Find and delete similar records in Postgres - django

Question

TL\DR version - Find and delete rows in Postgres with same date but different time, leaving one record per date.
Long read:
At some point we've migrated our app's backend to a newer version - this is a Django application - migrated from Python 2, Django 1.8 to Python 3 - Django 4, and with this update we're changed timezone for backend from UTC+2 to UTC+3. And now strange things happens - records which previously successfully have been read from db with queryset StatChildVisit.objects.filter(date=day, garden_group=garden_group) - (day is a python's date only not datetime) after update returns empty queryset, although records for that day are still in db. More so newly created records have different time in them - records created with old timezone looks like 2022-12-28 22:00:00.000000 +00:00 new records looks like 2022-12-28 21:00:00.000000 +00:00
Seems that bug happened because date field in django's model have been declared as DateTimeField -
class StatChildVisit(models.Model):
child = models.ForeignKey(Child, on_delete=models.CASCADE)
date = models.DateTimeField(default=timezone.now)
visit = models.BooleanField(_('Atended'), default=True)
disease = models.BooleanField(_('Sick'), default=False)
other_approved = models.BooleanField(_('Other approved'), default=False)
garden_group = models.ForeignKey(GardenGroup, verbose_name=_('Garden group'), editable=False, blank=True, null=True, on_delete=models.CASCADE)
rossecure_visit = models.ForeignKey('rossecure.Visits', editable=False, null=True, blank=True, on_delete=models.CASCADE)
class Meta:
verbose_name = _('Attendence')
verbose_name_plural = _('Attendence')
index_together = (
('date', 'garden_group'),
)
unique_together = (
('date', 'child'),
)
all records are always being created with date only (not datetime) passing to constructor
So we've decided to migrate this field to DateField, but after migration field type in DB is still 'timestamp with time zone', and besides, because this is a production database users after finding that some data looks like lost partially recreated records.
So now we have multiple records for same day but with different time which need to be deleted and because of constrains table column can not be altered with ALTER TABLE reports_statchildvisit ALTER COLUMN date TYPE date;
Because table have rather large records count (about 4 million) I think that problem should be solved via SQL side, and not Django side. My plan is to delete duplicates and then change column type to date.
I've tried to alter records with
update reports_statchildvisit
set date = date(date) + '21:00:00'::time
but because I've tried that after users created similar records script failed with ERROR: duplicate key value violates unique constraint
UPD: DDL on the SQL side looks like this:
create table public.reports_statchildvisit
(
id serial
primary key,
date timestamp with time zone not null,
visit boolean not null,
disease boolean not null,
child_id integer not null
constraint reports_statchil_child_id_30fbdf92a34d3fea_fk_children_child_id
references public.children_child
deferrable initially deferred,
garden_group_id integer
constraint repor_garden_group_id_ef61dd52421b5d2_fk_project_gardengroup_id
references public.project_gardengroup
deferrable initially deferred,
other_approved boolean not null,
rossecure_visit_id integer
constraint repo_rossecure_visit_id_488614f59207663f_fk_rossecure_visits_id
references public.rossecure_visits
deferrable initially deferred,
constraint reports_statchildvisit_date_3d6916481fe1e727_uniq
unique (date, child_id)
);
alter table public.reports_statchildvisit
owner to django;
create index reports_statchildvisit_10e12719
on public.reports_statchildvisit (garden_group_id);
create index reports_statchildvisit_42d2af72
on public.reports_statchildvisit (rossecure_visit_id);
create index reports_statchildvisit_date_66064e65c46d4137_idx
on public.reports_statchildvisit (date, garden_group_id);
create index reports_statchildvisit_f36263a3
on public.reports_statchildvisit (child_id);