Unique value in CollectionField form

Sometimes you want to prevent duplication of values within a collection. For example, let's say that you have a collection of items and want every item to have a unique firstName. You can achieve this by using the parent keyword in a validator with the $elemMatch operator, e.g.

{
  component: "CollectionField",
  name: "contacts",
  label: "Contacts",

  formFactory: {
    component: "Factory",

    product: {
      component: "Form",
      fields: [
        {
          // Unique id per item so that duplication is only considered
          // when the id is different
          name: "id",
          component: "UUIDField",
          label: "Id",
          hidden: true
        },
        {
          name: "firstName",
          component: "TextField",
          label: "First Name"
        },
        {
          name: "birthday",
          component: "DateField",
          label: "Birthday"
        }
      ],

      validators: [
        {
          where: {
            "parent.value": {
              $elemMatch: {
                $and: [
                  {
                    // Only consider duplication when the id doesn't match
                    id: {
                      $ne: "{{fields.id.value}}"
                    }
                  }
                  { name: "{{fields.firstName.value}}" }
                ]
              }
            }
          },
          error: {
            field: "firstName",
            error: "must be unique"
          }
        }
      ]
    }
  }

}