Why does MongoDB return empty arrays;

Question

First of all my code is obviously simplified. I create a new field named "followers", in order to store followers names, as array type.

USER SCHEMA

 var userSchema = new mongoose.Schema({
    local:{
        localId:{type:Number},
        username:{type:String},
        email:{type:String},          
        followers:{type:Array}       
    },
    facebook:{
        facebookId: {type:Number},     
        username:{type:String},
        email:{type:String},       
        followers:{type:Array}       
    },
    twitter:{
        twitterId: {type:Number},
        username:{type:String},
        email:{type:String},      
        followers:{type:Array} 
    }
});

DB record

{ "_id" : ObjectId("...."), "facebook" : { "facebookId" : 10215321125408144, "username" : "johnny", "email" : "johnny@hotmail.com}, "__v" : 0 }

But something weird comes up. When i make a query:

User.findOne({$or: [
              {'local.username': 'johnny' },
           { 'facebook.username': 'johnny'  },
            { 'twitter.username': 'johnny'  },
  ]}, function(err, user) {
            if(err){console.log(err);} 
              if(!user){
                console.log('cannot find user');
              }
              if(user){
                console.log('usrInfo is : '+user);
              return res.send(JSON.stringify(user));
            }
        });

i get as a result:

usrInfo is : { _id: 5a60de744b85e14051e03e2e,
  __v: 0,
  twitter: { followers: [] },
  facebook: 
   { facebookId: .....,
     username: 'johnny',
     email: 'johnny@hotmail.com',
    .............,
     followers: [] },
  local: { followers: [] } }

I cannot understand why

 twitter: { followers: [] } and   local: { followers: [] }

are showing up. Is there any way to avoid this; Because i'm new in MongoDB i would appreciate any explanation why MongoDB does that. Thanks

Answer 1

This issue occurs because, according to the Mongoose documentation, fields in your schema with type Array implicitly have a default value of []. Try setting the default value to undefined and see if that resolves your problem:

var userSchema = new mongoose.Schema({
    local:{
        localId:{type:Number},
        username:{type:String},
        email:{type:String},
        followers:{type:Array, default: undefined}
    },
    facebook:{
        facebookId: {type:Number},
        username:{type:String},
        email:{type:String},
        followers:{type:Array, default: undefined}
    },
    twitter:{
        twitterId: {type:Number},
        username:{type:String},
        email:{type:String},
        followers:{type:Array, default: undefined}
    }
});