On Jan 18, 2012, at 8:44 AM, Ronan Dunklau wrote:
> Hello.
>
> I'm having trouble mapping a relationship on a column_property.
>
> A similar problem has been reported in an old post on this mailing list,
> but with no answers:
> http://groups.google.com/group/sqlalchemy/browse_thread/thread/f7a13d9ca1494060/6fe890bb49c8dfa8#6fe890bb49c8dfa8
>
> I have a column containing a code, a substring of this code refers to
> another table's primary key.
>
> I tried the following solutions, but none works:
>
> 1) "Naive" joining on the attributes:
>
> Table1.rel_to_table2 = relationship(Table2,
> viewonly=True,
> uselist=False,
> primaryjoin=func.substring(Table1.code, 1, 3) == Table2.code)
>
> 2) Defining an intermediate column property:
>
> Table1.table2_code = column_property(func.substring(Table1.col1, 1, 3))
>
> Table1.rel_to_table2 = relationship(Table2,
> viewonly=True,
> uselist=False,
> primartyjoin=Table1.table2_code == Table2.code)
>
> 3) Adding a foreign_keys argument to the relationship, on the actual columns
>
> This results in an error:
>
> sqlalchemy/orm/properties.py", line 1458, in _refers_to_parent_table
> pt.is_derived_from(c.table) and \
> AttributeError: 'Function' object has no attribute 'table'
this is an advanced form of mapping that's not documented right now, as it's
been quasi-experimental for some time.
You wouldn't need column_property here, instead you do it like this ("table1"
and "table2" should be Table objects, not the classes, like Table2.__table__):
rel = relationship(Table2,
viewonly=True,
uselist=False,
_local_remote_pairs=[(table1.c.code,
table2.c.code)],
primaryjoin=func.substring(table1.c.code, 1, 3) ==
table2.c.code,
foreign_keys=[table1.c.code]
)
I'm not sure which side here you'd want to consider "foreign", that is, the
column which seeks to match an existing column. Switching "foreign_keys" to
point to table1.c.code or table2.c.code will cause relationship to consider it
as many-to-one, or one-to-many, respectively.
>
> Thank you!
>
>
> --
> Ronan Dunklau
>
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