old_contents: from springfield import Entity
  
  
  def entity_from_mongo(kls, values):
      """
      Construct an Entity of type `kls` from a Mongo document.
      """
      entity = kls()
      if hasattr(values, '__values__'):
          for key, val in values.__values__.items():
              if key == '_id':
                  key = 'id'
  
              if key in entity.__fields__:
                  entity.set(key, val)
      else:
          for key, val in values.items():
              if key == '_id':
                  key = 'id'
  
              if key in entity.__fields__:
                  entity.set(key, val)
      return entity
  
  
  def entity_to_mongo(entity):
      """
      Convert an Entity type into a structure able to be stored in Mongo.
      """
      data = {}
      for key, val in entity.__values__.iteritems():
          field = entity.__fields__[key]
  
          if isinstance(val, Entity):
              val = entity_to_mongo(val)
          else:
              val = field.flatten(val)
  
          if key == 'id':
              key = '_id'
  
          data[key] = val
  
      return data
  
new_contents: from springfield import Entity
  
  
  def entity_from_mongo(cls, values):
      """
      Construct an Entity of type `cls` from a Mongo document.
      """
      entity = cls()
      if hasattr(values, '__values__'):
          for key, val in values.__values__.items():
              if key == '_id':
                  key = 'id'
  
              if key in entity.__fields__:
                  entity.set(key, val)
      else:
          for key, val in values.items():
              if key == '_id':
                  key = 'id'
  
              if key in entity.__fields__:
                  entity.set(key, val)
      return entity
  
  
  def entity_to_mongo(entity):
      """
      Convert an Entity type into a structure able to be stored in Mongo.
      """
      data = {}
      for key, val in entity.__values__.iteritems():
          field = entity.__fields__[key]
  
          if isinstance(val, Entity):
              val = entity_to_mongo(val)
          else:
              val = field.flatten(val)
  
          if key == 'id':
              key = '_id'
  
          data[key] = val
  
      return data
  
subject: Use a more standard variable name for the class argument.