title: How to use aurelia-validation to array item
body: <p>please help me with <code>ensureForEach</code>, I cannot find documents of that point. I have a class:</p>
  
  <pre><code>export class EnterReplacements {
      replacements: Replacement[] = null;
  
      constructor(
          private router: Router,
          private eventAggregator: EventAggregator,
          private repoCreator: RepoCreator,
          protected validation: Validation
      ) {
          this.validation = validation.on(this)
              .ensureForEach('replacements')
              .ensure('value')
              .isNotEmpty()
              .etc(); // repeat all the same stuff as in the Item validation
      }
  }
  </code></pre>
  
  <p>It's get an error in runtime:</p>
  
  <pre><code>ERROR [app-router] Error: Error instantiating EnterReplacements. Check the inner error for details.
  </code></pre>
  
  <p>This's my html:</p>
  
  <pre><code>&lt;div class="col-xs-12 col-sm-6 col-lg-4" repeat.for="replacement of replacements"&gt;
      &lt;input  class="string-input" type="text" value.bind="replacement.value" placeholder.bind="replacement.friendlyName" change.delegate="$parent.onChanged()"/&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>I need to validate all <code>value</code> property of items in <code>replacements</code> array is not empty. Please help !!!</p>