Rails - Sep 2008 - validates presence of foreign_key & auto child save

Hi there,

You so you know, I''m fairly new to rails - I still have a lot learn.

For fun I''m trying to create a forum in rails. I have some problems
with
the create method of the topic resource. I''ve set up a virtual
attribute
called text for topic, to use for mass assignment:

In topics_controller.rb:
@topic = Topic.new(params[:topic])

In topic.rb:
def text=(text)
  posts.build(:text => text)
end

The topic model handles the text-attributes by building a post model
through its association.

This is pretty neat I think! It results in very readable and structured
code. But when I try to save the topic with @topic.save it gives me the
error that "Posts is invalid". This is because post.rb includes the
following:
validates_presence_of :topic_id

When the validation are run (just before the topic is actually saved)
rails also validates the children of the topic - that is the post model.
As this post has not yet defined a topic_id its validations fail.

The topic id will not be available until the topic is created in the
database - and the topic will not be created in the database until the
post validates - and the post will not validate until the topic id is
defined.

I hope you can see the cycle of dependencies.

If I just remove validates_presence_of :topic_id it works like a charm.
Rails even defines the posts topic id through the association set up
with posts.build(). However, to me it seems like a "dirty" solution to
the problem?

What would you do here? Any recommendations/ideas?

Thanks in advance.
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Rasmus Nielsen wrote:
> Hi there,
> 
> You so you know, I''m fairly new to rails - I still have a lot
learn.
> 
> For fun I''m trying to create a forum in rails. I have some
problems with
> the create method of the topic resource. I''ve set up a virtual
attribute
> called text for topic, to use for mass assignment:
> 
> In topics_controller.rb:
> @topic = Topic.new(params[:topic])
> 
> In topic.rb:
> def text=(text)
>   posts.build(:text => text)
> end
> 
> The topic model handles the text-attributes by building a post model
> through its association.
> 
> This is pretty neat I think! It results in very readable and structured
> code. But when I try to save the topic with @topic.save it gives me the
> error that "Posts is invalid". This is because post.rb includes
the
> following:
> validates_presence_of :topic_id
> 
> When the validation are run (just before the topic is actually saved)
> rails also validates the children of the topic - that is the post model.
> As this post has not yet defined a topic_id its validations fail.
> 
> The topic id will not be available until the topic is created in the
> database - and the topic will not be created in the database until the
> post validates - and the post will not validate until the topic id is
> defined.
> 
> I hope you can see the cycle of dependencies.
> 
> If I just remove validates_presence_of :topic_id it works like a charm.
> Rails even defines the posts topic id through the association set up
> with posts.build(). However, to me it seems like a "dirty"
solution to
> the problem?
> 
> What would you do here? Any recommendations/ideas?
> 
> Thanks in advance.
The usual solution is:

class Topic
   def text=(text)
     # Rails should be setting the belongs-to object automatically here
     posts.build(:text => text, :topic => self)
     text # Make sure you return something that''s expected
   end
end

class Post
  validates_presence_of :topic
end

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