title: How can I add to mongoose nested document, not overwrite it?
body: <p><strong>This is my mongoose model:</strong></p>
  
  <pre><code>...
  images: new Schema({
   poster: new Schema({
     small: String,
     medium: String,
     big: String
   }, { _id: false }),
   backdrop: new Schema({
     small: String,
     medium: String,
     big: String
   }, { _id: false })
  }, { _id: false }),
  ...
  </code></pre>
  
  <p><strong>This is what I'm doing:</strong><br>
  I'm getting poster, and update <code>images</code> like this:  </p>
  
  <pre><code>Movie.findOneAndUpdate({
  id: {
      imdb: movie.id.imdb
  }
  }, {
      images: {
          backdrop: {
              small: backdrops.small,
              medium: backdrops.medium,
              big: backdrops.big
          }
      }
  }
  </code></pre>
  
  <p>It works fine. then I get Backdrops and I update <code>images</code> like this:</p>
  
  <pre><code>Movie.findOneAndUpdate({
      id: {
          imdb: movie.id.imdb
      }
  }, {
      images: {
          backdrop: {
              small: backdrops.small,
              medium: backdrops.medium,
              big: backdrops.big
          }
      }
  }
  </code></pre>
  
  <p><strong>problem is</strong><br>
  It overwrites the whole <code>images</code> document, not adding <code>backdrops</code> along <code>poster</code>. How can I achieve that?</p>