title: An issue while creating multiple unique columns with GORM
body: <p>I have this Grails domain class:</p>
  
  <pre><code>class MyClass {
  
      static auditable = true;
  
      String description;
      Boolean isActive=true;
      Date deletedAt;
  
  
      static constraints = {
          description size: 1..250, blank: false, unique:['deletedAt', 'isActive'], index:'myclass_idx'
          deletedAt nullable: true, index:'myclass_idx'
          isActive index:'myclass_idx'
      }
  }
  </code></pre>
  
  <p>What I expect it to do is to create a constraint which won't allow duplicate records like these:</p>
  
  <pre>id| description | is_active | deleted_at  
  1 | desc1       | true      | (null)
  2 | desc1       | true      | (null)</pre>
  
  <p>but this stil happens. What am I missing here?</p>
  
  <p><strong>UPD:</strong> figured out that this happens only when there are null values compared to each other. If <code>deleted_at</code> contains two equal date values, the constraint works. </p>
  
  <p>Is there a workaround for that?</p>