On Jun 30, 2008, at 1:04 PM, Hans wrote:
>
> The problem is that I'd like to have sqlalchemy return KML for all
> geometry types by default, but I don't know how to setup my type to
> specify a SQL function that needs to be applied to the column in the
> select clause. Is this possible? Alternatively, it would be
> *perfect* to have additional *_kml properties added to the result
> objects (one for each GEOMETRY column), but this seems even more
> complex (?).
>
>
> I was expecting my convert_result_value method to operate on the SQL,
> but I am assuming from looking at the generated SQL that this method
> is actually going to operate on the raw data that was returned from
> SQL. So, is there a way to accomplish what I want?
>
TypeEngine currently has no hook that's consulted during SQL
rendering, which is where you're looking for this to happen. Its
not impossible to add such a feature but its not yet clear to me that
TypeEngine is the appropriate place for this decision to be made.
Its also not apparent yet how specifically within the SQL it would be
appropriate to render the function, and how labeling would work -
since SQLA, when it generates nested select statements, uses labels to
track each column named against a table outwards...such as "select
foo, bar from (select a as foo, b as bar from table)". In the case of
a SQL function, the column name becomes somewhat undefined until a
label is applied. SQLA nests SELECT statements very often.
You might want to try experimenting with a simple monkeypatch approach
just to see what kind of results you get - this is just a guess based
on a particular compiler hook we have that specifically processes
columns being rendered within a select:
from sqlalchemy.databases.postgres import PGCompiler
def label_select_column(self, select, column, asfrom):
if isinstance(col.type, MyGISType):
return func.some_kml_function(col).label(column.name)
else:
return super(self, PGCompiler).label_select_column(select,
column,
asfrom)
PGCompiler.label_select_column = label_select_column
if the above leads to decent results we can attempt to add a hook onto
TypeEngine perhaps.
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