form.haml

- form_for @screen do |f|
  - Asset.all.each do |asset|
    %label 
      = check_box_tag 'screen[asset_ids][]', asset.id, @screen.asset_ids.include?(asset.id)
      = asset.name
  = f.submit
   

models.rb

# "model A"
class Screen < ActiveRecord::Base
  has_and_belongs_to_many :assets
end

# "model B"
class Asset < ActiveRecord::Base
  has_and_belongs_to_many :screens
  default_scope :order => 'name'
end

    

summary.md

On the first submission of the form, I can freely add associated models. Every subsequent submission fails, however, with:

ActiveRecord::AssociationTypeMismatch
Asset(#1234567) expected, got Asset(#7654321)

Commenting out the default_scope command in Asset "solved" the problem. i.e., I didn't get the error anymore.

This is using:

• actionmailer (3.0.0.beta4)
• actionpack (3.0.0.beta4)
• activemodel (3.0.0.beta4)
• activerecord (3.0.0.beta4)
• activeresource (3.0.0.beta4)
• activesupport (3.0.0.beta4)
• arel (0.4.0)
• pg (0.9.0)
• rails (3.0.0.beta4)