add caching for StudentModule

--HG--
branch : dormsbee_performance

courseware/models.py

@@ -8,12 +8,18 @@ file and check it in at the same time as your model changes. To do that,
 2. ./manage.py schemamigration courseware --auto description_of_your_change
 3. Add the migration file created in mitx/courseware/migrations/
 
+
+ASSUMPTIONS: modules have unique IDs, even across different module_types
+
 """
 from django.db import models
+from django.core.cache import cache
 from django.contrib.auth.models import User
 
 from cache_toolbox import cache_model, cache_relation
 
+CACHE_TIMEOUT = 60 * 60 * 4 # Set the cache timeout to be four hours
+
 class StudentModule(models.Model):
     # For a homework problem, contains a JSON
     # object consisting of state
@@ -51,5 +57,28 @@ class StudentModule(models.Model):
     def __unicode__(self):
         return self.module_type+'/'+self.student.username+"/"+self.module_id+'/'+str(self.state)[:20]
 
+    @classmethod
+    def get_from_cache(cls, student, module_ids):
+        k = cls.key_for(student, module_ids)
+        student_modules = k.get(k)
+        if student_modules is None:
+            student_modules = StudentModule.objects.filter(student=student, module_id__in=module_ids)
+            # It's possible it really doesn't exist...
+            if student_modules is not None:
+                k.set(k, student_modules, CACHE_TIMEOUT)
+
+        return student_modules
+
+
+    @classmethod
+    def clear_cache_for(cls, student, module_ids):
+        k = cls.key_for(student_id, module_ids)
+        cache.delete(k)
+
+    @classmethod
+    def key_for(cls, student, module_ids):
+        module_ids_hash = md5(",".join(sorted(modules_ids))).hexdigest()
+        return "StudentModule-student_id:{0};module_ids_hash:{1}".format(student.id, module_ids_hash)
+
 
 cache_model(StudentModule)