Find where array size is equal to field

Question

I'm trying to find all documents where an array's size is equal to a field's value. For example:

This document should be found:

{
  arr: ["one", "two", "three"],
  expected: 3
}

But not this one:

{
  arr: ["one", "two", "three", "four"],
  expected: 2
}

I'm assuming I have to use some form of aggregation, so I've decided to use $expr:

$expr: { $eq: [{ $size: "$arr" }, { ??? }] }

This might be useful - https://stackoverflow.com/questions/7811163/query-for-documents-where-array-size-is-greater-than-1 — Mo A, Feb 06 '19 at 16:57
@MoA Unfortunately, it's not a predefined value, which is why I asked my question — APixel Visuals, Feb 06 '19 at 17:00

score 1 · Accepted Answer · answered Feb 06 '19 at 17:26

1

Similarly as you used the arr field using $ sign, you have to use for expected field

db.collection.find({ "$expr": { "$eq": [{ "$size": "$arr" }, "$expected"] })

answered Feb 06 '19 at 17:26

Ashh

1 Answers1