Custom validation in input field for unique value

Question

I've to use AngularJS 1.7 for a project where I have a form and have to validate an input field to be required, to have a particular pattern and to have unique values. For this I'm doing this :

  input#item-name(
    name="name",
    type="text",
    required,
    ng-pattern="$ctrl.pattern.id",
    ng-model="$ctrl.item.name",
  )

  div(ng-messages="addItemForm.name.$error")
    div(ng-messages-include="error-messages")

I've covered the required and the pattern one. But not able to include the unique validation.

I have an array with all the existing values :

this.existingItems = ['A','B','C];

I've tried something like this but did not work:

 input#item-name(
    name="name",
    type="text",
    required,
    ng-pattern="$ctrl.pattern.id",
    ng-model="$ctrl.item.name",
    ng-unique="$ctrl.existingItems",
  )

  div(ng-messages="addItemForm.name.$error")
    div(ng-messages-include="error-messages")

I've used this part of code inside a component and pointed "this" to "$ctrl".