title: How to use moongoose populate?
body: <p>I have two schemas one book and other user</p>
  
  <pre><code> let mongoose = require('mongoose'),
          Schema = mongoose.Schema;
  
      let book = new Schema({
          id: {type : Schema.Types.ObjectId, default: null},
          author: {type : Array, default: []},
          userId:{type:Array,default:[]},
          issued:[{type:Schema.Types.ObjectId,ref:'users'}]
      });
  
      let user = new Schema({
         id: {type : Schema.Types.ObjectId, default: null},
         name:{type : String, default: null},
         address:{type : String, default: null}
      });
  </code></pre>
  
  <p>Issued property in book schema carries the detail for the user who issued book.</p>
  
  <blockquote>
    <p>Issued Property can be populated by using userId of book Schema and
    matching userId of book schema with an Id of user Schema.</p>
  </blockquote>
  
  <pre><code>let user =  mongoose.model("Model", userSchema, "users");
  let book = mongoose.model("book", bookSchema);
  
    book.find({}).populate('users').exec(function (err, result) {
          console.log(result);
             //Query is failing what wrong i am doing.
      });
  </code></pre>
  
  <p>I am not able to determine how can I fetch user detail.</p>
  
  <p>I want object like:</p>
  
  <pre><code>{  
      id: 8888998898989,
      author: 'Author ',
      userId:['232323232332','ssssssss'],
      issued:[
               {'id':232323232332,'name':'User1'},
               {'id':ssssssss,'name':'User2'}
              ]
    }
  </code></pre>