How to set limit for array size in Mongoose schema

Question

Would you be kind to tell me is there any way to set limitation on array size while creating Mongoose schema. For example

 var peopleSchema = new Schema({
    name: {
        type: String,
        required: true,
        default: true
    },
   /* here I want to have limit: no more than 10 friends.
    Is it possible to define in schema?*/
    friends: [{
        type: Schema.Types.ObjectId,
        ref: 'peopleModel'
    }]
})

Answer 1

With a small tweak to your schema setup you can add a validate option:

var peopleSchema = new Schema({
  name: {
    type: String,
    required: true,
    default: true
  },
  friends: {
    type: [{
      type: Schema.Types.ObjectId,
      ref: 'peopleModel'
    }],
    validate: [arrayLimit, '{PATH} exceeds the limit of 10']
  }
});

function arrayLimit(val) {
  return val.length <= 10;
}

Answer 2

starting from mongo 3.6 you can add validation for a collection at server end, each document inserted/updated will be validated against the validator $jsonSchema, only the valid gets inserted, validation error will be for invalid documents

db.createCollection("people", {
   validator: {
      $jsonSchema: {
         bsonType: "object",
         required: [ "name" ],
         properties: {
            name: {
               bsonType: ["string"],
               description: "must be a string"
            },
            friends: {
               bsonType: ["array"],
               items : { bsonType: ["string"] },
               minItems: 0,
               maxItems: 10,
               description: "must be a array of string and max is 10"
            }
         }
      }
   }
});

collection

> db.people.find()

valid document

> db.people.insert({name: 'abc' , friends : ['1','2','3','4','5','6','7','8','9','10']})
WriteResult({ "nInserted" : 1 })

invalid document

> db.people.insert({name: 'def' , friends : ['1','2','3','4','5','6','7','8','9','10', '11']})
WriteResult({
    "nInserted" : 0,
    "writeError" : {
        "code" : 121,
        "errmsg" : "Document failed validation"
    }
})

find

> db.people.find()
{ "_id" : ObjectId("5a9779b60546616d5377ec1c"), "name" : "abc", "friends" : [ "1", "2", "3", "4", "5", "6", "7", "8", "9", "10" ] }
> 

Answer 3

You can use $slice modifier at the time you push new friend id into your array https://docs.mongodb.com/manual/reference/operator/update/slice/#up._S_slice

$push: {
  friends: {
   $each: [id],
   $slice: -10
  }
}

Answer 4

This is my schema and array limit on assignedToId via an external function.

const mongoose = require("mongoose");
    const Schema = mongoose.Schema;

const taskSchema = new Schema({
  parentTask: {
    trim: true,
    type: Schema.Types.ObjectId,
    ref: "task",
  },
  assignedToId: [{
    trim: true,
    type: Schema.Types.ObjectId,
    ref: "Employees",
  }],
  createdBy: {
    trim: true,
    type: Schema.Types.ObjectId,
    ref: "Employees",
    required: [true, "User ID is required"]
  },
  createdByName: {
    trim: true,
    type: String,
    required: [true, "Creater name is required"]
  },
},
  {
    timestamps: true
  });

// Validations for assignedTo employees' size
taskSchema.path('assignedToId').validate(function (value) {
  console.log(value.length)
  if (value.length > 10) {
    throw new Error("Assigned person's size can't be greater than 10!");
  }
});

const Tasks = mongoose.model("Tasks", taskSchema);

module.exports = Tasks;