Resolved decompression errors during tests

Note that this change will require deleting all CourseStructure data. This data should be regenerated using the generate_course_structure management command.

XCOM-513

common/djangoapps/util/models.py

@@ -4,12 +4,10 @@ import gzip
 import logging
 
 from django.db import models
-from django.db.models.signals import post_init
 from django.utils.text import compress_string
 
 from config_models.models import ConfigurationModel
 
-
 logger = logging.getLogger(__name__)  # pylint: disable=invalid-name
 
 
@@ -25,27 +23,31 @@ class RateLimitConfiguration(ConfigurationModel):
     pass
 
 
-def uncompress_string(s):
+def decompress_string(value):
     """
     Helper function to reverse CompressedTextField.get_prep_value.
     """
 
     try:
-        val = s.encode('utf').decode('base64')
+        val = value.encode('utf').decode('base64')
         zbuf = cStringIO.StringIO(val)
         zfile = gzip.GzipFile(fileobj=zbuf)
         ret = zfile.read()
         zfile.close()
     except Exception as e:
         logger.error('String decompression failed. There may be corrupted data in the database: %s', e)
-        ret = s
+        ret = value
     return ret
 
 
 class CompressedTextField(models.TextField):
-    """transparently compress data before hitting the db and uncompress after fetching"""
+    """ TextField that transparently compresses data when saving to the database, and decompresses the data
+    when retrieving it from the database. """
+
+    __metaclass__ = models.SubfieldBase
 
     def get_prep_value(self, value):
+        """ Compress the text data. """
         if value is not None:
             if isinstance(value, unicode):
                 value = value.encode('utf8')
@@ -53,28 +55,12 @@ class CompressedTextField(models.TextField):
             value = value.encode('base64').decode('utf8')
         return value
 
-    def post_init(self, instance=None, **kwargs):   # pylint: disable=unused-argument
-        value = self._get_val_from_obj(instance)
-        if value:
-            setattr(instance, self.attname, value)
-
-    def contribute_to_class(self, cls, name):
-        super(CompressedTextField, self).contribute_to_class(cls, name)
-        post_init.connect(self.post_init, sender=cls)
-
-    def _get_val_from_obj(self, obj):
-        if obj:
-            value = uncompress_string(getattr(obj, self.attname))
-            if value is not None:
-                try:
-                    value = value.decode('utf8')
-                except UnicodeDecodeError:
-                    pass
-                return value
-            else:
-                return self.get_default()
-        else:
-            return self.get_default()
+    def to_python(self, value):
+        """ Decompresses the value from the database. """
+        if isinstance(value, unicode):
+            value = decompress_string(value)
+
+        return value
 
     def south_field_triple(self):
         """Returns a suitable description of this field for South."""