modifications to Backbone textbook structure

Textbooks now have an empty chapterset by default, instead of one with one empty
chapter. The TextbookEdit view dynamically adds an empty chapter if the chapterset
is empty; the close button removes all empty chapters from the chapterset.

cms/static/js/models/textbook.js

@@ -2,7 +2,7 @@ CMS.Models.Textbook = Backbone.AssociatedModel.extend({
     defaults: function() {
         return {
             name: "",
-            chapters: new CMS.Collections.ChapterSet([{}]),
+            chapters: new CMS.Collections.ChapterSet(),
             showChapters: false
         };
     },

cms/static/js/views/textbook.js

@@ -78,6 +78,10 @@ CMS.Views.TextbookEdit = Backbone.View.extend({
             name: this.model.escape('name'),
             errors: null
         }));
+        var chapters = this.model.get('chapters');
+        if (chapters.length === 0) {
+            chapters.add([{}]);
+        }
         this.addAll();
         return this;
     },
@@ -109,6 +113,7 @@ CMS.Views.TextbookEdit = Backbone.View.extend({
         var that = this;
         _.each(this.$("li"), function(li, i) {
             var chapter = that.model.get('chapters').at(i);
+            if(!chapter) { return; }
             chapter.set({
                 "name": $(".chapter-name", li).val(),
                 "asset_path": $(".chapter-asset-path", li).val()
@@ -145,6 +150,11 @@ CMS.Views.TextbookEdit = Backbone.View.extend({
         // if the textbook has no content, remove it from the collection
         if(this.model.isEmpty()) {
             textbooks.remove(this.model);
+        } else {
+            // remove empty chapters from textbook
+            var chapters = this.model.get("chapters");
+            chapters.remove(chapters.filter(
+                function(chapter) { return chapter.isEmpty(); }));
         }
         textbooks.trigger('render');
         return this;