'this' equals 'global' when a property's type is an Array

[cocoasoda]

Do you want to request a feature or report a bug?
bug
What is the current behavior?
define a Schema like below

const Group = new mongoose.Schema({
    title: {
        type: String,
        required: true
    },
    members: {
        type: [
            { type: SchemaTypes.ObjectId, ref: 'User' }
        ],
        default: []
    },
    owner: {
        type: SchemaTypes.ObjectId,
        ref: 'User',
        required: true
    },
    managers: {
        type: [{
            type: SchemaTypes.ObjectId,
            ref: 'User'
        }],
        /* type: String works fine but not type: [String] */
        default: function() {
            console.log('this:',this === global);
            return [this.owner];
        }
    }
})

then create a document

const g = new Group({
        title: 'test',
        owner: users[0]._id,
    });

the document's members property is [ undefined ]
If the current behavior is a bug, please provide the steps to reproduce.

What is the expected behavior?
'this' should be the document itself, but this === global is true
Please mention your node.js, mongoose and MongoDB version.
nodejs v9.8.0 (Windows)
mongoose 5.0.11
mongoDB runs on Azure Cosmos DB maybe ^v3.4

Hi @cocoasoda. When you make the default value an empty array [] it returns a schemaType of Mixed [{}]. Arrays implicitly have a default value of [], so you can drop the default property on the members path. The docs are here

When I run your example I get this: false

6263.js

#!/usr/bin/env node
'use strict'

const mongoose = require('mongoose')
mongoose.connect('mongodb://localhost/test')
const Schema = mongoose.Schema

const groupSchema = new mongoose.Schema({
  title: {
    type: String,
    required: true
  },
  members: {
    type: [
      { type: Schema.Types.ObjectId, ref: 'User' }
    ],
    default: []
  },
  owner: {
    type: Schema.Types.ObjectId,
    ref: 'User',
    required: true
  },
  managers: {
    type: [{
      type: Schema.Types.ObjectId,
      ref: 'User'
    }],
    /* type: String works fine but not type: [String] */
    default: function () {
      console.log('this:', this === global)
      return [this.owner]
    }
  }
})

const Group = mongoose.model('group', groupSchema)

try {
  const g = new Group({
    title: 'test',
    owner: new mongoose.mongo.ObjectID()
  })
} catch (e) {
  console.error(e.message)
  mongoose.connection.close()
}

Output:

issues: ./6263.js
this: false            // it's undefined for me
Cannot read property 'owner' of undefined
issues: 

when a schema gets compiled by mongoose.model, the schema path defaults get set in the order they are declared in the object that you pass into new Schema, with one exception that I can see. If the schemaType is an array, that path gets it's defaults set first. So, this.owners wouldn't exist yet. I wrote a quick example to illustrate this:

#!/usr/bin/env node
'use strict'

const mongoose = require('mongoose')
const Schema = mongoose.Schema

const schema = new Schema({
  str: {
    type: String,
    default: function () {
      console.dir(`1) str: ${this}`)
    }
  },
  num: {
    type: Number,
    default: function () {
      console.dir(`2) num: ${this}`)
    }
  },
  mix: {
    type: Schema.Types.Mixed,
    default: function () {
      console.dir(`3) mix: ${this}`)
    }
  },
  arr: {
    type: Array,
    default: function () {
      console.dir(`4) arr: ${this}`)
      return 'hello'
    }
  },
  date: {
    type: Date,
    default: function () {
      console.dir(`5) date: ${this}`)
    }
  },
  buf: {
    type: Buffer,
    default: function () {
      console.dir(`6) buf: ${this}`)
    }
  },
  oid: {
    type: Schema.Types.ObjectId,
    default: function () {
      console.dir(`7) oid: ${this}`)
    }
  },
  dec: {
    type: Schema.Types.Decimal128,
    default: function () {
      console.dir(`8) dec: ${this}`)
    }
  },
  boo: {
    type: Boolean,
    default: function () {
      console.dir(`9) boo: ${this}`)
    }
  }
})

const Test = mongoose.model('test', schema)

const test = new Test({})

Output:

issues: ./allDefaultsOrders.js 
'4) arr: undefined'
'1) str: { arr: [ \'hello\' ], _id: 5ab546a88691337d77bb0b76 }'
'2) num: { arr: [ \'hello\' ], _id: 5ab546a88691337d77bb0b76 }'
'3) mix: { arr: [ \'hello\' ], _id: 5ab546a88691337d77bb0b76 }'
'5) date: { arr: [ \'hello\' ], _id: 5ab546a88691337d77bb0b76 }'
'6) buf: { arr: [ \'hello\' ], _id: 5ab546a88691337d77bb0b76 }'
'7) oid: { arr: [ \'hello\' ], _id: 5ab546a88691337d77bb0b76 }'
'8) dec: { arr: [ \'hello\' ], _id: 5ab546a88691337d77bb0b76 }'
'9) boo: { arr: [ \'hello\' ], _id: 5ab546a88691337d77bb0b76 }'

You can move the addition of the owner to the manager's array to a pre save hook like this:

#!/usr/bin/env node
'use strict'

const mongoose = require('mongoose')
mongoose.connect('mongodb://localhost/test')
const Schema = mongoose.Schema

const groupSchema = new mongoose.Schema({
  title: {
    type: String,
    required: true
  },
  members: {
    type: [
      { type: Schema.Types.ObjectId, ref: 'User' }
    ],
    default: []
  },
  owner: {
    type: Schema.Types.ObjectId,
    ref: 'User',
    required: true
  },
  managers: {
    type: [{
      type: Schema.Types.ObjectId,
      ref: 'User'
    }]
  }
})

groupSchema.pre('save', function () {
  if (this.isNew) {
    this.managers.push(this.owner)
  }
})

const Group = mongoose.model('group', groupSchema)

const g = new Group({
  title: 'test',
  owner: new mongoose.mongo.ObjectID()
})

g.save(function (err, doc) {
  if (err) { return console.error(err) }
  doc.members.push(new mongoose.mongo.ObjectID())
  doc.save((err, secondSave) => {
    if (err) { return console.error(err) }
    console.log(secondSave)
    return mongoose.connection.close()
  })
})

Output:

issues: ./6263.js 
{ members: [ 5ab60c4a7560e30ad763d8e5 ],
  managers: [ 5ab60c4a7560e30ad763d8e3 ],
  _id: 5ab60c4a7560e30ad763d8e4,
  title: 'test',
  owner: 5ab60c4a7560e30ad763d8e3,
  __v: 1 }
issues: 

[EDIT] I added a test in the pre save hook for new document, so the owner will only get pushed on initial creation.

[cocoasoda]

@lineus Thanks for your help and advice. Pre-save hook runs very well for me.
BTW I forgot to use strict mode that 'this' exposed to 'global'.