has_many through and partials

Question

I have a User model, a Post model, and an Interest model.

---

User has_many posts through interests

User has_many interests

Post has_many users through interests

Post has_many interests

Interest belongs to Post

Interest belongs to User  

---

Application_Controller is as follows:

class ApplicationController < ActionController::Base
  before_filter :login_from_cookie
  before_filter :find_user_interests
  helper :all # include all helpers, all the time
  session :session_key => '_blah_session'

  include AuthenticatedSystem   

  def find_user_interests
    @user_interests = current_user ? current_user.interests : []  
    true
  end

end

---

Application.html.erb has as follows:

<%= render :partial => "users/interests", :object => @user_interests %>

_interests.html.erb partial is as follows:

ul
  <% unless current_user.nil? then -%>  
    <% @user_interests.each do |interest| -%>
      li<%= interest.post.title %>/li
    <% end %>
  <% end -%>   
/ul    

---

Given all this when I at localhost:3000/posts/1 my partial shows up fine, but when in localhost:3000/posts I get an error undefined method 'title' for nil:NilClass thus an error in the line li<%= interest.post.title %>/li shown above in the _interests.html.erb partial.

What the heck would be the issue?

TIA

Answer 1

That just means that one of the interests doesn't have an associated post on the other end. Most likely it was deleted. This could have been prevented by the following:

class Post < ActiveRecord::Base
  has_many :interests, :dependent => :destroy
end

In the meantime you should clean up the orphans in the database.

Edit: You claim this was already in your model, but if it was then it's not clear how you could have an orphaned Interest as the error indicates. Maybe it was created before you added the dependent clause? Again, go delete the orphans via SQL and then try again. If the problem resurfaces later you must be deleting without callbacks somewhere.

Regarding your size problem. You could be using current_user.interests.count. This is due to some magic with Rails associations. count is a special method on a Rails association that runs SQL. length is just an array method telling you how many items are in the array. Rails associations have a few special methods, but the rest of them they pass through to the array object transparently.

Further critiques: when you pass :object => @user_interests you are setting a variable with the name of the partial. So you could reference the local variable interests in the partial. However you are referencing @user_interests, so passing the object is not necessary. All else being equal, passing the object and using a local variable is probably better (it's more explicit, more of a functional-programming style), but in this case you are not making use of that.

Finally, stylewise, I may be wrong since I don't have the full context, but in general I would put the logged_in condition in the template rather than setting user_interests to an empty array if there is no logged in user. This would allow you to reference current_user.interests.count in the template and independently set the interests to be displayed (eg. for pagination) in @user_interests.