Question

So I have a simple post model that has_many tags :through post_tags. This allows for a many to many relationship. However I haven’t been able to get the form_for and fields_for to work. I am really stuck because I can’t find any documentation on the form_for helper with a has_many :through relationship. I watched the rails cast, read previous stack questions, and even researched in the Rails 3 Way. Anywho, here’s what I got.

 class Post < ActiveRecord::Base
  belongs_to :blog

  has_many :post_tags, :dependent => :destroy
  has_many :tag, :through => :post_tags, :dependent => :destroy 

  has_many :post_categories, :dependent => :destroy
  has_many :category, :through => :post_categories, :dependent => :destroy 

  attr_accessible(:title, ...)

  accepts_nested_attributes_for :tag, :allow_destroy => true   

end

and

class PostTag < ActiveRecord::Base
  belongs_to :tag
  belongs_to :post
end

and

class Tag < ActiveRecord::Base
    has_many :post_tags
    has_many :post, :through => :post_tags
end

and my controller code is

def new
    @title = "Create a New Post"
    @user = User.find(params[:user_id])
    @blog = @user.blog
    @post = @blog.post.new
    @post.ptype = params[:type]
    3.times { @post.tag.build}
  end

  def create
    @user = User.find(params[:user_id])
    @blog = @user.blog
    @post = @blog.post.new(params[:post])

    if @post.save
        ...
    end
  end

and the form is

<%= form_for([@user,@blog,@post],:url => user_blogs_posts_path, :html => {:multipart=>true}) do |p| %>
    <%= render 'shared/error_messages', :object => p.object %>
    ... 
        <%= p.fields_for :tag do |t|%>
            <%=t.label :tag %>
            <%=t.text_field :tag %>
        <% end %>

        <%=p.submit%>
<% end %>

and the error is

Can't mass-assign protected attributes: tag_attributes

Answer 1

Add :tag_attributes to attr_accessible in Post

attr_accessible :title, ..., :tag_attributes, ...