MongoDB query on n'th element(variable index) of subdocument array

Question

How can I query on the nth element of a subdocument array in MongoDB where n is variable?

Suppose, I've documents like bellow:

{
  list:[
    {
      a: true,
      b: 'abc'
    },
    {
      a: false,
      b: 'def'
    },
    {
      a: true,
      b: 'ghi'
    },
  ]
}

Query 1: I need to find all documents which have a: false on the 1st element of list(i.e. 'list.0.a': false)
Query 2: I need to find all documents which have a: false on the 2nd element of list(i.e. 'list.1.a': false)
Query 3: I need to find all documents which have a: false on the 3rd element of list(i.e. 'list.2.a': false)

Why not just run query like db.collection.find({ "list.1.a": false}) — sushant mehta
– sushant mehta, Commented Aug 22, 2019 at 5:14
@Caconde I tried deconstructing array with unwind but it doesn't help. @mickl 's answer works perfectly! — Mostafiz Rahman
– Mostafiz Rahman, Commented Aug 22, 2019 at 6:28

mickl · Accepted Answer · 2019-08-22 01:34:24Z

2

You can use $expr to use aggregation expressions in your query, $let to define temporary variable, $arrayElemAt to take nth element of an array:

db.collection.find({
    $expr: {
        $let: {
            vars: { fst: { $arrayElemAt: [ "$list", 0 ] } },
            in: { $eq: [ "$$fst.a", false ] }
        }
    }
})

answered Aug 22, 2019 at 1:34

mickl

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2 Comments

Mostafiz Rahman Over a year ago

This works perfectly! Thanks! Anyway, is there any performance issue with $expr operator?

mickl Over a year ago

@MostafizRahman I don't think so - it's just because there are two types of operators: the old ones for querying and the new ones from aggregation so it's a kind of bridge between these two worlds

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