[Sqlalchemy-tickets] [sqlalchemy] #2872: AliasedClass.__getattr__() creates malformed QueryableAttribute

[sqlalchemy <mi...@zz...>]

#2872: AliasedClass.__getattr__() creates malformed QueryableAttribute
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 Reporter:  elic    |           Owner:  zzzeek
     Type:  defect  |          Status:  new
 Priority:  medium  |       Milestone:  0.9.xx
Component:  orm     |        Severity:  no triage selected yet
 Keywords:          |  Progress State:  awaiting triage
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 This an odd border case that few others will probably find themselves in,
 but I ''think'' it's a bug in `sqlalchemy.orm.util.AliasedClass` (even
 it's not, I'd still love if the behavior could be slightly tweaked).

 Under 0.8.3 & 0.9b1, when `AliasedClass.__getattr__()` is retrieving the
 attr's value, if it encounters a `QueryableAttribute`, it adapts it
 accordingly.  If it encounters a foreign descriptor, it calls
 `descriptor.__get__(None, cls)`, then adapts the result if it's a
 `PropComparator`, but otherwise returns it unchanged.

 The situation I find myself in is that I have a descriptor which is itself
 returning a `QueryableAttribute` instance (in particular, it's storing and
 returning the `InstrumentedAttribute` that the mapper sets on the class
 attribute).  Since `QueryableAttribute` inherits from `PropComparator`,
 the current code tries to adapt it as one, which results in a wildly
 messed-up object, since `QueryableAttribute`'s call signature is very
 different from what the `PropComparator`-adapter code is expecting.

 My proposed change adds a check for descriptors returning
 `QueryableAttribute` instances, and adapts them the same as if they were
 stored directly in the class. Attached is a patch (`queryable.patch`) for
 0.9, and a similar change seems to fix 0.8.

 I'm happy to explain in more detail how I arrived at this situation, but I
 didn't want to make this bug report overly long. Also attached is
 `test_proxy_descriptor.py`, which was distilled from my actual use case,
 and which demonstrates the error.

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Ticket URL: <http://www.sqlalchemy.org/trac/ticket/2872>
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