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.