Sorting and Ordering¶
Stdnet can sort instances of a model in three different ways:
- Explicit sorting using the Query.sort_by method.
- Implicit sorting via the Metaclass.ordering attribute of the model metaclass.
- Incremental sorting, a variant of the implicit sorting for models which require to keep track how many times instances with the same id are created.
Explicit Sorting¶
Sorting is usually achieved by using the Query.sort_by() method with a field name as parameter. Lets consider the following model:
class SportActivity(odm.StdNet):
person = odm.SymbolField()
activity = odm.SymbolField()
dt = odm.DateTimeField()
models = odm.Router()
models.register(SportActivity)
To obtained a sorted query on dates for a given person:
qs = models.sportactivity.filter(person='pippo').sort_by('-dt')
The negative sign in front of dt indicates descending order.
Implicit Sorting¶
Implicit sorting is achieved by setting the Metaclass.ordering attribute in the model Meta class. Let’s consider the following Log model example:
class Log(odm.StdModel):
'''A database log entry'''
timestamp = odm.DateTimeField(default=datetime.now)
level = odm.SymbolField()
msg = odm.CharField()
source = odm.CharField()
host = odm.CharField()
user = odm.SymbolField(required=False)
client = odm.CharField()
class Meta:
ordering = '-timestamp'
models.register(Log)
It makes lots of sense to have the log entries always sorted in a descending order with respect to the timestamp field. This solution always returns Query in this order, without the need to call sort_by method.
Note
Implicit sorting is a much faster solution than explicit sorting, since there is no sorting step involved (which is a N log(N) time complexity algorithm). Instead, the order is maintained by using sorted sets as indices rather than sets.
