title: Remove sub-document from Mongo with mongoose
body: <p>I am trying to remove an item from a collection that is stored in a mongoose document. My document looks like this: </p>
  
  <pre><code>{
    "__v": 3,
    "_id": "5221040475f174d59a000005",
    "items": [
      {
        "sku": 1321654654613213,
        "name": "goldfish",
        "quantity": 12,
        "_id": "52224ed5bd9d340000000003"
      },
      {
        "sku": 12,
        "name": "goldfish",
        "quantity": 13,
        "_id": "52225dcbf2f1e40000000003"
      },
      {
        "sku": 1299,
        "name": "goldfish",
        "quantity": 13,
        "_id": "522260b6f2f1e40000000004"
      }
    ]
  }
  </code></pre>
  
  <p>I want to remove the goldfish with the sku of 12. I am doing the following: </p>
  
  <pre><code>var inventory = res.locals.content;
  inventory.items.remove( {sku: req.params.itemSku}, function (err, item) {
    if (err) {
      console.log('error occurred', err);
      res.send('error');
    }
    else {
      res.send('Item found and deleted');
      return; 
    }
  });
  </code></pre>
  
  <p>when I do this, I get the error "TypeError: Cannot read property 'equals' of undefined".  I don't understand why.</p>