AR has a subclass of AM:PresenceValidator.

This allows us to mark the parent object as invalid if all associated objects
in a presence validated association are marked for destruction.

See: rails#6812

activerecord/CHANGELOG.md

@@ -31,6 +31,13 @@
 
     *kennyj*
 
+*   Changed validates_presence_of on an association so that children objects
+    do not validate as being present if they are marked for destruction. This
+    prevents you from saving the parent successfully and thus putting the parent
+    in an invalid state.
+
+    *Nick Monje & Brent Wheeldon*
+
 *   `FinderMethods#exists?` now returns `false` with the `false` argument.
 
     *Egor Lynko*

activerecord/lib/active_record/validations.rb

@@ -81,3 +81,4 @@ def perform_validations(options={})
 
 require "active_record/validations/associated"
 require "active_record/validations/uniqueness"
+require "active_record/validations/presence"

activerecord/lib/active_record/validations/presence.rb

@@ -0,0 +1,64 @@
+module ActiveRecord
+  module Validations
+    class PresenceValidator < ActiveModel::Validations::PresenceValidator
+      def validate(record)
+        super
+        attributes.each do |attribute|
+          next unless record.class.reflect_on_association(attribute)
+          value = record.send(attribute)
+          if Array(value).all? { |r| r.marked_for_destruction? }
+            record.errors.add(attribute, :blank, options)
+          end
+        end
+      end
+    end
+
+    module ClassMethods
+      # Validates that the specified attributes are not blank (as defined by
+      # Object#blank?), and, if the attribute is an association, that the
+      # associated object is not marked for destruction. Happens by default
+      # on save.
+      #
+      #   class Person < ActiveRecord::Base
+      #     has_one :face
+      #     validates_presence_of :face
+      #   end
+      #
+      # The face attribute must be in the object and it cannot be blank or marked
+      # for destruction.
+      #
+      # If you want to validate the presence of a boolean field (where the real values
+      # are true and false), you will want to use
+      # <tt>validates_inclusion_of :field_name, :in => [true, false]</tt>.
+      #
+      # This is due to the way Object#blank? handles boolean values:
+      # <tt>false.blank? # => true</tt>.
+      #
+      # This validator defers to the ActiveModel validation for presence, adding the
+      # check to see that an associated object is not marked for destruction. This
+      # prevents the parent object from validating successfully and saving, which then
+      # deletes the associated object, thus putting the parent object into an invalid
+      # state.
+      #
+      # Configuration options:
+      # * <tt>:message</tt> - A custom error message (default is: "can't be blank").
+      # * <tt>:on</tt> - Specifies when this validation is active. Runs in all
+      #   validation contexts by default (+nil+), other options are <tt>:create</tt>
+      #   and <tt>:update</tt>.
+      # * <tt>:if</tt> - Specifies a method, proc or string to call to determine if
+      #   the validation should occur (e.g. <tt>:if => :allow_validation</tt>, or
+      #   <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>). The method, proc
+      #   or string should return or evaluate to a true or false value.
+      # * <tt>:unless</tt> - Specifies a method, proc or string to call to determine
+      #   if the validation should not occur (e.g. <tt>:unless => :skip_validation</tt>,
+      #   or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>). The method,
+      #   proc or string should return or evaluate to a true or false value.
+      # * <tt>:strict</tt> - Specifies whether validation should be strict.
+      #   See <tt>ActiveModel::Validation#validates!</tt> for more information.
+      #
+      def validates_presence_of(*attr_names)
+        validates_with PresenceValidator, _merge_attributes(attr_names)
+      end
+    end
+  end
+end

activerecord/test/cases/validations/presence_validation_test.rb

@@ -0,0 +1,44 @@
+# encoding: utf-8
+require "cases/helper"
+require 'models/man'
+require 'models/face'
+require 'models/interest'
+
+class PresenceValidationTest < ActiveRecord::TestCase
+  class Boy < Man; end
+
+  repair_validations(Boy)
+
+  def test_validates_presence_of_non_association
+    Boy.validates_presence_of(:name)
+    b = Boy.new
+    assert b.invalid?
+
+    b.name = "Alex"
+    assert b.valid?
+  end
+
+  def test_validates_presence_of_has_one_marked_for_destruction
+    Boy.validates_presence_of(:face)
+    b = Boy.new
+    f = Face.new
+    b.face = f
+    assert b.valid?
+
+    f.mark_for_destruction
+    assert b.invalid?
+  end
+
+  def test_validates_presence_of_has_many_marked_for_destruction
+    Boy.validates_presence_of(:interests)
+    b = Boy.new
+    b.interests << [i1 = Interest.new, i2 = Interest.new]
+    assert b.valid?
+
+    i1.mark_for_destruction
+    assert b.valid?
+
+    i2.mark_for_destruction
+    assert b.invalid?
+  end
+end

guides/source/active_record_validations_callbacks.textile

@@ -373,6 +373,8 @@ class LineItem < ActiveRecord::Base
 end
 </ruby>
 
+If you validate the presence of an object associated via a +has_one+ or +has_many+ relationship, it will check that the object is neither +blank?+ nor +marked_for_destruction?+.
+
 Since +false.blank?+ is true, if you want to validate the presence of a boolean field you should use <tt>validates :field_name, :inclusion => { :in => [true, false] }</tt>.
 
 The default error message is "_can't be empty_".