Question

I have 2 models like the following

    Class Post  
      has_many :comments, :dependent => :destroy 
    end  

    Class Comment  
      validates_presence_of :post
      validates_presence_of :comment
      belongs_to :post  
    end  
  

In Comments controller,

    def create
      comment = @post.comments.build(params[:comment])
      if comment.save
        // some code
      else
        // some code
      end
    end

When the comment is invalid as per the validation, the comment is not saved. But when the @post object is accessed in the view, it contains a comment object with nil id. This did not happen in Rails 2.3.11. We are upgraded to Rails 3.1 and then now to Rails 3.2. This comment object with nil id disappears when I do @post.reload. We are using REE.

I tried to interchange build and new methods. It had the same result as build. Similar behavior is found across our application. Is it the expected behavior or am I doing something wrong?

Answer 1

This seems like expected behaviour to me.

via http://guides.rubyonrails.org/association_basics.html#belongs_to-association-reference

4.1.1.3 build_association(attributes = {})

The build_association method returns a new object of the associated
type. This object will be instantiated from the passed attributes, and
the link through this object’s foreign key will be set, but the
associated object will not yet be saved.

When you call @post.comments.build(...), Rails:

1. Creates a new Comment object
2. sets comment.post_id to @post.id.
3. Inserts it into the comments array (in memory).

When the validation fails, it doesn’t delete the comment and the comment persists in the in-memory comments array. When @post gets to your view, @post.comments still includes that badly validated comment.

As for how to deal with it, I’m not sure. Maybe you could do something like (in your controller)… (Feels pretty ugly though.)

def create
  comment = @post.comments.build(params[:comment])
  if comment.save
    // some code
  else
    @bad_comment = @post.comments.pop
  end
end