[Vuejs]-Forcing v-validate to update rules (with Vue)

Vue.js is unable to track updates on nested reference types.

Try:

let fields = [...this.fields]
fields[0].rules = cities
this.fields = fields

Use Vue.set to track changes : https://v2.vuejs.org/v2/guide/reactivity.html

Vue.set(this.fields[0], 'rules', cities);