Re: [Sqlalchemy-tickets] [sqlalchemy] #2876: Using relationship with primaryjoin in declared_attr fails

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

#2876: Using relationship with primaryjoin in declared_attr fails
-----------------------------------+------------------------------------
      Reporter:  wichert           |      Owner:  zzzeek
          Type:  defect            |     Status:  closed
      Priority:  medium            |  Milestone:
     Component:  declarative       |   Severity:  no triage selected yet
    Resolution:  worksforme        |   Keywords:
Progress State:  completed/closed  |
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Changes (by zzzeek):

 * status:  new => closed
 * resolution:   => worksforme
 * status_field:  awaiting triage => completed/closed


Comment:

 TL;DR; - this situation is documented at
 http://docs.sqlalchemy.org/en/rel_0_9/orm/extensions/declarative.html
 #mixing-in-relationships

     relationship() definitions which require explicit primaryjoin,
 order_by etc. expressions should use the string forms for these arguments,
 so that they are evaluated as late as possible. To reference the mixin
 class in these expressions, use the given cls to get its name:


 The error message here is improved in 0.9, where you will get:

 {{{
   File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/sql/schema.py", line
 1485, in _resolve_col_tokens
     "this ForeignKey's parent column is not yet associated "
 sqlalchemy.exc.InvalidRequestError: this ForeignKey's parent column is not
 yet associated with a Table.
 }}}

 Your method avatar_original_id creates a new `Column` object *each time it
 is called*.  When you call it within your `avatar_original` method, you
 get a `Column` back, but '''that Column object is unknown to any other
 aspect of the program'''.

 So the class gets mapped normally, declarative calls upon
 `avatar_original_id` to get at the `Column` that will actually be mapped,
 and the one you've put in your `relationship()` has no parent table.

 The solution - except in the most simplistic cases, strings or lambdas
 should always be used for relationship arguments in declarative:

 {{{
 #!python


     @declared_attr
     def avatar_original(cls):
         return relationship(Image, cascade='all',
                 primaryjoin=lambda: (cls.avatar_original_id == Image.id))
 }}}

 or

 {{{
 #!python
     @declared_attr
     def avatar_original(cls):
         return relationship(Image, cascade='all',
                 primaryjoin="%s.avatar_original_id == Image.id" %
 cls.__name__)

 }}}

 the latter is documented in the last example at:
 http://docs.sqlalchemy.org/en/rel_0_9/orm/extensions/declarative.html
 #mixing-in-relationships

 the "lambda:" form should probably be added to the docs as it is handy.

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Ticket URL: <http://www.sqlalchemy.org/trac/ticket/2876#comment:1>
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