Check arrays field size with gt and lt

[Rutenioh]

Hello,

I am trying to add a rule using MongoDB conditions to check if an array is greater or less than a specific number.

I have done several tests, the first one being that the field is greater than the number:

"conditions" : {
    "arrayField" : {
        "$lte" : 5
    }
}

This won't work because it checks the value, not the size.

I then tried using $expr and $where, but I can't find a way to implement it.
Our way to do the is:

ForbiddenError.from(ability).throwUnlessCan(action,
      subjectCasl(subject, subjectDTO || {}),
      field));

We load the rule definition from db and use it this way. The rule definition is:

DefinitionItem {
    action: string;
    conditions?: Record<string, unknown>;
    fields?: string[];
    inverted?: boolean;
    reason?: string;
    subject: string;
}

and we create the ability:

const rule = //load from db
const ability = createMongoAbility(rule.definition)

Thank you, and if you need more information, please let me know.