On May 15, 2014, at 12:28 AM, Randy Syring <ra...@thesyrings.us> wrote:
> I am trying to get SQLAlchemy to let my database's foreign keys "on delete
> cascade" do the cleanup on the association table between two objects. I have
> setup the cascade and passive_delete options on the relationship as seems
> appropriate from the docs. However, when a related object is loaded into the
> collection of a primary object and the primary object is deleted from the
> session, then SQLAlchemy issues a delete statement for the related object.
there's a setting known as "passive_deletes='all'" which is intended not just
to block the relationship from emitting a SELECT for rows to be deleted, but
also prevents the DELETE from taking place entirely. However, the "all" part
of this feature is not implemented for many-to-many relationships right now -
while a many-to-many with passive_deletes=True will forego emitting a SELECT
for unloaded rows, it still emits DELETE for those rows which it knows about in
memory. It does at least emit the DELETE within an "executemany" so is an
efficient operation.
The delete for the related object here only occurs when cascade="delete" is
set. If you didn't have this cascade set up, the delete of the primary object
would only delete the association rows, which is appropriate. if these
association rows are actually mapped elsewhere in some more elaborate way then
you shouldn't be using "secondary" in this way, you'd use the association
object pattern.
>
> I have a code example to reproduce the problem at stackoverflow:
> http://stackoverflow.com/questions/23669198/passive-deletes-in-sqlalchemy-with-a-many-to-many-relationship-dont-prevent-del
>
> Is there a way to configure SQLAlchemy to never emit a DELETE for a related
> object even when that object is loaded in a collection of an entity that is
> deleted from the session?
OK so here, the "related object" is *not* being deleted. That would be "tags".
the "blog_tags_table" is not an object, it is a many-to-many table.
from that code given you would just apply viewonly=True to the relationship.
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