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

    }
});

User.findOneAndUpdate({$or: [
              {'local.username': username }, // 'or' condition works perfect
           { 'facebook.username': username },
            { 'twitter.username': username },
  ]},{
    email:req.body.email // doesn't work...here i don't know what to do...
  },function(err, user) {
            if(err){console.log(err);}
              if(!user){
                console.log('cannot find user');
              }
              if(user){
                console.log(user);
                   return res.send(JSON.stringify(user));

            }
        });

User.findOneAndUpdate({$or: [
          {'local.username': username }, // 'or' condition works perfect
       { 'facebook.username': username },
        { 'twitter.username': username },
]},{
  $set:{email:req.body.email} // doesn't work...here i don't know what to do...
},function(err, user) {
        if(err){console.log(err);}
          if(!user){
            console.log('cannot find user');
          }
          if(user){
            console.log(user);
               return res.send(JSON.stringify(user));

        }
    });