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}
    }
  ]
}

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Answer 1

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.