Typing in databases using empty field

My question is about databases pattern (in my case mongo)

is it a good pattern to type, using an empty field ?

for example I have a text object:

• id: id
• text: string
• commentId: id

if comment is empty than it's not a comment it's a answer

But I think this one is better:

• id: id
• text: string
• commentId: id
• type : ['comment', 'answer']

I my opinion, this is a pattern because it only use commentId for what it is, a storage for id and not a typing variable.

What is your opinion ?