How To Create Mongoose Schema with Array of Object IDs?

I have defined a mongoose user schema:

var userSchema = mongoose.Schema({
  email: { type: String, required: true, unique: true},
  password: { type: String, required: true},
  name: {
      first: { type: String, required: true, trim: true},
      last: { type: String, required: true, trim: true}
  },
  phone: Number,
  lists: [listSchema],
  friends: [mongoose.Types.ObjectId],
  accessToken: { type: String } // Used for Remember Me
});

var listSchema = new mongoose.Schema({
    name: String,
    description: String,
    contents: [contentSchema],
    created: {type: Date, default:Date.now}
});
var contentSchema = new mongoose.Schema({
    name: String,
    quantity: String,
    complete: Boolean
});

exports.User = mongoose.model('User', userSchema);

the friends parameter is defined as an array of Object IDs. So in other words, a user will have an array containing the IDs of other users. I am not sure if this is the proper notation for doing this.

I am trying to push a new Friend to the friend array of the current user:

user = req.user;
  console.log("adding friend to db");
  models.User.findOne({'email': req.params.email}, '_id', function(err, newFriend){
    models.User.findOne({'_id': user._id}, function(err, user){
      if (err) { return next(err); }
      user.friends.push(newFriend);
    });
  });

however this gives me the following error:

TypeError: Object 531975a04179b4200064daf0 has no method 'cast'

If you want to use Mongoose populate feature, you should do:

var userSchema = mongoose.Schema({
  email: { type: String, required: true, unique: true},
  password: { type: String, required: true},
  name: {
      first: { type: String, required: true, trim: true},
      last: { type: String, required: true, trim: true}
  },
  phone: Number,
  lists: [listSchema],
  friends: [{ type : ObjectId, ref: 'User' }],
  accessToken: { type: String } // Used for Remember Me
});
exports.User = mongoose.model('User', userSchema);

This way you can do this query:

var User = schemas.User;
User
 .find()
 .populate('friends')
 .exec(...)

You'll see that each User will have an array of Users (this user's friends).

And the correct way to insert is like Gabor said:

user.friends.push(newFriend._id);

I'm new to Mongoose myself, so I'm not entirely sure this is right. However, you appear to have written:

friends: [mongoose.Types.ObjectId],

I believe the property you're looking for is actually found here:

friends: [mongoose.Schema.Types.ObjectId],

It may be that the docs have changed since you posted this question though. Apologies if that's the case. Please see the Mongoose SchemaTypes docs for more info and examples.

I would try this.

user.friends.push(newFriend._id);

or

friends: [userSchema],

but i'm not sure if this is correct.