#17427: DatabaseWrapper no longer hashable-> failure in
test_connections_thread_local
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Reporter: vsajip | Owner: nobody
Type: Uncategorized | Status: new
Component: Database layer | Version: SVN
(models, ORM) | Resolution:
Severity: Normal | Triage Stage:
Keywords: | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by akaariai):
I did a little investigation, and to me it seems the correct fix is to not
define `__eq__` or `__hash__` at all, that is, they are inherited from
object, and thus only same instance is equal.
The current equality test using alias isn't a sane test anymore.
Previously you had to try extra-hard to get two different instances in one
thread to point to the same alias. Now this is much easier to do. So, if
you have two DBWrappers pointing to the same alias but using different
connections, I don't see a reason why they should be equal. Other
possibility would be to base the the equality on the underlying connection
equality, that is self.connection == other.connection.
Previously if you transferred the DBWrapper in connections[some_alias]
from thread 1 to thread 2, the transferred DBWrapper would have pointed to
the same connection in thread 2 (because the DBWrapper instance was
threading.local). So, if you tested self.alias == other.alias you would be
testing that the connection is the same, too. This is no longer true.
I hope the above explanation makes some sense. Making this change will
require some minor changes to Django code. But the bigger deal is if this
breaks user code in backwards incompatible way. User code relying on
.alias equality is no longer correct, and instance equality will give the
same result as long as you don't transfer connections between threads or
anything like that. So, no problems should be caused to users by this.
--
Ticket URL: <https://code.djangoproject.com/ticket/17427#comment:5>
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