    *Yves Senn*
+
 *   Ensure `first!` and friends work on loaded associations.
 
     Fixes #18237.
diff --git a/activerecord/lib/active_record/validations.rb b/activerecord/lib/active_record/validations.rb
index a6c8ff7f3a..f27adc9c40 100644
--- a/activerecord/lib/active_record/validations.rb
+++ b/activerecord/lib/active_record/validations.rb
@@ -88,3 +88,4 @@ end
 require "active_record/validations/associated"
 require "active_record/validations/uniqueness"
 require "active_record/validations/presence"
+require "active_record/validations/length"
diff --git a/activerecord/lib/active_record/validations/length.rb b/activerecord/lib/active_record/validations/length.rb
new file mode 100644
index 0000000000..ef5a6cbbe7
--- /dev/null
+++ b/activerecord/lib/active_record/validations/length.rb
@@ -0,0 +1,21 @@
+module ActiveRecord
+  module Validations
+    class LengthValidator < ActiveModel::Validations::LengthValidator # :nodoc:
+      def validate_each(record, attribute, association_or_value)
+        if association_or_value.respond_to?(:loaded?) && association_or_value.loaded?
+          association_or_value = association_or_value.target.reject(&:marked_for_destruction?)
+        end
+        super
+      end
+    end
+
+    module ClassMethods
+      # See <tt>ActiveModel::Validation::LengthValidator</tt> for more information.
+      def validates_length_of(*attr_names)
+        validates_with LengthValidator, _merge_attributes(attr_names)
+      end
+
+      alias_method :validates_size_of, :validates_length_of
+    end
+  end
+end
diff --git a/activerecord/test/cases/validations/length_validation_test.rb b/activerecord/test/cases/validations/length_validation_test.rb
index 4a92da38ce..2c0e282761 100644
--- a/activerecord/test/cases/validations/length_validation_test.rb
+++ b/activerecord/test/cases/validations/length_validation_test.rb
@@ -2,6 +2,7 @@
 require "cases/helper"
 require 'models/owner'
 require 'models/pet'
+require 'models/person'
 
 class LengthValidationTest < ActiveRecord::TestCase
   fixtures :owners
@@ -44,4 +45,21 @@ class LengthValidationTest < ActiveRecord::TestCase
       assert o.valid?
     end
   end
+
+  def test_validates_size_of_reprects_records_marked_for_destruction
+    assert_nothing_raised { Owner.validates_size_of :pets, minimum: 1 }
+    owner = Owner.new
+    assert_not owner.save
+    assert owner.errors[:pets].any?
+    pet = owner.pets.build
+    assert owner.valid?
+    assert owner.save
+
+    pet_count = Pet.count
+    assert_not owner.update_attributes pets_attributes: [ {_destroy: 1, id: pet.id} ]
+    assert_not owner.valid?
+    assert owner.errors[:pets].any?
+    assert_equal pet_count, Pet.count
+  end
+
 end
diff --git a/activerecord/test/models/owner.rb b/activerecord/test/models/owner.rb
index 2e3a9a3681..cedb774b10 100644
--- a/activerecord/test/models/owner.rb
+++ b/activerecord/test/models/owner.rb
@@ -17,6 +17,8 @@ class Owner < ActiveRecord::Base
 
   after_commit :execute_blocks
 
+  accepts_nested_attributes_for :pets, allow_destroy: true
+
   def blocks
     @blocks ||= []
   end