I'd like to set up a sharding strategy where shards are chosen based
on this code:
SHARD_COUNT = 5
databases = dict((i,create_engine('sqlite://shard_%s.db' % i)) for i
in xrange(SHARD_COUNT))
def shard_chooser(mapper, instance, clause=None):
return instance.primary_key_id % SHARD_COUNT
Ie, I have 5 shards, and I choose the shard based on primary_key % 5.
To do this, I need to ensure primary keys are globally unique, so I
use a function similar to the id_generator function in the shard
example here:
http://www.sqlalchemy.org/trac/browser/sqlalchemy/trunk/examples/sharding/attribute_shard.py
and use that function in my Table definitions with the 'default' named
argument.
The problem with this approach, is that the shard_chooser function is
called before the primary_key is created and set on the instance, and
therefore raises a TypeError.
return instance.user_id % SHARD_COUNT
TypeError: unsupported operand type(s) for %: 'NoneType' and 'int'
Is there any way around this issue, other than explicitly calling an
id_generator function, and setting the primary key before the instance
gets passed to the shard chooser?
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