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" : "[email protected]}, "__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: '[email protected]',
    .............,
     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

B. Fleming · Accepted Answer · 2018-01-18 18:43:23Z

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}
    }
});

answered Jan 18, 2018 at 18:43

B. Fleming

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2 Comments

Johnk Overlord Over a year ago

Thanks, it works. But if you want to explain this a little bit more.. [ ] means empty. Why MongoDB returns an empty array on non empty query?

B. Fleming Over a year ago

I'm not sure what you mean by "non-empty" in this context. In your specific example, do you mean "non-existent"? If so, it's because of the way your schema is defined. The default value for arrays is an empty array, so when you perform a query and receive the document, any arrays with that default value will be automatically included in your results as an empty array despite not existing in your document. This is likely Mongoose-specific functionality, provided so that you don't have to worry about whether or not an array exists to perform basic array ops like $push and $pull.

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