title: Laravel ORM get all books from a list of authors
body: <p>I have a model for book and one for author.
  The book model has a belongsTo author relationship, and author hasMany books.</p>
  
  <p>I have a <code>sql</code> query that gives me some authors and now i want all those authors books together in one list.</p>
  
  <p>I thought I could write:</p>
  
  <pre><code>$authors-&gt;books
  </code></pre>
  
  <p>But this gives me the error</p>
  
  <pre><code>Undefined property: Illuminate\Database\Eloquent\Collection::$books 
  </code></pre>
  
  <p>I know I can call <code>$author-&gt;books</code> if it is a single author I have, but what if I like in this case a collection of authors, how can I get all their books together?</p>