6. Cascading

This is the one toughest section to write.

MongoDB, as far as everyone knows, does not support cascading operations like the way MySQL and other vendors do with cascading deletion. Nevertheless, Tori supports cascading through the database abstraction layer (DBAL).

Warning

Cascading persistence and removal via DBAL has high probability of degrading performance with large dataset as in order to calculate a dependency graph, all data must be loaded into the memory space of the computing process. This introduces a spike in memory and network usage.

This feature is introduced for convenience sake but should be used sparingly or accounted for potential performance degration.

Here is a sample scenario.

Suppose I have two types of objects: a sport team and a player. When a team is updated, removed or refreshed, the associated player should be treated the same way as the team. Here is a sample code.

from passerine.db.entity import entity
from passerine.db.mapper import CascadingType as c

@entity
class Player(object):
    pass # omit the usual setup decribed in the basic usage.

@link(
    target=Player,
    mapped_by='player',
    cascading=[c.PERSIST, c.DELETE, c.REFRESH]
)
@entity
class Team(object):
    pass # omit the usual setup decribed in the basic usage.

Now, whatever operation is used on a Team entity, associated Player entites are subject to the same operation.