python - Programming error with order by and distinct on Django with PostgreSQL -

there lot of similar posts none hit @ trying to. distinct has use same fields order_by, fine.

so have following query:

q = mymodel.objects.order_by('field1', 'field2', '-tstamp').distinct('field1', 'field2') 

ultimately trying find latest entry in table combinations of field1 , field2. order_by think should, , that's great. when distinct following error.

programmingerror: select distinct on expressions must match initial order expressions 

ultimately seems sql problem (not django one). looking @ django docs distinct, shows can , can't work. that

q = mymodel.objects.order_by('field1', 'field2', '-tstamp').distinct('field1') 

will work (...and does). don't understand when add on field2 in same order done in order_by still same result. appreciated

edit: notice if do

q = mymodel.objects.order_by('field1', 'field2', '-tstamp').distinct('field1', 'field2', 'tstamp') # or without - in order_by 

it still raises same error, though docs suggest should work fine

was able query run using pk's in order_by()

q = mymodel.objects.order_by('field1__pk', 'field2__pk', '-tstamp').distinct('field1__pk', 'field2__pk') 

apparently when not ordinary types orderby doesn't play super nice. using pk's of objects seems work