Handle comment context in cases of missing parent thread.

TNL-4944

models/comment.rb

@@ -141,7 +141,14 @@ class Comment < Content
   end
 
   def context
-    self.comment_thread_id ? self.comment_thread.context : nil
+    if self.comment_thread_id
+      t = CommentThread.find self.comment_thread_id
+      if t
+        t.context
+      end
+    end
+  rescue Mongoid::Errors::DocumentNotFound
+    nil
   end
 
   def course_context?

spec/models/comment_spec.rb

@@ -36,6 +36,14 @@ describe Comment do
         expect(comment.context).to eq("course")
       end
     end
+
+    context 'without valid parent thread' do
+      it 'returns nil' do
+        comment = make_comment(author, course_thread, "comment")
+        comment.comment_thread_id = 'not a thread'
+        expect(comment.context).to eq(nil)
+      end
+    end
   end
 
   describe '#course_context?' do
@@ -52,6 +60,14 @@ describe Comment do
         expect(comment.course_context?).to be_true
       end
     end
+
+    context 'without valid parent thread' do
+      it 'returns false' do
+        comment = make_comment(author, course_thread, "comment")
+        comment.comment_thread_id = 'not a thread'
+        expect(comment.course_context?).to be_false
+      end
+    end
   end
 
   describe '#standalone_context?' do
@@ -68,6 +84,15 @@ describe Comment do
         expect(comment.standalone_context?).to be_false
       end
     end
+
+    context 'without valid parent thread' do
+      it 'returns false' do
+        comment = make_comment(author, course_thread, "comment")
+        comment.comment_thread_id = 'not a thread'
+        expect(comment.standalone_context?).to be_false
+      end
+    end
+
   end
 
   describe '#child_count' do