Alex jg Alex jg - 1 year ago 245
Python Question

Using PostgreSQL Aggregate ORDER BY with sqlalchemy

I have a query which uses PostgreSQL's syntax for using ORDER BY in an aggregate function, something like this:

SELECT some_agg_func(a ORDER BY b DESC) FROM table;

Does anyone know of anyway to do this with the sqlalchemy expression language?

Answer Source

You need to use SQLAlchemy's compiler extension to achieve this. Here's an example for Postgres string_agg function:

from sqlalchemy.sql.expression import ColumnElement, _literal_as_column
from sqlalchemy.dialects import postgresql
from sqlalchemy.ext.compiler import compiles

class string_agg(ColumnElement):
    def __init__(self, expr, separator, order_by=None):
        self.type = Text()
        self.expr = _literal_as_column(expr)
        self.separator = literal(separator)
        self.order_by = _literal_as_column(order_by)

    def _from_objects(self):
        return self.expr._from_objects

@compiles(string_agg, 'postgresql')
def compile_string_agg(element, compiler, **kwargs):
    head = 'string_agg(%s, %s' % (
    if element.order_by is not None:
        tail = ' ORDER BY %s)' % compiler.process(element.order_by)
        tail = ')'
    return head + tail

query = session.query(string_agg(, ', ',
# Print compiled SQL query.
print query.statement.compile(dialect=postgresql.dialect())
# Run the query and print result.
print query.scalar()
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