title: Grails 3.2 - List field names in domain class that are not nullable
body: <p>How can I get a list of field names for a domain class that are not nullable?</p>
  
  <p>For instance, in the following domain:</p>
  
  <pre><code>class MyDomain {
      String field1
      String field2
      String field3
      String field4
  
      static constraints = {
          field2 nullable: true
          field3 nullable: true
      }
  }
  </code></pre>
  
  <p>How can I get back the list <code>['field1','field4']</code> in a controller?</p>
  
  <p>I'm validating rows in a CSV, and some of the row information is different from what is stored in the domain, so it would be preferable to get a List of String names rather than bind to a command object with exclusions.</p>