title: Many to Many relationship in Firebase
body: <p>I have a Firebase database. I have Companies and Contractors. A Contractor can work for more than one Company and a Company can have multiple Contractors. This is a straightforward many to many relationship. I want to be able to answer the questions about Companies and Contractors: </p>
  
  <ol>
  <li>Given a Company, who are the current Contractors. </li>
  <li>Given a Contractor what Companies are they working for. </li>
  </ol>
  
  <p>What are the alternatives for structuring the data within firebase?</p>