Problem binding 2 selects with angularjs and json data

I have a controller:

app.controller("AllController", function($scope, $http){
    $scope.listSkills = function() {
        $http.get("/showSkills").success(function(skills_from_database){
            $scope.skills = skills_from_database.list;
        })
    }
});

This is the html with both selects, but only the first one works:

<select name="skill_category" id="skill_category" form="AddSkillForm" onclick="showNewCatInput()">
  <option value="newCat">New category</option>
  <option ng-repeat="category in skills" value="{{category.categoryName}}">{{category.categoryName}}</option>
</select>


<select name="skill_subcategory" id="skill_subcategory" form="AddSkillForm" onclick="showNewSubCatInput()">
  <option value="newSubCat">New subcategory</option>
  <option ng-repeat="subcategory in category.subcategories" value="{{subcategory.subcategoryName}}">{{subcategory.subcategoryName}}</option>
</select>

So I know that to get all the subcategories I need the category selected on the first select, but I don't know how to do it. As I read, I think that I need to use ng-model, but even if I could connect the 2 selects, it would still not work the second one.

That is because I can't access the subcategory with this:

ng-repeat="subcategory in category.subcategories"

Simply because "category" doesn't exist there, so where it comes? from this other code that works:

<div ng-repeat="category in skills">
  <h1>{{category.categoryName}}</h1>
  <div ng-repeat="subcategory in category.subcategories">
    <h2>{{subcategory.subcategoryName}}</h2>
  </div>
</div>

As you can see, I know I have to access to "category in skills" first, but what I need is not all categories in skills, I need just the selected category.

So to summarize the problems:

Don't know how to use ng-model to bind 2 selects and don't know how to access the subcategories from the selected category

The json data and angular scope can be found in this other post that I made yesterday, there you can also see the html/angular code that works to get the category and subcategory. Stackoverflow post