title: TypeError: Cannot read property '_id' of undefined
body: <p>I am getting error "TypeError: Cannot read property '_id' of undefined" on a simple post request to save a document to the collection called <code>books</code>,
  My payload looks like this:</p>
  
  <pre><code>{
      "name": "practical view",
      "author": "DN",
      "location": "room 50"
  }
  </code></pre>
  
  <p>And I am just doing <code>db.books.save()</code> in my route in express.
  Since I am not passing the id, this should ideally work, but not in this case.</p>
  
  <p>Below is the full error dump I am getting on my node server:</p>
  
  <pre><code>C:\NodeProjects\ExpressTutorial\Library\node_modules\mongojs\node_modules\mongodb\lib\mongodb\mongo_client.js:411
            throw err
                  ^
  TypeError: Cannot read property '_id' of undefined
      at Collection.save (C:\NodeProjects\ExpressTutorial\Library\node_modules\mongojs\node_modules\mongodb\lib\mongodb\collection.js:393:15)
      at C:\NodeProjects\ExpressTutorial\Library\node_modules\mongojs\index.js:203:65
      at apply (C:\NodeProjects\ExpressTutorial\Library\node_modules\mongojs\node_modules\thunky\index.js:16:28)
      at C:\NodeProjects\ExpressTutorial\Library\node_modules\mongojs\node_modules\thunky\index.js:20:25
      at Db.collection (C:\NodeProjects\ExpressTutorial\Library\node_modules\mongojs\node_modules\mongodb\lib\mongodb\db.js:488:44)
      at C:\NodeProjects\ExpressTutorial\Library\node_modules\mongojs\index.js:268:7  
      at apply (C:\NodeProjects\ExpressTutorial\Library\node_modules\mongojs\node_modules\thunky\index.js:16:28)
      at C:\NodeProjects\ExpressTutorial\Library\node_modules\mongojs\node_modules\thunky\index.js:20:25
      at C:\NodeProjects\ExpressTutorial\Library\node_modules\mongojs\index.js:300:4
      at C:\NodeProjects\ExpressTutorial\Library\node_modules\mongojs\node_modules\mongodb\lib\mongodb\mongo_client.js:408:11
  31 Aug 00:14:30 - [nodemon] app crashed - waiting for file changes before starting...
  </code></pre>