how do I select ng-option select angularjs?

Question

When edit the product, I want this ng-option to be selected automatically. How do I do that?

<div ng-if="field.name=='ProductId'" class="input-group inputFill">
    <select ng-disabled="defaultSaveButtons[$index]"
            class="form-control inputFill2"
            ng-model="field.value"
            ng-options="product.id as product.name for product in dropdown.productOptions  | orderBy:'name'
                track by product.id ">
    </select>
</div>