Using comparable property as a selected element in SQL is a use case
which hasn't been addressed as of yet - the main usage was so far just
for producing WHERE/ORDER BY/etc. expressions. In particular, the
comparable property doesn't have any query setup capabilities (i.e.
where it puts itself into a SQL expression as a column to be loaded)
nor any ability to receive result rows in an ORM entity loading
context. This example though is just a column loading context.
So for the exact thing you're trying to do, it would probably work if
you changed your __clause_element__ method to return a concatenation,
since column elements within sess.query(X) are expected to be scalar
ColumnElement objects:
def __clause_element__(self):
r = None
for col in self.prop.descriptor.columns:
if r is None:
r = col
else:
r = r + text("' '") + col
But really, if you're using comparable property as a loader, I
wouldn't use comparable property anyway. It's much easier to achieve
as a regular column_property, which has full blown ORM capabilities,
meaning that it works as an entity attribute loader as well:
mapper(MyClass, mytable, properties ={
'full_name':column_property(mytable.c.first_name + text("' '") +
mytable.c.last_name)
})
The comparison generated by the above property would look like:
WHERE table.firstname || ' ' || table.lastname = ?
If you wanted to generate the criterion using AND the way your custom
prop comparator does, you can plug your comparator in using
comparator_factory - this option is available in rc1 but has not yet
been documented (its a TODO for the 0.5.0 milestone):
mapper(MyClass, mytable, properties ={
'full_name':column_property(mytable.c.first_name + text("' '") +
mytable.c.last_name, comparator_factory =
JoinedValuesProperty.Comparator)
})
You still would want your __clause_element__ to return the SQL
concatenation expression.
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