Expected "[" or AggregationStage but "{" found.: MongoDB aggregation doesn't let me use $toLower inside a $eq

Question

I wrote a MongoDB pipeline that has this code in it:

  {
    $eq: [
      {
        "$toLower": "HELLO"
      },
      "hello"
    ]
  }

And here's a screenshot of it in Mongo Compass

I am expecting it to simply return true, and "$match" everything (for now). Eventually I will swap "HELLO" with a field name etc.

Does anyone know why I am getting this error?

Answer 1

$match does not accept raw aggregation expressions. Instead, use a $expr query expression to include aggregation expression in $match.

https://docs.mongodb.com/manual/reference/operator/aggregation/match/index.html#pipe._S_match

$expr: {
    $eq: [
      {
        $toLower: "HELLO"
      },
      "hello"
    ]
}

Aggregate command Find method