Re: [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:  high      |  Milestone:  0.9.0
     Component:  orm       |   Severity:  major - 1-3 hours
    Resolution:            |   Keywords:
Progress State:  in queue  |
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Comment (by elic):

 > ... Not really sure about that metaclass test though, just a simple
 descriptor is all we need to illustrate?

 I agree, my test was kinda overwrought... that `@hybrid_property` test is
 much better, and anything that passes that should work fine for my use-
 case.  (So much of sqlalchemy I need to study, I wasn't aware of the
 hybrid decorators until now! Gonna go read docs cover to cover this
 weekend).

 ----

 ''sidenote: why the odd test...''

 My test was extracted from a custom declarative layer, which puts it's
 custom descriptor on the same attribute as one instrumented by SQLAlchemy
 (rather having the descriptor on a different attribute, as in your test).
 I thought that or some other unnoticed aspect might be important, so
 wanted to replicate the bug as I found it, despite the crazy :)

 To explain the metaclass: it's being used to intercept the
 class-`setattr()` call made by `ClassManager.instrument_attribute()`, and
 store the `InstrumentedAttribute` internally.  This allows the custom
 descriptor to wrap the `InstrumentedAttribute` at the instance level, but
 return the `InstrumentedAttribute` unchanged at the class level. (Looking
 back, I should probably update my code to use
 `sqlalchemy.ext.instrumentation` instead of the metaclass hack, as I
 wasn't aware of that feature when I started using sqlalchemy).

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Ticket URL: <http://www.sqlalchemy.org/trac/ticket/2872#comment:4>
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