Angularjs pass ng-repeat checkbox value undefined

This question asked already but that not solve my issue.

Am trying to pass checkbox value to controller but i got undefined message. Am beginner in Angularjs.

This is my HTML code:

<form class="form-horizontal" name="addForm" novalidate ng-submit="addData(addForm)">
    <div class="form-group">
        <div class="col-sm-3" ng-repeat="directories in formData.getMainDirectories">
            <input type="checkbox" ng-model="directories.main_directories_id.selected" value=""><span>  </span>
        </div>
    </div>
    <button type="submit" class="btn btn-primary btn-md" button-spinner="loading" ng-disabled="loading">Submit</button>
</form>

My .js code:

$scope.addData = function(form)
{
    $scope.errors = [];
    alert(form.main_directories_id); // alert here
    if(form.$valid)
    {
        $rootScope.loading = true;

        webServices.upload('create', $scope.formData).then(function(getData) {
            $rootScope.loading = false;
            if (getData.status == 200) {
                $sessionStorage.successmessage = getData.data.message;
                localStorage.directory = '';
                $scope.goback();
            } else if (getData.status == 401) {
                $scope.errors = utility.getError(getData.data.message);
                $scope.showerrors();
            } else {
                $rootScope.$emit("showerror", getData);
            }
        });
    }
}

$scope.getMainDirectories = function()
    {
        webServices.get('getMainDirectories').then(function(getData) 
        {
            $rootScope.loading = false;
            if (getData.status == 200) 
            {
                $scope.formData.getMainDirectories = getData.data;
            } else {
                $rootScope.$emit("showerror", getData);
            }
            //console.log(getData.data);
        });
    }

Image: