title: Mongo group and push: pushing all fields
body: <p>Is there an easy way to "$push" all fields of a document?
  For example:</p>
  
  <p>Say I have a Mongo collection of books:</p>
  
  <pre><code>{author: "tolstoy", title:"war &amp; peace", price:100, pages:800}
  {author: "tolstoy", title:"Ivan Ilyich", price:50,  pages:100}
  </code></pre>
  
  <p>I'd like to group them by author - for each author, list his <em>entire</em> book objects:</p>
  
  <pre><code>{ author: "tolstoy",
    books: [
       {author: "tolstoy", title:"war &amp; peace", price:100, pages:800}
       {author: "tolstoy", title:"Ivan Ilyich", price:50,  pages:100}
    ]
  }
  </code></pre>
  
  <p>I can achieve this by explicitly pushing all fields:</p>
  
  <pre><code>{$group: {
       _id: "$author",
       books:{$push: {author:"$author", title:"$title", price:"$price", pages:"$pages"}},
  }}
  </code></pre>
  
  <p>But is there any shortcut, something in the lines of:</p>
  
  <pre><code>// Fictional syntax...
  {$group: {
      _id: "$author",
      books:{$push: "$.*"},
  }}
  </code></pre>