When a student joins a classroom, they also join all course instances for that classroom which are free to join

app/views/courses/StudentCoursesView.coffee

@@ -27,8 +27,11 @@ module.exports = class StudentCoursesView extends RootView
     @supermodel.loadCollection(@classrooms, 'classrooms', { data: {memberID: me.id} })
     @courses = new CocoCollection([], { url: "/db/course", model: Course})
     @supermodel.loadCollection(@courses, 'courses')
+      
+  onLoaded: ->
     if (@classCode = utils.getQueryVariable('_cc', false)) and not me.isAnonymous()
       @joinClass()
+    super()
 
   onClickJoinClassButton: (e) ->
     return @openModalView new AuthModal() if me.isAnonymous()
@@ -57,19 +60,32 @@ module.exports = class StudentCoursesView extends RootView
     })
 
   onJoinClassroomSuccess: (data, textStatus, jqxhr) ->
-#    application.tracker?.trackEvent 'Redeemed course prepaid code', {prepaidCode: prepaidID}
+    classroom = new Classroom(data)
-#    me.fetch(cache: false).always =>
+    application.tracker?.trackEvent 'Joined classroom', {
-#      if data?.length > 0 && data[0].courseID && data[0]._id
+      classroomID: classroom.id, 
-#        courseID = data[0].courseID
+      classroomName: classroom.get('name')
-#        courseInstanceID = data[0]._id
+      ownerID: classroom.get('ownerID')
-#        route = "/courses/#{courseID}/#{courseInstanceID}"
+    }
-#        viewArgs = [{}, courseID, courseInstanceID]
+    @classrooms.add(classroom)
-#        Backbone.Mediator.publish 'router:navigate',
+    @render()
-#          route: route
-#          viewClass: 'views/courses/CourseDetailsView'
-#          viewArgs: viewArgs
-#      else
-#        @state = 'unknown_error'
-#        @stateMessage = "Database error."
-#        @render?()
     
+    classroomCourseInstances = new CocoCollection([], { url: "/db/course_instance", model: CourseInstance })
+    classroomCourseInstances.fetch({ data: {classroomID: classroom.id} })
+    @listenToOnce classroomCourseInstances, 'sync', ->
+      
+      # join any course instances in the classroom which are free to join
+      jqxhrs = []
+      for courseInstance in classroomCourseInstances.models
+        course = @courses.get(courseInstance.get('courseID'))
+        if course.get('free')
+          jqxhrs.push $.ajax({
+            method: 'POST'
+            url: _.result(courseInstance, 'url') + '/members'
+            data: { userID: me.id }
+            context: @
+            success: (data) ->
+              @courseInstances.add(data)
+          })
+      $.when(jqxhrs...).done =>
+        @state = ''
+        @render()

server/classrooms/Classroom.coffee

@@ -31,6 +31,12 @@ ClassroomSchema.pre('save', (next) ->
     next()
 )
 
+ClassroomSchema.methods.isOwner = (userID) ->
+  return userID.equals(@get('ownerID'))
+  
+ClassroomSchema.methods.isMember = (userID) ->
+  return _.any @get('members') or [], (memberID) -> userID.equals(memberID)
+
 ClassroomSchema.statics.jsonSchema = jsonSchema
 
 module.exports = Classroom = mongoose.model 'classroom', ClassroomSchema, 'classrooms'

server/courses/course_instance_handler.coffee

@@ -52,6 +52,8 @@ CourseInstanceHandler = class CourseInstanceHandler extends Handler
         ownsCourseInstance = courseInstance.get('ownerID').equals(req.user.get('_id'))
         addingSelf = userID is req.user.id
         return @sendForbiddenError(res) unless ownsCourseInstance or addingSelf
+        alreadyInCourseInstance = _.any courseInstance.get('members') or [], (memberID) -> memberID.toString() is userID
+        return @sendSuccess(res, @formatEntity(req, courseInstance)) if alreadyInCourseInstance
         Prepaid.find({ 'redeemers.userID': mongoose.Types.ObjectId(userID) }).count (err, userIsPrepaid) =>
           return @sendDatabaseError(res, err) if err
           Course.findById courseInstance.get('courseID'), (err, course) =>
@@ -193,6 +195,14 @@ CourseInstanceHandler = class CourseInstanceHandler extends Handler
       CourseInstance.find {members: mongoose.Types.ObjectId(memberID)}, (err, courseInstances) =>
         return @sendDatabaseError(res, err) if err
         return @sendSuccess(res, (@formatEntity(req, courseInstance) for courseInstance in courseInstances))
+    else if classroomID = req.query.classroomID
+      return @sendForbiddenError(res) unless req.user
+      return @sendBadInputError(res, 'Bad memberID') unless utils.isID classroomID
+      Classroom.findById classroomID, (err, classroom) =>
+        return @sendForbiddenError(res) unless classroom.isMember(req.user._id) or classroom.isOwner(req.user._id)
+        CourseInstance.find {classroomID: mongoose.Types.ObjectId(classroomID)}, (err, courseInstances) =>
+          return @sendDatabaseError(res, err) if err
+          return @sendSuccess(res, (@formatEntity(req, courseInstance) for courseInstance in courseInstances))
     else
       super(arguments...)