Rails - Jun 2005 - HABTM and saving data

Hi, all! First-time poster, be gentle.

OK, I am doing a very simple application with articles and categories 
only. I have the requisite models as so:

class Article < ActiveRecord::Base
    has_and_belongs_to_many :categories
end

class Category < ActiveRecord::Base
    has_and_belongs_to_many :articles
end

and I also have the corresponding DB tables which are named:
articles
categories
articles_categories

I am able to create and edit and retrieve articles and categories just 
fine. What is eluding me is the ability to create the HABTM 
relationships and update them from the views. That is, I can display the 
categories as checkboxes when adding a new article or editing an 
article. However, I do not understand how to take the values of which 
categories the user has chosen and insert them into the appropriate 
place in my ''create'' action and thusly update them in the
''update'' action.

The checkboxes that I have look like this in the view:
<% @categories.each do |cat| %>
    <input type="checkbox" id="<%= cat.name %>"
name="categories[]"
value="<%= cat.id %>" />
    <%= cat.name %>
<% end %>

And the parameters returned when submitting the form are something like 
this:
{"article"=>{"body"=>"Category testing is
fun.", "title"=>"Wow, the 4th
Article", "author"=>"Me"},
"categories"=>["3", "4"]}

So, I know that I am just totally missing this conceptually. In 
addition, in the edit view I would need to find out which categories are 
associated with a particular article and fill those checkboxes on display.

I have looked at several online tutorials and such, but none of them 
seem to really show this though it seems very basic to the whole Rails 
construct. Once I get it all working, I should probably type it up for 
the wiki as it seems like an area that is underserviced.

Thanks for any assistance you can provide.

-- 
Alex Ezell

On Jun 15, 2005, at 7:03 AM, Alex Ezell wrote:
> Hi, all! First-time poster, be gentle.
Did you see Doug Alcorn''s post about HABTM just a few minutes before  
yours, or did you just subscribe to the list?

The basic idea for saving the association between two HABTM-assocated  
tables is to:
1. Clear the association
2. Append all of the objects

Something like this:

     article.categories.clear
     article.categories <<
Category.find(@params[''category''])

The second line can deal with @params[''category''] as a single
id or
as an array of ids.

Duane Johnson
(canadaduane)




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Duane Johnson wrote:
> Did you see Doug Alcorn''s post about HABTM just a few minutes
before
> yours, or did you just subscribe to the list?
I have been subscribed for several days, but I did not receive his post. 
I wonder if I am not getting all the messages. I will take a look at the 
archive on Gmane and read it.
> The basic idea for saving the association between two HABTM-assocated 
> tables is to:
> 1. Clear the association
> 2. Append all of the objects
>
> Something like this:
>
>     article.categories.clear
>     article.categories <<
Category.find(@params[''category''])
>
> The second line can deal with @params[''category''] as a
single id or as
> an array of ids.
This is precisely the bit I didn''t understand. Thanks so much!

When I add this to my controller such as:

def create()
        @article = Article.new(@params[''article''])
        if @article.save
            @article.categories.clear
           
@article.categories<<Category.find(@params[''category''])
<--- line 22
            flash[''notice''] = ''Article was
successfully posted.''
            redirect_to :action => ''index''
        else
              flash[''notice''] = ''There was a problem
posting your
article. Please try again.''
              render_action ''edit''
        end
    end

I end up with this error:

Couldn''t find Category without an ID
app/controllers/article_controller.rb:22:in `create''

And this SQL statement in the log seems to be the culprit:
SELECT t.*, j.* FROM articles_categories j, categories t WHERE t.id = 
j.category_id AND j.article_id = 7

Why does it need to select from articles_categories? Shouldn''t it
select
the category id from "categories" and then insert that and the article
id from "articles" into articles_categories?

Thanks for your time.

-- 
Alex Ezell