Error is:
NoMethodError in CollectionsController#update

You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.push
Rails.root: /home/john/Rails/testapp

Application Trace | Framework Trace | Full Trace
app/controllers/collections_controller.rb:59:in `block in update'
app/controllers/collections_controller.rb:58:in `update'


class Collection
  references_and_referenced_in_many :products
end

class Products
  references_and_referenced_in_many :collections
end

class CollectionsController < ApplicationController
  # PUT /collections/1
  # PUT /collections/1.xml
  def update
    @collection = Collection.find(params[:id])
    respond_to do |format|
      if @collection.update_attributes(params[:collection])
        format.html { redirect_to(@collection, :notice => 'Collection was successfully updated.') }
        format.xml  { head :ok }
      else
        format.html { render :action => "edit" }
        format.xml  { render :xml => @collection.errors, :status => :unprocessable_entity }
      end
    end
  end
end


then the form:
= form_for @collection do |f|
  - for product in @store.products
    #product[product]
      = check_box_tag "collection[products][]", product._id, @collection.products.include?(product)
      = product.name
  = f.submit "Update products"

if you set the file field to "collection[product_ids][]" it generates the same error too.