How to apply DISTINCT on only date part of datetime field in sqlalchemy python? - python-3.x

Question

I need to query my database and return the result by applying Distinct on only date part of datetime field.
My code is:
#blueprint.route('/<field_id>/timeline', methods=['GET'])
#blueprint.response(field_timeline_paged_schema)
def get_field_timeline(
field_id,
page=1,
size=10,
order_by=['capture_datetime desc'],
**kwargs
):
session = flask.g.session
field = fetch_field(session, parse_uuid(field_id))
if field:
query = session.query(
func.distinct(cast(Capture.capture_datetime, Date)),
Capture.capture_datetime.label('event_date'),
Capture.tags['visibility'].label('visibility')
).filter(Capture.field_id == parse_uuid(field_id))
return paginate(
query=query,
order_by=order_by,
page=page,
size=size
)
However this returns the following error:
(psycopg2.errors.InvalidColumnReference) for SELECT DISTINCT, ORDER BY expressions must appear in select list
The resulting query is:
SELECT distinct(CAST(tenant_resson.capture.capture_datetime AS DATE)) AS distinct_1, CAST(tenant_resson.capture.capture_datetime AS DATE) AS event_date, tenant_resson.capture.tags -> %(tags_1)s AS visibility
FROM tenant_resson.capture
WHERE tenant_resson.capture.field_id = %(field_id_1)s
Error is:
Query error - {'error': ProgrammingError('(psycopg2.errors.InvalidColumnReference) SELECT DISTINCT ON expressions must match initial ORDER BY expressions\nLINE 2: FROM (SELECT DISTINCT ON (CAST(tenant_resson.capture.capture...\n ^\n',)
How to resolve this issue? Cast is not working for order_by.

Answer 1

I am not familiar with sqlalchemy but this resulting query works as you expect. Please note the DISTINCT ON.
Maybe there is a way in sqlalchemy to execute non-trivial parameterized queries? This would give you the extra benefit to be able to test and optimize the query upfront.
SELECT DISTINCT ON (CAST(tenant_resson.capture.capture_datetime AS DATE))
CAST(tenant_resson.capture.capture_datetime AS DATE) AS event_date,
tenant_resson.capture.tags -> %(tags_1)s AS visibility
FROM tenant_resson.capture
WHERE tenant_resson.capture.field_id = %(field_id_1)s;
You can order by event_date if your business logic needs.

Answer 2

The query posted by #Stefanov.sm is correct. In SQLAlchemy terms it would be
query = (
session.query(
Capture.capture_datetime.label('event_date'),
Capture.tags['visibility'].label('visibility')
).distinct(cast(Capture.capture_datetime, Date))\
.filter(Capture.field_id == parse_uuid(field_id))
)
See the docs for more information

Answer 3

I needed to add order_by to my query. Now it works fine.
query = session.query(
cast(Capture.capture_datetime, Date).label('event_date'),
Capture.tags['visibility'].label('visibility')
).filter(Capture.field_id == parse_uuid(field_id)) \
.distinct(cast(Capture.capture_datetime, Date)) \
.order_by(cast(Capture.capture_datetime, Date).desc())