Nested (children) comments within a single parent comment in MongoDB

Question

Question: Do I nest my child comments within a single parent comment or not?

Overview:

• Creating comment system

• Comments can have children

• Right now each comment child or parent is stored as one record in a collection called SubmissionCommentsSchema

• Child comments have a key => value of parent_id => Object ID where Object ID references a parent comment that has a parent_id of null.

Schema looks like:

SubmissionCommentsSchema = new Schema({
  id : Schema.ObjectId,
  submission_id : {
    type: String
  },
  parent_id : {
    type: String,
    default: null
  },
  comment : {
    type: String,
    required: true
  },
  user_id: {
    type: String,
    required: true
  },
  username : {
    type: String,
    required: true
  },
  created : {
    type: Date,
    default: Date.now
  },
  reply : {
    type: Boolean,
    default: false,
    required: true
  },
  deleted : {
    type: Boolean,
    default: false
  }
});

Example of a parent comment:

  { submission_id: '51899313634afe0000000051',
    comment: 'asdfadsf',
    user_id: '516e173f48670b44d20004dc',
    username: 'bobcobb',
    _id: 51899338634afe0000000055,
    deleted: false,
    reply: false,
    created: Tue May 07 2013 16:50:16 GMT-0700 (PDT),
    parent_id: null },

Example of a child comment referencing the above parent comment:

  { submission_id: '51899313634afe0000000051',
    comment: 'Testing one two four',
    user_id: '516b45f8ac6a1b488e000001',
    username: 'testing',
    _id: 519d93a83867470000000146,
    deleted: false,
    reply: false,
    created: Wed May 22 2013 20:57:28 GMT-0700 (PDT),
    parent_id: '51899338634afe0000000055' },

• When serving these to a view, I pull them all back and loop through them to see which have parent_ids !== null. If so, then I put those in a list and append them to the parent comment and then dump those in the view.

Should I just store the children comments within the original parent comment as a nested array?

Answer 1

Probably not since you may well overflow the document size limit if a particular thread receives many replies.

What you might want to do however is add an integer depth field to the comments so you can easily fetch, say, the top 2 levels, display them, and then load subsequent levels only as the user expands the comment tree.

Sometimes it's also worth maintaining an array of ancestor ids so you can quickly fetch any sub-tree, or given a deep-link to a specific comment can quickly figure out what comments you need to load 'above' it to display the minimally-expanded comment tree that includes it.