unit of work logging can be enabled:
import logging
logging.basicConfig()
logging.getLogger('sqlalchemy.orm.unitofwork').setLevel(logging.DEBUG)
when a class has a relation to itself, a dependency is created during
flush:
DEBUG:sqlalchemy.orm.unitofwork.UOWTransaction:Dependent tuples:
(Forum->Forum)
it means, we can no longer insert all Forum rows in the order they
were received - we have to check each object for dependencies on a
parent Forum object and modify the insert order in that way.
A sort is then performed among all three Forum objects, where "sort"
is a topological dependency sort. The ordering of this sort is not
deterministic. in this case, there are no dependencies at all so the
sort turns up an ordering that is mostly random. but topological
doesn't take "insert order" into account at this level.
the resulting list is then grouped into a tree-like structure, which
is intended to create "batches" of objects, where within each batch,
the uow can again fall back upon the insert ordering among those
rows. the "tree" step is total craziness which I wrote at some point
and was rewritten by ants aasma, our resident uber genius, and would
be great to get rid of.
Here's what the code has to say:
# rationale for "tree" sort as opposed to a straight
# dependency - keep non-dependent objects
# grouped together, so that insert ordering as determined
# by session.add() is maintained.
# An alternative might be to represent the "insert order"
# as part of the topological sort itself, which would
# eliminate the need for this step (but may make the original
# topological sort more expensive)
so in theory topological could be improved to take "insert order" into
account, we can lose the unnecessary tree stage which doesn't really
solve the issue completely anyway, and that would probably improve
this test.
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