Add support for belongs_to's optional option to handle data integ…

…rity.

By adding this support, model can able to save foreign_key to either
`nil` value or to the valid foreign_key value which means
foreign_key value must present in the foreign table.

    class Post < ActiveRecord::Base
      belongs_to :author, optional: :with_integrity
    end

    author = Author.create!
    post = Post.create!(author_id: author.id)

    post.author_id = 0 # Invalid id which is not present in database
    post.save! # Should raise 'ActiveRecord::RecordInvalid' error

    post.author_id = nil
    post.save! # No Error

Based on this issue's discussion rails#26557. Added this.

activerecord/lib/active_record/associations/builder/belongs_to.rb

@@ -127,6 +127,20 @@ def self.define_validations(model, reflection)
 
       if reflection.options[:optional].nil?
         required = model.belongs_to_required_by_default
+      elsif reflection.options[:optional] == :with_integrity
+        callback = lambda do |record|
+          fk       = reflection.foreign_key
+          fk_value = record.public_send(fk)
+          association_klass = record.association(reflection.name).klass
+          association_pk    = association_klass.primary_key
+
+          if fk_value.present? && association_klass.find_by("#{association_pk}": fk_value).nil?
+            record.errors.add(fk, :invalid)
+          end
+        end
+
+        model.validate callback
+        required = false
       else
         required = !reflection.options[:optional]
       end

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -99,6 +99,32 @@ def self.name; "Temp"; end
     ActiveRecord::Base.belongs_to_required_by_default = original_value
   end
 
+  def test_optional_relation_with_integrity_option
+    original_value = ActiveRecord::Base.belongs_to_required_by_default
+    ActiveRecord::Base.belongs_to_required_by_default = true
+
+    model = Class.new(ActiveRecord::Base) do
+      self.table_name = "posts"
+      def self.name; "Temp"; end
+      belongs_to :author, optional: :with_integrity
+    end
+
+    author1 = Author.create!(name: "abc")
+    author2 = Author.create!(name: "xyz")
+
+    m = model.create!(title: "post title", body: "haha", author_id: author1.id)
+    m.author_id = 0
+
+    assert_raises(ActiveRecord::RecordInvalid) do
+      assert m.save!
+    end
+
+    m.author_id = author2.id
+    assert m.save
+  ensure
+    ActiveRecord::Base.belongs_to_required_by_default = original_value
+  end
+
   def test_required_belongs_to_config
     original_value = ActiveRecord::Base.belongs_to_required_by_default
     ActiveRecord::Base.belongs_to_required_by_default = true