title: Simple-schema conditionally required field within array of objects
body: <p>I'm trying to make a field conditionally required, however, I'm having trouble accessing the field value when it's within an array of objects. <code>this.field</code> returns undefined. Basically what I want to do is check if the <code>classes.$.active</code> field is false. If it is false I want the <code>monday</code> field to set to false. If I can successfully check the value of <code>active</code> I can do the rest. </p>
  
  <pre><code>classes: {
    type: Array,
    optional: true,
  },
  'classes.$': {
    type: Object,
    optional: true,
  },
  'classes.$.monday': {
    type: Boolean,
    optional: true,
    custom() {
      console.log('test', this.field('classes.$.active').value);
    },
  }
  </code></pre>