from django.db import transaction

from django.db.models import F

def set_position_for_new_obj(obj, position_field, insert_position='last'):

"""

set a new instance's position field, update other objects if needed

:param obj:

:param insert_position:

:return obj: instance, with position field set, not saved

"""

if insert_position == 'last':

pos = get_last_position(obj.__class__, position_field) + 1

else:

qs = obj.__class__._default_manager.get_queryset()

qs.update(**{position_field: F(position_field) + 1})

pos = 1

setattr(obj, position_field, pos)

return obj

def get_last_position(model_cls, position_field):

objects = model_cls._default_manager.get_queryset()

# result = objects.aggregate(last_position=Max(position_field))

# return result['last_position'] or 0

return objects.all().count()

def reorder_all(model_cls, position_field):

with transaction.atomic():

pos = 1

for o in model_cls._default_manager.get_queryset():

setattr(o, position_field, pos)

o.save()

pos += 1

def position_object(obj, position_field, new_position, commit=True):

"""

set a new position for an object

:param obj:

:param position_field:

:param new_position:

:return obj:

"""

# calculate, return, cleanup if necessary

obj_start_position = getattr(obj, position_field, None)

if new_position == obj_start_position:

return obj

base_qs = obj.__class__._default_manager.get_queryset()

last_position = get_last_position(obj.__class__, position_field)

if new_position > last_position:

new_position = last_position

if new_position == 0:

new_position = 1

# the actual work

if new_position > obj_start_position:

# down/right in the list, bigger position value than before

update_kwargs = {position_field: F(position_field) - 1}

update_start_position = obj_start_position

update_end_position = new_position

else:

# up/left in the list

update_kwargs = {position_field: F(position_field) + 1}

update_start_position = new_position

update_end_position = obj_start_position

update_qs_filter_kwargs = {

'%s__gte' % position_field: update_start_position,

'%s__lte' % position_field: update_end_position

}

update_qs = base_qs.filter(**update_qs_filter_kwargs)

update_qs = update_qs.exclude(pk=obj.pk)

with transaction.atomic():

update_qs.update(**update_kwargs)

setattr(obj, position_field, new_position)

if commit:

obj.save()

return obj