AR specific length validator to respect `marked_for_destruction`.

Closes #7247.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/test/models/owner.rb

2 files changed, 22 insertions, 0 deletions

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