MongoDB Aggregation Project check if array contains

Question

I have following document:

{
    _id : 21353456,
    username : "xy",
    text : "asdf",
    comments : [
        {
            username : "User1",
            text : "hi",
        },
        {
            username : "User2",
            text : "hi1",
        },
        {
            username : "User3",
            text : "hi2",
        },
        {
            username : "User4",
            text : "hi3",
        }

    ]
}

Now I want to get the username, text and comments with aggregation and project. In addition I also want a boolean if the comments array contains an username with "User1". I have this, but it doesn't work.

db.posttest.aggregate(
   [
     {
       $project:
          {
            username: 1,
            text: 1,
            comments : 1,
            hasComment: { $eq: [ "comments.$.username", "User1" ] },
            _id: 0
          }
     }
   ]
)

Answer 1

To achieve this you would need to first unwind the comments, and then use a group with a little trick. If you want to omit the _id, then you would also need to do a simple project. Here is the full aggregation pipeline:

db.posttest.aggregate([
  { $unwind : "$comments" },
  { $group : {
    _id : "$_id",
    username : { $first : "$username" },
    text : { $first : "$text" },
    comments : { $push : "$comments" },
    hasComments : { $max : { $eq : [ "$comments.username", "User1" ] } }
  }},
  { $project : { _id : false } }
])

An explanation is following.

First, we need to get rid of an array (comments). To do this we unwind the record; it gives us four records:

{
  "_id" : 21353456,
  "username" : "xy",
  "text" : "asdf",
  "comments" : {
    "username" : "User1",
    "text" : "hi"
  }
},
{
  "_id" : 21353456,
  "username" : "xy",
  "text" : "asdf",
  "comments" : {
    "username" : "User2",
    "text" : "hi1"
  }
},
{
  "_id" : 21353456,
  "username" : "xy",
  "text" : "asdf",
  "comments" : {
    "username" : "User3",
    "text" : "hi2"
  }
},
{
  "_id" : 21353456,
  "username" : "xy",
  "text" : "asdf",
  "comments" : {
    "username" : "User4",
    "text" : "hi3"
  }
}

Now we can group all the records into one applying a function to each field. First, we need to give criteria, the 'group by' field (or set of fields). In our case, it is simply the id: _id: "$_id".

Then, for each field, we need to make a decision on how to include it into the resulting record. We have few fields: username, text, and comments. For each four records the username and text are the same, so we can easily pick any of them, namely $first or $last.

comments, however, are different. We want to preserve all of them so that we $push each one back.

The hasComments is a bit tricky here: we need to check if at least one comment.username contains the username. We can use $eq: [...] here, it will give us some array, e.g. [true, false, false, false] or [false, false, true, false]. We would need to pick which value goes into the resulting record. In this case, we can use neither $first nor $last. However, $max will give us an appropriate result.