Queryset ".dates(...)" returns a non distinct DateTimeQueryset of datetime objects

Question

For a QuerySet of blog entries, I want to create a DateQuerySet of the months in which those posts were made. The query is:

dates = Entry.published.all().dates('pub_date', 'month')

According to the docs

"month" returns a list of all distinct year/month values for the field.

If I have 4 entries with dates :

(2012, Feb, 3rd)
(2012, Feb, 2nd)
(2012, Jan, 24th)
(2011, Dec, 28th)

I expect to get 3 datetime objects returned; one for Dec, Jan, Feb, instead I get 4 returned, one for each of the original dates

Is this expected behaviour? I've tried adding distinct() to the query, but it still returns every date.

UPDATE

A simple way to fix this is make a Set from the DateQuerySet:

dates = Entry.published.all().dates('pub_date', 'month')
return set(dates)

This removes the duplicates datetime objects but I still don't understand why this is happening (or if I am misunderstanding how dates() works)

I am curious, what does dates = Entry.published.dates('pub_date', 'month') return? — akonsu
It returns a DateQuerySet which is basically a list of datetime objects. It returns one for every entry in the original queryset, regardless of whether or not the occur in the same month of not — Timmy O'Mahony
does it also return duplicate dates as does the version with the all() call? — akonsu
No, but the published manager inherits from the defaults and simply filters out Entries from the future — Timmy O'Mahony

Yuji 'Tomita' Tomita Yuji 'Tomita' Tomita · Accepted Answer · 2012-02-09T18:26:00

Your symptoms sound exactly like a GROUP BY query with ordering.

You can print query.query to see if there's ordering being applied to a field that would be added to the SELECT, thus making all of those distinct as well.

Entry.published.dates('pub_date', 'month').order_by()

Queryset ".dates(...)" returns a non distinct DateTimeQueryset of datetime objects

2 Answers