Has many_through, how to show only unassociated fields for a category?

Hi everybody,

I'm using ruby 1.9.1, with rails 2.3.*. I have an application of
products and categories, but I want each type of product to have
specific attributes. So, basically, according to the chosen category,
I want different fields to be shown for the user.

So, before actually making the association of the product with the
attribute, I want to create an administration system where the
administrators could actually select wich fields they want for each
category, in an N:N association, usign has many through
(categoryFields is the middle table). So, up to now, the structure is:

class Field < ActiveRecord::Base
    has_many :categoryFields, :dependent => :destroy
    has_many :categories, :through => :categoryFields

class CategoryField < ActiveRecord::Base
    belongs_to :field
    belongs_to :category

class Category < ActiveRecord::Base
has_many :categoryFields, :dependent => :destroy
has_many :fields, :through => :categoryFields

And this works perfectly.
so, now, in my Category Edit view, I want to show a list of ALL
AVAILABLE FIELDS (i.e. only those not already associated to that
specific category) at the right, and in the left, all the ASSOCIATED
FIELDS for that category. Using a simple drag-and-drop, the user can
drag fields from the right to insert them in category, or move them to
the right to remove them from that specific category.

The problem is that I don't want that any field in the left column
also appear in the right column (each field can only be used once in
that specific category, since those attributes are unique). So, once
you have associated a field to a category, it will not appear in the
list of available fields (right list). I'm coming from PHP and this is
my first rails application (I know, it's kinda complicated for a

So, what should I do, to exclude those values?

The edit method in the category controller:
@category = Category.find(params[:id])
@categories = Category.roots #the categories acts as nested set, so
we have to fetch all available parent categories
@fields = Field.all

Now, the view do the following:
  <div class="category_fields span-12 last">
  <% if !@category.fields.empty? # this loops through all teh
ASSOCIATED fields and display their names%>
    <% @category.fields.each do |field| %>
    <% end %>
  <% end %>
  <div class="fields_available">
    <% if !@fields.empty? # this loops through ALL THE fields and
display their names %>
    <% @fields.each do |field| %>
    <% end %>
  <% end %>

So, where should I place the "filter" so that only available fields
will appear in the second div? Shoud I place it in the controller, as
a condition somehow, and how? Or should I use some array-function to
"pop out" the already associated fields from the @fields array? Wich
is better from the point of view of performance, and how do I do each
of them?

Thanks a lot in advice, for any help
Samuel Brandão

Fields.all gives you an array of all the fields in the database
@category.fields gives you an array of all the fields for the given
category (since you have the has_many :fields, :through
=> :categoryFields association)

How do you get the fields that are present in the first array but not
in the second? Subtraction.

@fields_not_chosen_yet = Fields.all - @category.fields

Btw, you may want to exchange :categoryFields for :category_fields.

Hi Sharagoz, thank you for your reply. However, I had already tried
array subtraction once, but with no success.

I tried for example:

associatedFields = @category.fields
allFields = Field.all

available = allFields - associatedFields

however it did not work.
When I try:


[#, #, #, #]


[#, #, #, #, #, #, #, #]

It seems to me that those resulting variables (allFields and
associatedFields) are not of the specific class Array, or if they are,
the subtraction is not working properly. I'm gonna do some more tests,
to make sure i'm not screwing somewhere else.

thanks in advice for any help (at least I know i'm in the right
direction :D)

Is there ANY way to make this subtraction/selection through SQL

associatedFields = @category.fields
allFields = Field.all

available = allFields - associatedFields

Field.all :conditions => ["id not in (?)", @category.fields.all.map(&:id)]

Field.where(["id not in (?)", @category.fields.all.map(&:id)]) if rails 3

Ivan Nastyukhin