Validate presence of 'poll_id' attribute on Poll::Question model

Fixes #1831

On branch aperez-validate-poll-question-is-selected
  Changes to be committed:
    modified:   app/models/poll/question.rb
    modified:   spec/models/poll/question_spec.rb
    modified:   spec/features/admin/poll/polls_spec.rb

app/models/poll/question.rb

@@ -20,6 +20,7 @@ class Poll::Question < ActiveRecord::Base
 
   validates :title, presence: true
   validates :author, presence: true
+  validates :poll_id, presence: true
 
   validates :title, length: { minimum: 4 }
   validates :description, length: { maximum: Poll::Question.description_max_length }

spec/features/admin/poll/polls_spec.rb

@@ -183,7 +183,7 @@ feature 'Admin polls' do
 
       scenario 'Add question to poll', :js do
         poll = create(:poll)
-        question = create(:poll_question, poll: nil, title: 'Should we rebuild the city?')
+        question = create(:poll_question, title: 'Should we rebuild the city?')
 
         visit admin_poll_path(poll)
 

spec/models/poll/question_spec.rb

@@ -9,6 +9,20 @@ RSpec.describe Poll::Question, type: :model do
     end
   end
 
+  describe "#poll_question_id" do
+    it "should be invalid if a poll is not selected" do
+      q = create(:poll_question)
+      q.poll_id = nil
+      expect(q).to_not be_valid
+    end
+
+    it "should be valid if a poll is selected" do
+      q = create(:poll_question)
+      q.poll_id = 1
+      expect(q).to be_valid
+    end
+  end
+
   describe "#copy_attributes_from_proposal" do
     it "copies the attributes from the proposal" do
       create_list(:geozone, 3)