MongoDB: match non-empty doc in array

Question

I have a collection structured thusly:

{
  _id: 1,
  score: [
    {
      foo: 'a',
      bar: 0,
      user: {user1: 0, user2: 7}
    }
  ]
}

I need to find all documents that have at least one 'score' (element in score array) that has a certain value of 'bar' and a non-empty 'user' sub-document.

This is what I came up with (and it seemed like it should work):

db.col.find({score: {"$elemMatch": {bar:0, user: {"$not":{}} }}})

But, I get this error:

error: { "$err" : "$not cannot be empty", "code" : 13030 }

Any other way to do this?

Is that exactly how it's structured? Seems a bit off. > var doc = { score: [ foo: 'a', bar: 0, user: { user1: 0, user2: 7 } ] }; Thu Jul 7 00:43:42 SyntaxError: missing ] after element list (shell):1 — Justin Jenkins
– Justin Jenkins, Commented Jul 7, 2011 at 7:44

Dmitri · Accepted Answer · 2011-07-27 00:02:19Z

111

Figured it out: { 'score.user': { "$gt": {} } } will match non-empty docs.

answered Jul 27, 2011 at 0:02

Dmitri

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4 Comments

zVictor Over a year ago

In this case, score.user is an object. Would it also work to check if score, an array, is empty?

Dmitri Over a year ago

It might, but for arrays you're probably better off using: { foo: { $not: { $size: 0 } } }

zVictor Over a year ago

excelent! I tested, and both worked. Do you know why second option would be better?

Dmitri Over a year ago

@knoxxs: see the docs on sort order: docs.mongodb.org/manual/reference/bson-types - an empty doc sorts before any non-empty doc, but a null will sort before any (including empty) doc.

Justin Jenkins · Accepted Answer · 2011-07-07 09:08:27Z

3

I'm not sure I quite understand your schema, but perhaps the most straight forward way would be to not have an "empty" value for score.user ?

Instead purposely not have that field in your document if it has no content?

Then your query could be something like ...

> db.test.find({ "score" : { "$elemMatch" : { bar : 0, "user" : {"$exists": true }}}})

i.e. looking for a value in score.bar that you want (0 in this case) checking for the mear existence ($exists, see docs) of score.user (and if it has a value, then it'll exist?)

editied: oops I missed the $elemMatch you had ...

edited Jul 7, 2011 at 9:08

answered Jul 7, 2011 at 9:00

Justin Jenkins

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1 Comment

Dmitri Over a year ago

Yes, I've thought of doing it that way, the main reason I'd prefer a query that matches both non-existent and empty user is that individual user scores can be removed and I'd rather not have to check whether the subdoc is empty and needs to be removed every time.

cwb · Accepted Answer · 2011-07-07 12:56:48Z

You probably want to add an auxiliary array that keeps track of the users in the user document:

{
  _id: 1,
  score: [
    {
      foo: 'a',
      bar: 0,
      users: ["user1", "user2"],
      user: {user1: 0, user2: 7}
    }
  ]
}

Then you can add new users atomically:

> db.test.update({_id: 1, score: { $elemMatch: {bar: 0}}},                       
... {$set: {'score.$.user.user3': 10}, $addToSet: {'score.$.users': "user3"}})

Remove users:

> db.test.update({_id: 1, score: { $elemMatch: {bar: 0}}},
... {$unset: {'score.$.user.user3': 1}, $pop: {'score.$.users': "user3"}})

Query scores:

> db.test.find({_id: 1, score: {$elemMatch: {bar: 0, users: {$not: {$size: 0}}}}})

If you know you'll only be adding non-existent users and removing existent users from the user document, you can simplify users to a counter instead of an array, but the above is more resilient.

user2665694 · Accepted Answer · 2011-07-07 07:37:47Z

0

Look at the $size operator for checking array sizes.

answered Jul 7, 2011 at 7:37

user2665694

1 Comment

Dmitri Over a year ago

I'm not checking the size of any arrays. I left out a set of braces before, may be more clear now. An equivalent operator for documents/objects would work, but that doesn't seem to exist.

Rafiq · Accepted Answer · 2022-08-24 17:25:10Z

 $group: {
        _id: '$_id',
        tasks: {
          $addToSet: {
            $cond: {
              if: {
                $eq: [
                  {
                    $ifNull: ['$tasks.id', ''],
                  },
                  '',
                ],
              },
              then: '$$REMOVE',
              else: {
                id: '$tasks.id',
                description: '$tasks.description',
                assignee: {
                  $cond: {
                    if: {
                      $eq: [
                        {
                          $ifNull: ['$tasks.assignee._id', ''],
                        },
                        '',
                      ],
                    },
                    then: undefined,
                    else: {
                      id: '$tasks.assignee._id',
                      name: '$tasks.assignee.name',
                    },
                  },
                },
              },
            },
          },
        },

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