title: Issue with grails fixtures and domain class validation
body: <p>I am new to fixtures in grails and have bumped into this problem in my code.  I am trying to use fixtures to build an instance of the following domain objects:</p>
  
  <pre><code>class Tree {
      static constraints = {
          relationships(
              validator: { relationships, tree-&gt;
                  relationships &amp;&amp; 
                  !(relationships.isEmpty()) &amp;&amp; 
                  relationships.every { it.validate() 
              }
          )
      }
      static hasMany = [
          relationships: Branch
      ]
  }
  
  class Branch {
      Tree tree
      static constraints = {}
  }
  </code></pre>
  
  <p>I have tried these implementations, all of which result in a validation error:</p>
  
  <pre><code>fixture {
      Build {
          //oak(Tree)
          //oakBranch(Branch){ tree = oak }
  
          //oakBranch(Branch){ tree = ref("oak") }
          //oak(Tree)
  
          //oak(Tree){ relationships = [ ref("oakBranch") ] }
          //oakBranch(Branch)
      }
  }
  </code></pre>
  
  <p>All implementations return the same error: "on field of 'relationships': rejected value [null]".  Any help will be much appreciated, thanks.</p>