Mongo query based on string length inside array

Question

I have a Mongo db structure that looks like this and I would like to retrieve all the entries where blood type length is bigger than 2.

{
    "_id" : ObjectId("5dba8987b4a39c13bc104a23"),
    "contact" : {
        "firstName" : "Mark",
        "lastName" : "Doe",
        "parentsBloodType" : [ 
            {
                "type" : "AB+",
            }, 
            {
                "type" : "A+",
            }, 
        ],
    },
    "createdAt" : ISODate("2019-10-31T07:13:11.278Z"),
    "updatedAt" : ISODate("2019-11-26T09:59:41.611Z")
}

{
    "_id" : ObjectId("5dba898fdsw54c132c104a23"),
    "contact" : {
        "firstName" : "Helen",
        "lastName" : "Fly",
        "parentsBloodType" : [ 
            {
                "type" : "O-",
            }, 
            {
                "type" : "AB-",
            }, 
        ],
    },
    "createdAt" : ISODate("2019-10-31T08:13:11.278Z"),
    "updatedAt" : ISODate("2019-11-26T09:59:41.611Z")
}

I tried the following query:

db.users.find({"contact.parentsBloodType.type" : {$exists:true}, "$expr": { "$gt": [ { "$strLenCP": "$contact.parentsBloodType.type" }, 2 ] } })

But I got this error:

Error: error: {
    "ok" : 0,
    "errmsg" : "$strLenCP requires a string argument, found: array",
    "code" : 34471,
    "codeName" : "Location34471"
}

How can I correctly do this?

Do you want the whole document in the output or just the array elements that match the "contact.parentsBloodType.type"'s length>2? — prasad_
– prasad_, Commented Dec 13, 2019 at 8:54

prasad_ · Accepted Answer · 2019-12-13 09:30:30Z

5

This query will match documents where the type field length is 3 or greater:

db.users.find( { "contact.parentsBloodType.type" : { $exists: true }, "contact.parentsBloodType.type": { $regex: /.{3,}/ } } )

NOTES: The regular expression .{3,} means any character (specified by dot (.) metacharacter) with length of 3 or greater (specified by {3,}). Also, see MongoDB's $regex query operator.

edited Dec 13, 2019 at 9:30

answered Dec 13, 2019 at 9:17

prasad_

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2 Comments

chakwok Over a year ago

Does this query works if contact.parentsBloodType is an array of 2 [{type: "a"}, {type: "abc"}]? Wouldn't the document be returned?

prasad_ Over a year ago

It works, because the "abc" has a string length of 3 (which is greater than 2).

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