Can't update rails has_many :through relationship

I try:

@item.associations.update_attributes(:tag_id=>params[:tag])

and

@item.associations.tag_id=params[:tag]

Both give me undefined method errors for update_attributes and tag_id=, respectively. Here's my setup:

class Item < ActiveRecord::Base
  has_many :associations,:foreign_key=>"item_id",:dependent=>:destroy
  has_many :reverse_associations,:foreign_key=>"tag_id",:class_name=>"Association"
  has_many :tags,:through=>:associations
end

class Tag < ActiveRecord::Base
  has_many :associations,:foreign_key=>"tag_id",:dependent=>:destroy
  has_many :reverse_associations,:foreign_key=>"item_id",:class_name=>"Association"
  has_many :items,:through=>:associations
  attr_accessible :name
end

class Association < ActiveRecord::Base
  belong开发者_如何学Pythons_to :item
  belongs_to :tag
end

What am I doing wrong?

You're trying to update tag_id on the entire @item.associations collection instead of updating a single Assocation instance.

The proper way to solve this depends on what you're trying to accomplish. To update the tag_id for all associations in @item.association, try:

@item.associations.each do |association|
  association.update_attributes(:tag_id => params[:tag])
end

If you want to update the tag id for a specific Association, then you somehow need to get that association first:

# Just picking the first association for the item as an example.
# You should make sure to retrieve the association that you actually
# want to update.
retagged_association = @item.associations.first

# Now, retag the association
retagged_association.update_attributes(:tag_id => params[:tag])