title: Mongo/Mongoose Invalid atomic update value error
body: <p>I am trying to write to write an update to a Mongo document using the Mongoose findOneAndUpdate function. Essentially, I have a document that has an array of another Schema in it, and when I attempt to append more of those schema type, I get the following error:</p>
  
  <pre><code>[Error: Invalid atomic update value for $__. Expected an object, received object]
  </code></pre>
  
  <p>I'm having a hard time figuring out what this error even means, much less what its source is. </p>
  
  <p>The data I'm attempting to update is as follows:</p>
  
  <pre><code>{ section_id: 51e427ac550dabbb0900000d,
  version_id: 7,
  last_editor_id: 51ca0c4b5b0669307000000e,
  changelog: 'Added modules via merge function.',
  committed: true,
  _id: 51e45c559b85903d0f00000a,
  __v: 0,
  modules: 
  [ { orderId: 0,
  type: 'test',
  tags: [],
  data: [],
  images: [],
  content: ["Some Content Here"] },
  { orderId: 1,
  type: 'test',
  tags: [],
  data: [],
  images: [],
  content: ["Some Content Here"] },
  { orderId: 2,
  type: 'test',
  tags: [],
  data: [],
  images: [],
  content: ["Some Content Here"] },
  { orderId: 3,
  type: 'test',
  tags: [],
  data: [],
  images: [],
  content: ["Some Content Here"] },
  { orderId: 4,
  type: 'test',
  tags: [],
  data: [],
  images: [],
  content: ["Some Content Here"] },
  { orderId: 5,
  type: 'test',
  tags: [],
  data: [],
  images: [],
  content: ["Some Content Here"] } ] }
  </code></pre>
  
  <p>The only difference is that when I retrieve it, there are three fewer modules, and I append some new ones to the array.</p>
  
  <p>Would love to hear any thoughts, at least as to what the error means!</p>