Mongodb find array length greater than specified size

Question

I have this Mongoose model schema

const postSchema = new Schema({

  title: String,

  headline: [{
    kind: String,
    id: Schema.Types.ObjectId,
    content: String,
    relevance: Number,
    _id: false
  }],

});

I want to find models in the database where the headline array has a length greater than x

I have this query:

 const query = {
        'headline.kind': 'topic',
        'headline.id': topicId,
        'headline':{
            '$size':{
                '$gt': x
            }
        }
    };

but when I use this I get:

 { MongooseError: Cast to number failed for value "{ '$gt': 2 }" at path "headline"
    at CastError (/home/oleg/WebstormProjects/lectal/api/node_modules/mongoose/lib/error/cast.js:26:11)

anyone know the right way to construct this query? (In my code I just hardcoded the number 2 for x.)

Neil Lunn · Accepted Answer · 2017-06-09 23:25:39Z

For the most efficient way you can do this with a "dot notation" by specifying the n position. Which is basically saying if an array has "at least" n + 1 elements then return it.

So rather than writing { "$gt": 2 } you instead look for the presence of the index value of the "third" element ( from zero index third is 2 ):

{ "headline.2": { "$exists": true } }

The $exists operator is looking for the presence of an element at that given index, and when the condition is met then the array must be of at least that length and therefore "greater than two"

Also noting that your query condition want's to match several properties on an array element, and for this you actually use $elemMatch, otherwise the conditions actually apply to matching any element in the array, and not just the one with "both" conditions as $elemMatch does.

{
  "headline": { "$elemMatch": { "kind": "topic", "id": topicId } },
  "headline.2": { "$exists": true }
}

to do that "dynamically", we merely build the query generating the key from the parameter:

const query = {
  "headline": { "$elemMatch": { "kind": "topic", "id": topicId } }
};

query['headline.'+x] = { "$exists": true }

Adrian Mole · Accepted Answer · 2020-07-21 14:49:16Z

3

$expr:{$gte: [{$size: "$preferredArray"}, specifiedsize]}

If you're using aggregate its better and faster; the entire query would look like this:

db.collection.aggregate([
 
  {
      $match: {
          preferred criteria:1,
          $expr:{$gte: [{$size: "$diagnosis"}, 2]}
      }
          
      
  },

edited Jul 21, 2020 at 14:49

Adrian Mole

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answered Jul 21, 2020 at 14:47

PatronusSeeker

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Comments

maxpaj · Accepted Answer · 2017-01-29 08:00:41Z

2

From what I've read it is not possible to do that using a $size and $gt.

Try using $where - http://mongoosejs.com/docs/api.html#query_Query-%24where

answered Jan 29, 2017 at 8:00

maxpaj

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