FIX: Avoid exception when rendering a poll in a trashed post

Maintain the poll belongs_to post relation when a post is trashed

plugins/poll/app/models/poll.rb

@@ -4,7 +4,7 @@ class Poll < ActiveRecord::Base
   # because we want to use the 'type' column and don't want to use STI
   self.inheritance_column = nil
 
-  belongs_to :post
+  belongs_to :post, -> { unscope(:where) }
 
   has_many :poll_options, -> { order(:id) }, dependent: :destroy
   has_many :poll_votes
@@ -51,7 +51,7 @@ class Poll < ActiveRecord::Base
   end
 
   def is_me?(user)
-    user && post.user&.id == user&.id
+    user && post && post.user&.id == user&.id
   end
 
   def has_voted?(user)

plugins/poll/spec/models/poll_spec.rb

@@ -69,4 +69,18 @@ describe ::DiscoursePoll::Poll do
       expect(poll.can_see_results?(user)).to eq(true)
     end
   end
+
+  describe 'when post is trashed' do
+    it "maintains the association" do
+      user = Fabricate(:user)
+      post = Fabricate(:post, raw: "[poll results=staff_only]\n- A\n- B\n[/poll]", user: user)
+      poll = post.polls.first
+
+      post.trash!
+      poll.reload
+
+      expect(poll.post).to eq(post)
+    end
+
+  end
 end