I have an object Option which is like a type, it only has a property: name (it could have more properties in the future).
Instances of Option have a unique name.
Many objects may have zero or more of Option instances.
For example:
class Consumer:
options = models.ManyToManyField(Option, blank=True, help_text="the options")
But then, in order to create the Option instances for Consumer's many-to-many options relationship, I need to create a new Option instance and add it to options.
This however "breaks" my uniqueness: Now I have two with the same name! And so forth for every instance of Option I create.for Many-to-Many links. Instead of the 4 I need, I have now 68 in my DB...
I believe I have fundamentally misunderstood Many-To-Many, and / or mis-designed this relationship...
Can anybody help?
EDIT: Here's how I set options in an example:
def enable_option(request, pk=0, option_pk=0, *args, **kwargs):
consumer = get_object_or_404(Consumer, pk=pk)
option = get_object_or_404(Option, pk=option_pk)
new_option = Option()
new_option.name = option.name // I know I am breaking my own rule...but when I read the consumer options, I need the exact same name! Still, I believe I am modeling wrong
new_option.save()
consumer.options.add(new_option)
consumer.save()
return HttpResponse()
I don't really understand why you create a new Option here. You get the existing one; you can just add that to the relationship:
consumer = get_object_or_404(Consumer, pk=pk)
option = get_object_or_404(Option, pk=option_pk)
consumer.options.add(option)
You don't even need to call save, as modifying an m2m does not change the instance itself.
Related
I need to bulk_create the historical records for each object deleted in a queryset. I've coded it correct I think as follows.
def bulk_delete_with_history(objects, model, batch_size=None, default_user=None, default_change_reason="", default_date=None):
"""
The package `simple_history` logs the deletion one object at a time.
This does it in bulk.
"""
model_manager = model._default_manager
model_manager.filter(pk__in=[obj.pk for obj in objects]).delete()
history_manager = get_history_manager_for_model(model)
history_type = "-"
historical_instances = []
for instance in objects:
history_user = getattr(
instance,
"_history_user",
default_user or history_manager.model.get_default_history_user(
instance),
)
row = history_manager.model(
history_date=getattr(
instance, "_history_date", default_date or timezone.now()
),
history_user=history_user,
history_change_reason=get_change_reason_from_object(
instance) or default_change_reason,
history_type=history_type,
**{
field.attname: getattr(instance, field.attname)
for field in instance._meta.fields
if field.name not in history_manager.model._history_excluded_fields
}
)
if hasattr(history_manager.model, "history_relation"):
row.history_relation_id = instance.pk
historical_instances.append(row)
return history_manager.bulk_create(
historical_instances, batch_size=batch_size
)
The problem though is I need to disconnect the post_delete signal so that a historical record isn't created by simple history before i do it all at once.
I've tried this but it doesn't work.
models.signals.post_delete.disconnect(HistoricalRecords.post_delete, sender=Customer)
Where Customer is just a class I'm using to test this utility function.
Can anybody advise? Thanks in advance.
Asked the question on their github page also - https://github.com/jazzband/django-simple-history/issues/717
I made a classic mistake with disconnecting signals. One must of course disconnect with the object that was connected. This is discussed here in more detail - django signal disconnect not working
This now works because I am disconnecting with reference to the correct object.
receiver_object = models.signals.post_delete._live_receivers(Customer)[0]
models.signals.post_delete.disconnect(receiver_object, sender=Customer)
Still, I think it would be nice if django-simple-history provided a bulk_delete utility function like they provide one for bulk_create and bulk_update.
To make a long story short, I am very grateful for hints on how I can accomplish the following. I have an app A that I don't want to change. I have an app B that needs to select data from A or to request data to be added/changed if necessary. Think of B as an app to suggest data that should end in A only after review/approval. By itself, B is pretty useless. Furthermore, a significant amount of what B's users will enter needs to be rejected. That's why I want A to be protected so to say.
# in app A
class Some_A_Model(models.Model): #e.g., think artist
some_value = models.TextField()
# in app B
class MyCustomField(models.ForeignKey):
...
class Some_B_Model(models.Model): # e.g., think personal preference
best_A = MyCustomField('Some_A_Model')
worst_A = MyCustomField('Some_A_Model')
how_many_times_I_like_the one_better_than_the_other = models.FloatField()
class Mediator(models.Model):
# already exists: generic foreign key
content_type = models.ForeignKey(
ContentType,
on_delete=models.CASCADE
)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey(
'content_type',
'object_id'
)
#does not yet exist or needs to be changed:
add_or_change = PickledObjectField()
Django should create a form for Some_B_Model where I can select instances of Some_A_Model for best_A and worst_A, respectively; if, however, my intended best_A is not yet in A's database, I want to be able to request this item to be added. And if I find worst_A is present but has a typo, I want to be able to request this item to be corrected. An editor should be required to review/edit the data entered in B and either reject or release all the associated changes to A's database as an atomic transaction. I don't want any garbage in A and refrain from adding some status field to track what is considered valid, requiring filtering all the time. If it's in A, it must be good.
I figured I need to define a MyCustomField, which could be a customized ForeignKey. In addition, I need some intermediate model ('mediator' maybe?) that MyCustomField would actually be pointing to and that can hold a (generic) ForeignKey to the item I selected, and a pickled instance of the item I would like to see added to A's database (e.g., a pickled, unsaved instance of Some_A_model), or both to request a change. Note that I consider using PickledObjectField from 'django-picklefield', but this is not a must.
As there is only some documentation on custom model fields but not on the further steps regarding form fields and widgets, it seems I have to dig through django's source to find out how to tie my intended functionality into its magic. That's where I am hoping for some comments and hints. Does my plan sound reasonable to you? Is this a known pattern, and if so, what is it called? Maybe someone has already done this or there is a plugin I could look into? What alternatives would you consider?
Many thanks in advance!
Best regards
I'm trying to render a form with a combo that shows related entities. Therefore I'm using a ModelChoiceField.
This approach works well, until I needed to limit which entities to show. If I use a simple query expression it also works well, but things break if I use a raw SQL query.
So my code that works, sets the queryset to a filter expression.
class ReservationForm(forms.Form):
location_time_slot = ModelChoiceField(queryset=LocationTimeSlot.objects.all(), empty_label="Select your prefered time")
def __init__(self,*args,**kwargs):
city_id = kwargs.pop("city_id") # client is the parameter passed from views.py
super(ReservationForm, self).__init__(*args,**kwargs)
# TODO: move this to a manager
self.fields['location_time_slot'].queryset = LocationTimeSlot.objects.filter(city__id = city_id )
BUT, if I change that to a raw query I start having problems. Code that does not work:
class ReservationForm(forms.Form):
location_time_slot = ModelChoiceField(queryset=LocationTimeSlot.objects.all(), empty_label="Select your prefered time")
def __init__(self,*args,**kwargs):
city_id = kwargs.pop("city_id") # client is the parameter passed from views.py
super(ReservationForm, self).__init__(*args,**kwargs)
# TODO: move this to a manager
query = """SELECT ts.id, ts.datetime_to, ts.datetime_from, ts.available_reserves, l.name, l.'order'
FROM reservations_locationtimeslot AS ts
INNER JOIN reservations_location AS l ON l.id = ts.location_id
WHERE l.city_id = %s
AND ts.available_reserves > 0
AND ts.datetime_from > datetime() """
time_slots = LocationTimeSlot.objects.raw(query, [city_id])
self.fields['location_time_slot'].queryset = time_slots
The first error I get when trying to render the widget is: 'RawQuerySet' object has no attribute 'all'
I could solve that one thanks to one of the commets in enter link description here, by doing:
time_slots.all = time_slots.__iter__ # Dummy fix to allow default form rendering with raw SQL
But now I'm getting something similar when posting the form:
'RawQuerySet' object has no attribute 'get'
Is there a proper way to prepare a RawQuerySet to be used by ModelChoiceField?
Thanks!
Are you sure you actually need a raw query there? Just looking at that query, I can't see any reason you can't just do it with filter(location__city=city_id, available_reserves__gte=0, datetime_from__gt=datetime.datetime.now()).
Raw query sets are missing a number of methods that are defined on conventional query sets, so just dropping them in place isn't likely to work without writing your own definitions for all those methods.
I temporarily fixed the problem adding the missing methods.
The way I'm currently using the ModelChoiceField I only needed to add the all() and get() methods, but in different scenarios you might need to add some other methods as well. Also this is not a perfect solution because:
1) Defining the get method this way migth produce incorrect results. I think the get() method is used to validate that the selected option is within the options returned by all(). The way I temporarily implemented it only validates that the id exists in the table.
2) I guess the get method is less performant specified this way.
If anyone can think of a better solution, please let me know.
So my temporary solution:
class LocationTimeSlotManager(models.Manager):
def availableSlots(self, city_id):
query = """SELECT ts.id, ts.datetime_to, ts.datetime_from, ts.available_reserves, l.name, l.'order'
FROM reservations_locationtimeslot AS ts
.....
.....
MORE SQL """
time_slots = LocationTimeSlot.objects.raw(query, [city_id])
# Dummy fix to allow default form rendering with raw SQL
time_slots.all = time_slots.__iter__
time_slots.get = LocationTimeSlot.objects.get
return time_slots
I have two tables namely Stuff and Boss. Based on the slug(user_type) from the user, I define which table is going to be used.
def Person_info(request,user_type):
if user_type=="Staff":
item=Staff()
elif user_type=="Boss":
item=Boss()
.................
Then, I need to get the last id for item from its table.
But, I am having "Manager isn't accessible via Staff instances" When I try to get the last id of Staff table.
How can I bypass this problem ?
You are querying using the instance, which is incorrect.
Change your code as below:
def Person_info(request,user_type):
if user_type=="Staff":
# Note no () at the end, which makes the item an instance by instantiating it, not a class by assigning it
item=Staff
elif user_type=="Boss":
item=Boss
....
I have a database of exhibition listings related by foreign key to a database of venues where they take place. Django templates access the venue information in the query results through listing.venue.name, listing.venue.url, and so on.
However, some exhibitions take place in temporary venues, and that information is stored in the same database, in what would be listing.temp_venue_url and such. Because it seems wasteful and sad to put conditionals all over the templates, I want to move the info for temporary venues to where the templates are expecting info for regular venues. This didn't work:
def transfer_temp_values(listings):
for listing in listings:
if listing.temp_venue:
listing.venue = Venue
listing.venue.name = listing.temp_venue
listing.venue.url = listing.temp_venue_url
listing.venue.state = listing.temp_venue_state
listing.venue.location = listing.temp_venue_location
The error surprised me:
ValueError at /[...]/
Cannot assign "<class 'myproject.gsa.models.Venue'>": "Exhibition.venue" must be a "Venue" instance.
I rather thought it was. How do I go about accomplishing this?
The error message is because you have assigned the class Venue to the listing, rather than an instance of it. You need to call the class to get an instance:
listing.venue = Venue()