Rails 3.2: undefined method for []:ActiveRecord::Relation -

i have product model has many categories , category has many attributes.

class product < activerecord::base       has_many :categories, :dependent => :destroy   accepts_nested_attributes_for :categories, :reject_if => lambda { |a| a[:name].blank? }, :allow_destroy => true    attr_accessible :categories_attributes, :comments, :name, :price, :available, :image, :remote_image_url end  class category < activerecord::base   belongs_to :product   has_many :attributes, :dependent => :destroy   attr_accessible :name, :attributes_attributes    accepts_nested_attributes_for :attributes, :reject_if => lambda { |a| a[:name].blank? }, :allow_destroy => true end  class attribute < activerecord::base   belongs_to :category    attr_accessible :name, :value, :is_key end 

my problem that, when try updating product model, following error:

undefined method `keys' []:activerecord::relation 

i have no keys attribute in model, have is_key in attribute model, thinking problem comes from.

i using following form product update:

.form_container   = nested_form_for @product, :html => {:class => 'form'} |f|     - if @product.errors.any?       #error_explanation         %h2= "#{pluralize(@product.errors.count, "error")} prohibited product being saved:"         %ul           - @product.errors.full_messages.each |msg|             %li= msg      .title       product fields      .field       = f.label :name       = f.text_field :name, :class => :required     .field       = f.label :price       = f.text_field :price, :class => :required     .field       = f.label :comments       = f.text_area :comments     .field       = f.label :product_type       = select_tag 'product_type', options_from_collection_for_select(@product_types, 'id', 'name'), { :class => '' }     %br/     .title       image      .field       = f.file_field :image, :class => :file     .field       = f.label :remote_image_url, 'or image url'       = f.text_field :remote_image_url     %br/      .title       specifications     .field       = f.fields_for :categories |category|         = render 'category_fields', :f => category     .field       = f.link_to_add "add category", :categories, :class => 'button_link'     .field       = f.label :available       = f.check_box :available, {checked: true}     .actions       = f.submit submit_button_name, :class => :submit 

change line

has_many :attributes, :dependent => :destroy 

to

has_many :category_attributes, :dependent => :destroy,  :class_name => 'attribute'    

along rest attr_accessible etc. avoid conflict build in attributes rails method