title: Cast to string failed for array
body: <p>So, I'm trying to create a document from the elements of an array but it constantly throws <code>Document Validation Error</code>.</p>
  
  <p>The schema is like this:</p>
  
  <pre><code>userId: String,
  terms: [{
    value: String,
    system: String
  }],
  group: String
  </code></pre>
  
  <p>I have an array of <code>values</code> for <code>terms</code>, <code>['hello', 'hey', 'hi']</code></p>
  
  <p>Now, I want to be able to <code>create</code> a document using the <code>values</code> from the array like this:</p>
  
  <pre><code>{
    userId: 'foo',
    terms: [{value: 'hello'}, {value: 'hey'}, {value: 'hi'}],
    group: 'bar'
  }
  </code></pre>
  
  <p>The way I'm trying to do it but failing:</p>
  
  <pre><code>let arr = ['hello', 'hey', 'hi'];
  Document.create({userId: 'somevalue', terms: {value: {$in: arr}}}) // validation error
  </code></pre>