```ruby
module MongoModel
  module DocumentParent
    extend ActiveSupport::Concern
    
    def parent_document
      @_parent_document.is_a?(Proc) ? @_parent_document.call(self) : @_parent_document
    end
    
    def parent_document=(doc)
      @_parent_document = doc
    end
  end
end
```

Fix parent_document proc on Ruby 1.9
user
```ruby
module MongoModel
  module DocumentParent
    extend ActiveSupport::Concern
    
    def parent_document
      if @_parent_document.respond_to?(:call)
        case @_parent_document.arity
        when 0 then @_parent_document.call
        else        @_parent_document.call(self)
        end
      else
        @_parent_document
      end
    end
    
    def parent_document=(doc)
      @_parent_document = doc
    end
  end
end
```
assistant