title: Mongoose: Remove object from array based on ID (Cast Error)
body: <p>I have a model that looks like this:</p>
  
  <pre><code>mongoose.Schema({
    username: String,
    posts: [{ type: Schema.Types.ObjectId, ref: 'Post' }]
  });
  </code></pre>
  
  <p>I have an endpoint that I want to pass an ObjectID:</p>
  
  <pre><code>app.delete('/post', function(req, res) {
    User.findOne({ _id: req.user._id}, function(err, result) {
      result.pull({ _id: req.body.post_id });
    });
  });
  </code></pre>
  
  <p>Feels like it should work, but I'm getting this error:</p>
  
  <p><code>CastError: Cast to ObjectId failed for value "[object Object]"</code></p>
  
  <p>What am I doing wrong?</p>