未定义的方法，用于nil：NilClass错误

Question

I'm receiving an undefined method `site_name' for nil:NilClass error for the following. Would appreciate any help. I'm trying to list the site_name in the Site table and i'm not sure how best to resolve the issue.

class Site < ActiveRecord::Base
      attr_accessible :site_id, :site_name, :region

      has_many :trials, :foreign_key => "site_id"

    end

    class Trial < ActiveRecord::Base
      attr_accessible :trial_id, :site_id, :year, :trial_type

      scope :year, ->(year) { where(year: year) }
      scope :trial_id, ->(trial_id) { where(trial_id: trial_id) }

      belongs_to :site

    end

My Controller:

class TrialsController < ApplicationController

  def index
    @years = Trial.group('year').order('year DESC')
    end

  def trial
    @trial = Trial.trial_id(params[:trial_id])
  end

**def list
@list = Trial.year(params[:year]).order(:region_id).joins(:sites).where('sites.site_id' => :site_id)

end**

end

My view:

    <% @list.each do |list| %>
        <tr>
        <td>
      <%= link_to list.site.site_name, trial_trials_path(trial_id: list.trial_id) %>
        </td>
        <td>    
        <%= link_to list.trial_type, trial_trials_path(trial_id: list.trial_id) unless list.trial_type.blank? %>
        </td>
        <td>    
        <%= link_to list.trial_type, trial_trials_path(trial_id: list.trial_id) %>
        </td>
        <td>    
        <%= link_to list.planted_at, trial_trials_path(trial_id: list.trial_id) unless list.planted_at.blank? %>
        </td>
        <td>    
        <%= link_to list.picked_at, trial_trials_path(trial_id: list.trial_id) unless list.picked_at.blank? %>
        </td>
        </tr>
        <% end %>

Answer 1

What if you just change the line to this:

<%= link_to list.site.site_name, trial_trials_path(trial_id: list.trial_id) if list.site.try(:site_name) %>

Also, you could do a Site.where(site_name: nil) in the Rails console to see which site doesn't have a name.

Answer 2

if you don't want page crush even if there is not site for list, you could try to use this

list.site.try(:site_name)

but I think that you should use another flow, like

1. Do not let to create list without site.
2. Do not list lists without sites.

Answer 3

You have to decide about your default site name, eg. "Example.com" . And then you simply define it on nil:

def nil.site_name
  "Example.com"
end

And the error will go away.