fix for empty create group dialog

fixes CNVS-9490

trigger render event on the collection so that we don't accidentally
re-render the entire IndexView

test plan
=========
with draft state enabled:
- navigate to the assignments index page and edit an assignment group
  from the gear menu
- click the button to add a new assignment group
- verify that the modal has a form in it and that a new assignment group
  can be added

Change-Id: I877dc6daddfcb4a2b6f67dfd862dc2cb5328bcc3
Reviewed-on: https://gerrit.instructure.com/26420
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Matthew Irish <mirish@instructure.com>

app/coffeescripts/views/assignments/AssignmentGroupListView.coffee

@@ -60,6 +60,7 @@ define [
     attachCollection: ->
       super
       @itemViewOptions = course: @course
+      @collection.on 'render', @render
       @collection.on 'add', @render
       @collection.on 'remove', @render
       @collection.on 'change:groupWeights', @render

app/coffeescripts/views/assignments/CreateGroupView.coffee

@@ -41,13 +41,16 @@ define [
 
     onSaveSuccess: ->
       super
-      if @assignmentGroup # meaning we are editing
-        @model.collection.view.render()
+      # meaning we are editing
+      if @assignmentGroup
+        # trigger instead of calling render directly
+        @model.collection.trigger 'render', @model.collection
       else
         @assignmentGroups.add(@model)
         @model = new AssignmentGroup(assignments: [])
 
-      @render()
+        @render()
+
 
     getFormData: ->
       data = super

spec/coffeescripts/views/assignments/CreateGroupViewSpec.coffee

@@ -17,10 +17,12 @@ define [
   assignmentGroups = ->
     @groups = new AssignmentGroupCollection([group(), group()])
 
-  createView = ->
-    view = new CreateGroupView
+  createView = (hasAssignmentGroup=true)->
+    args =
       assignmentGroups: assignmentGroups()
-      assignmentGroup: @groups.first()
+      assignmentGroup: @groups.first() if hasAssignmentGroup
+
+    view = new CreateGroupView(args)
 
   module 'CreateGroupView'
 
@@ -59,3 +61,18 @@ define [
     errors = view.validateFormData(data)
     ok errors
     equal _.keys(errors).length, 1
+
+
+  test 'it should trigger a render event on save success when editing', ->
+    triggerSpy = sinon.spy(AssignmentGroupCollection::, 'trigger')
+    view = createView()
+    view.onSaveSuccess()
+    ok triggerSpy.calledWith 'render'
+    triggerSpy.restore()
+
+  test 'it should call render on save success if adding an assignmentGroup', ->
+    view = createView(false)
+    renderStub = sinon.stub(view, 'render')
+    calls = renderStub.callCount
+    view.onSaveSuccess()
+    equal renderStub.callCount, calls + 1