title: How to create rails association
body: <p>How to create rails associations like this:
  <img src="https://i.stack.imgur.com/CS1Ha.png" alt="enter image description here"></p>
  
  <p>and example will show like this:
  <img src="https://i.stack.imgur.com/qeGG3.png" alt="enter image description here"></p>