How to get and filter data

Everything works fine in my main function, even the filter. but the problem is that whenever I click on a filter type and switch to a different filter type, it gets filtered from the existing filtered data instead of fetching new data from the server and filter ...

if i add a fetch call on my filter function, it fetch all data without filtering ... how can i fix this.?

my code:

    $(document).ready(function(){
    var school = {};

    school.model = Backbone.Model.extend({
        defaults:{
            name:'no name',
            age:'no age'
        }
    });

    school.collect = Backbone.Collection.extend({
        model:school.model,
        url:'js/school.json',
        initialize:function(){
            this.sortVar = "name"
        }
    });

    school.view = Backbone.View.extend({
        tagName:'div',
        className:'member',
        template:$('#newTemp').html(),
        render:function(){
            var temp = _.template(this.template);
            this.$el.html(temp(this.model.toJSON()));
            return this;
        }
    });

    school.views = Backbone.View.extend({
        el:$('#content'),
        events:{
            'click #newData' : 'newArrival',
            'click #showName' : 'showByName',
            'click #showAge' : 'showByAge'
        },
        initialize:function(){
            _.bindAll(this);
            this.collection = new school.collect;
            this.collection.bind('reset', this.render);
            this.collection.fetch();
            this.childViews = [];
        },
        newArrival:function(){
            this.collection.fetch(); //it works fine, i use this for update
        },
        showByName:function(){
// this.collection.fetch(); //but i can't do this, it removes filtered data..
            this.sortVar = 'name';
            var filterType = _.filter(this.collection.models, function(item){
                return item.get('name') != '';
            })
            this.collection.reset(filterType); //resets without fetching (filtering  from existing datas..)
        },
        showByAge:function(){
// this.collection.fetch(); //but i can't do this, it removes filtered data..
            this.sortVar = 'age';
            var filterType = _.filter(this.collection.models,function(item){
                return item.get('age') != 0;
            })
            this.collection.reset(filterType); //resets without fetching (filtering from existing datas..)
        },
        render:function(){

            _.each(this.childViews, function(old){
                old.remove();    
            });

            this.childViews = [];

            var that = this;
            _.each(this.collection.models, function(item){
                that.renderItem(item);    
            });
        },
        renderItem:function(item){
            var newItem = new school.view({model:item});
            this.$el.append(newItem.render().el);
            this.childViews.push(newItem);
        }
    });

    var newSchool = new school.views;
    });

      

        
        
        
      

    

thanks in advance, also i have 2 more add methods that sort name and age while showing data.