Question 115493 in Mongodb Question posted in
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MongoDB – Compare the field value with array size

I’m trying to return all documents which satisfy document.field.value > document.array.length. I’m trying to do this using MongoClient in ExpressJS and was not able to find answers on SO.

Here is what I have tried but this gives an empty list.

const db = ref()
const c = db.collection(t)
const docs = await c.find({ 
                "stop.time":{$gt: new Date().toISOString()},
                "stop.expect": {$gt: { $size: "stop.riders"}}
            })
console.log(docs)

Also tried replacing

"stop.expect": {$gt: { $size: "stop.riders"}}

with the below code which does not compile

$expr: {$lt: [{$size: "stop.riders"}, "stop.expect"]}

Sample data:

{ "stop": { "expect":3, "riders": ["asd", "erw", "wer"] } },
{ "stop": { "expect":4, "riders": ["asq", "frw", "wbr"] } }
Tags:

2

Answers


  1. 0

    To filter the query with a complex query involving the calculation, you need to use the $expr operator, which allows the aggregation operators.

    Next, within the $expr operator, to refer to the field, you need to add the prefix $.

    db.collection.find({
  $expr: {
    $lt: [
      {
        $size: "$stop.riders"
      },
      "$stop.expect"
    ]
  }
})

    Demo @ Mongo Playground

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  2. 0

    To compare fields within a document to each other, you must use $expr. It would look like this:

    const docs = await c.find({ 
  $expr: { $gt: ["$stop.expect", { $size: "$stop.riders" }] },
});

    I omitted the stop.time condition because it’s not in your sample data and it’s unclear what type of field it is, if it actually is in your data (whether it’s a Date or String would be very important).

    Note that this sort of query is unable to use an index.

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