MongoDB relationships: embed or reference?

I’m new to MongoDB–coming from a relational database background. I want to design a question structure with some comments, but I don’t know which relationship to use for comments: embed or reference?

A question with some comments, like stackoverflow, would have a structure like this:

Question
    title="aaa"
    content = bbb'
    comments = ???

At first, I want to use embeded comments (I think embed is recommended in MongoDB), like this:

Question
    title="aaa"
    content="bbb"
    comments = [ { content="xxx", createdAt="yyy"}, 
                 { content="xxx", createdAt="yyy"}, 
                 { content="xxx", createdAt="yyy"} ]

It clear, but I’m worried about this case: If I want to edit a specified comment, how do I get its content and its question? There is no _id to let me find one, nor question_ref to let me find its question. (I’m so newbie, that I don’t know if there’s any way to do this without _id and question_ref.)

Do I have to use ref not embed? Then I have to create a new collection for comments?