Deleting non existing record should raise an error in sqlalchemy

Question

Why deleting non-existing record does not raise an error in sqlalchemy. no feedback on whether the record was deleted or not.

session.query(Element).filter(Element.id==ElementId).delete()

Thank you for your clarification.

Answer 1

I don't think it is an error. For example it is perfectly legal to issue a query to delete records in sql that "don't exist"

If i have a table 'posts' with a column 'id'. with no records

DELETE FROM posts WHERE ID > 0;

It is perfectly valid sql, there is no error, even though there are no rows

I am not too familiar with sqlalchmey but could you check to see if value exists first?

element = session.query(Element).filter(Element.id==ElementId).first()
if element:
   # delete element
else:
   # raise exception

The above will issue an additional query though...

Also, if you want a delete method that raises error you can create your own session class Change SQLAlchemy's Session.delete() behaviour and override delete

As zzzeek points out delete with a criteria

Returns the number of rows deleted, excluding any cascades.

Which is another option for seeing if any rows are deleted

Answer 2

Actually, sqlalchemy delete operation returns number of affected rows. So you can check and if affected rows is 0 then you can raise error.

effected_rows = session.query(Element).filter(Element.id==ElementId).delete()
if effected_rows == 0:
    # raise exception
else:
    # delete element

This works for me.