title: TypeError: Cannot read property 'push' of undefined Mongoose
body: <p>I am getting the below error which doing a app in Node.js using express.
  I am using Mongoose for my DB operations below i have detailed my design</p>
  
  <p>Party.js</p>
  
  <pre><code>var mongoose = require("mongoose");
  
  var partySchema = new mongoose.Schema({
      partyName: String,
      songs: [
           {
                   type: mongoose.Schema.Types.ObjectId,
                  ref: "Song"
           }
     ]
  });
  
  module.exports = mongoose.model("Party", partySchema);
  </code></pre>
  
  <p>Song.js</p>
  
  <pre><code>var mongoose = require("mongoose");
  
  var songsSchema = new mongoose.Schema({
      videoId: String,
      videoName: String
  });
  
  module.exports = mongoose.model("Song", songsSchema);
  </code></pre>
  
  <p>My App.js</p>
  
  <pre><code>app.post("/search/addSong", function(req, res) {
      Party.find({partyName:"hello"},function(err,party){
          if(err){
              console.log("Failed in find");
          } else {
              console.log(party);
              // console.log(typeof(req.body.videoId));
              var videoId = req.body.videoId;
  
              var newSong = [ {
                  videoId:req.body.videoId,
                  videoName:req.body.videoName
              }
              ];
              Song.create(newSong, function(err, createdSong){
         if(err){
             console.log("Error creating a new party");
         } else  {
             console.log(createdSong);
             party.songs.push(createdSong);// ERROR ON THIS LINE
             party.save();
             res.redirect("/search");
         }
      });
          }
      });
      res.render("addSong");
  });
  </code></pre>
  
  <p>I am able to create collection objects of the Party and Song individually, when I add the song to the party queue, I get the following error:</p>
  
  <pre><code>TypeError: Cannot read property 'push' of undefined
  </code></pre>
  
  <p>Can anyone please let me know what I a missing in here..!!</p>
  
  <p>Thanks in advance.!!</p>