<form ng-submit="createSubCategory(formData)">
    <div ng-repeat="sub_category in event_sub_categories">
        <ion-checkbox ng-model="formData.sub_category" ng-true-value="'{{sub_category}}'">
          {{sub_category}}</ion-checkbox>
    </div>
    <button type="submit" class="button button-block button-positive">Continue</button>
</form>

<ion-checkbox ng-repeat="sub_category in event_sub_categories" ng-model="formData[sub_category]">