When using the value attribute with an angular variable be sure to include the curly braces {{}}

Do this:

<option ng-repeat="category in categories" value="{{category.id}}"> {{category.name}} </option>

Dont Do This:

<option ng-repeat="category in categories" value="category.id"> {{category.name}} </option>

This happens because the value="" attribute is not an angular attribute therefore wont automatically reference angular.