4 files changed, 106 insertions, 0 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 9b5310b6d5..3351013b16 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   `validates_absence_of` respects `marked_for_destruction?`.
+
+    Fixes #20449.
+
+    *Yves Senn*
+
 *   Include the `Enumerable` module in `ActiveRecord::Relation`
 
     *Sean Griffin & bogdan*
diff --git a/activerecord/lib/active_record/validations.rb b/activerecord/lib/active_record/validations.rb
index e227212827..34d96b19fe 100644
--- a/activerecord/lib/active_record/validations.rb
+++ b/activerecord/lib/active_record/validations.rb
@@ -76,4 +76,5 @@ end
 require "active_record/validations/associated"
 require "active_record/validations/uniqueness"
 require "active_record/validations/presence"
+require "active_record/validations/absence"
 require "active_record/validations/length"
diff --git a/activerecord/lib/active_record/validations/absence.rb b/activerecord/lib/active_record/validations/absence.rb
new file mode 100644
index 0000000000..2e19e6dc5c
--- /dev/null
+++ b/activerecord/lib/active_record/validations/absence.rb
@@ -0,0 +1,24 @@
+module ActiveRecord
+  module Validations
+    class AbsenceValidator < ActiveModel::Validations::AbsenceValidator # :nodoc:
+      def validate_each(record, attribute, association_or_value)
+        return unless should_validate?(record)
+        if record.class._reflect_on_association(attribute)
+          association_or_value = Array.wrap(association_or_value).reject(&:marked_for_destruction?)
+        end
+        super
+      end
+    end
+
+    module ClassMethods
+      # Validates that the specified attributes are not present (as defined by
+      # Object#present?). If the attribute is an association, the associated object
+      # is considered absent if it was marked for destruction.
+      #
+      # See ActiveModel::Validations::HelperMethods.validates_absence_of for more information.
+      def validates_absence_of(*attr_names)
+        validates_with AbsenceValidator, _merge_attributes(attr_names)
+      end
+    end
+  end
+end
diff --git a/activerecord/test/cases/validations/absence_validation_test.rb b/activerecord/test/cases/validations/absence_validation_test.rb
new file mode 100644
index 0000000000..dd43ee358c
--- /dev/null
+++ b/activerecord/test/cases/validations/absence_validation_test.rb
@@ -0,0 +1,75 @@
+require "cases/helper"
+require 'models/face'
+require 'models/interest'
+require 'models/man'
+require 'models/topic'
+
+class AbsenceValidationTest < ActiveRecord::TestCase
+  def test_non_association
+    boy_klass = Class.new(Man) do
+      def self.name; "Boy" end
+      validates_absence_of :name
+    end
+
+    assert boy_klass.new.valid?
+    assert_not boy_klass.new(name: "Alex").valid?
+  end
+
+  def test_has_one_marked_for_destruction
+    boy_klass = Class.new(Man) do
+      def self.name; "Boy" end
+      validates_absence_of :face
+    end
+
+    boy = boy_klass.new(face: Face.new)
+    assert_not boy.valid?, "should not be valid if has_one association is present"
+    assert_equal 1, boy.errors[:face].size, "should only add one error"
+
+    boy.face.mark_for_destruction
+    assert boy.valid?, "should be valid if association is marked for destruction"
+  end
+
+  def test_has_many_marked_for_destruction
+    boy_klass = Class.new(Man) do
+      def self.name; "Boy" end
+      validates_absence_of :interests
+    end
+    boy = boy_klass.new
+    boy.interests << [i1 = Interest.new, i2 = Interest.new]
+    assert_not boy.valid?, "should not be valid if has_many association is present"
+
+    i1.mark_for_destruction
+    assert_not boy.valid?, "should not be valid if has_many association is present"
+
+    i2.mark_for_destruction
+    assert boy.valid?
+  end
+
+  def test_does_not_call_to_a_on_associations
+    boy_klass = Class.new(Man) do
+      def self.name; "Boy" end
+      validates_absence_of :face
+    end
+
+    face_with_to_a = Face.new
+    def face_with_to_a.to_a; ['(/)', '(\)']; end
+
+    assert_nothing_raised { boy_klass.new(face: face_with_to_a).valid? }
+  end
+
+  def test_does_not_validate_if_parent_record_is_validate_false
+    repair_validations(Interest) do
+      Interest.validates_absence_of(:topic)
+      interest = Interest.new(topic: Topic.new(title: "Math"))
+      interest.save!(validate: false)
+      assert interest.persisted?
+
+      man = Man.new(interest_ids: [interest.id])
+      man.save!
+
+      assert_equal man.interests.size, 1
+      assert interest.valid?
+      assert man.valid?
+    end
+  end
+end