title: In Objection.js, what's the benefit of setting up relationMappings?
body: <p>I'm kind of confused about what relationMappings do inside a Objection.js Model class.</p>
  
  <p>I thought once we setup the relationMapping inside a Model, we will get related data in every query. But, it turns out that I still only the Model properties itself.</p>
  
  <p>Is there anything else I should use to get related data in query?</p>