title: Mongoose instance .save() not working
body: <p>I have a problem with Mongoose and MongoDb</p>
  
  <p>It is very interesting that only <code>Model.update</code> works and <code>save</code> never works and does not even fire callback.</p>
  
  <p>Mongoose: 4.4.5
  MongoDB: 3.0.8</p>
  
  <p><strong>Express Route</strong></p>
  
  <pre><code>var mongoose = require('mongoose');
  mongoose.connect("mongodb://127.0.0.1:27017/db");
  var db = mongoose.connection;
  db.on('error', console.error.bind(console, 'connection error:'));
  db.once('open', function(callback) {
      console.log("connection to db open")
  });
  var User = require("../models/user.js");
  </code></pre>
  
  <p><strong>User Model</strong></p>
  
  <pre><code>var user = new Schema({
      uid: { type: Number, required: true, unique: true},
      hwid: { type: String, default:""},
      bol:{type:String,default:""}
  });
  </code></pre>
  
  <p><strong>Update Enpoint</strong></p>
  
  <blockquote>
    <p>Working version: <strong>Model.update()</strong></p>
  </blockquote>
  
  <pre><code>User.update({_id: id}, {
      uid: 5, 
  }, function(err, numberAffected, rawResponse) {
      console.log(err);
  })
  </code></pre>
  
  <blockquote>
    <p>Not working version and I have to solve this: <strong>Object.save()</strong></p>
  </blockquote>
  
  <pre><code>User.find({_id:id}, function(err,user){
      if(err){
           console.log(err);
      }
      if(!user){
           console.log("No user");
      }else{
          user.uid = 5;
          user.save(function(err,news){
              console.log("Tried to save...");
          });
      }
      console.log("At least worked");
  })
  </code></pre>
  
  <p>Even callback is not firing. Connection successfully opens. It never invokes callback. </p>
  
  <hr>
  
  <ol>
  <li>Tried to use <code>var User = connection.model('User', schema)</code> didn't work.</li>
  </ol>