How to set Pre selected data in dynamic select option in IONIC

I face a strange problem where I need to show Pre selected data(which
also come from server) in select option. The problem that I need to show
select option based on key and value option.


<div class="list list-inset">
        <span class="input-label">Permisstion</span>
        <select ng-model="permisstion" >
         <option ng-repeat="(key, value) in Roles" id="{{key}}" value="{{value}}">{{value}}</option>


"Roles": {
    "21": "Admin",
    "22": "Main Manager",
    "23": "Branch Manager",
    "26": "Side Manager"

I don’t no how to show Pre selected data in select option and I try a lot but till now I don’t get success.
Please help.