I have two models that go as follows (for simplicity's sake):
class Bank(models.Model):
name = models.CharField(max_length=250)
class Person(models.Model):
bank = models.ForeignKey(Bank)
savings = models.IntegerField()
debts = models.IntegerField()
def get_net_worth(self):
return self.savings - self.debts
And I want to write a method that lets me know the "total net worth" of the people in a bank, like:
class Bank(models.Model):
...
def get_net_worth(self):
return self.person_set.all().aggregate(Sum('get_net_worth'))
But of course Django complains that such field does not exist:
django.core.exceptions.FieldError: Cannot resolve keyword 'get_net_worth' into field.
Is there a way to achieve this without having to explicitly store the "net worth" in the database?
Edit:
In case you wonder, one can just do
def get_net_worth(self):
person_queryset = self.person_set.all()
return person_queryset.aggregate(Sum('savings'))['savings__sum'] - person_queryset.aggregate(Sum('debts'))['debts__sum']
But I do have to somehow use the get_net_worth method on every "Person".
Edit 2:
Just to clarify things up: These two are not my actual models, they are over-simplified examples of what I want to achieve, and what I essentially want to do is NOT to do arithmetic between two fields, but rather use a model's method as the field for the aggregate function (of course a method can do some more complex stuff before returning a value), therefore this one is not a duplicate of the other question.
Related
Is there a by the book way of allowing a user to add columns to a sites database table. For example, if the site was about animals, one user might want to have stats like, 'walks per week' and 'type of food' about their breed of dog. but another user might want to keep track of how much milk their goat is producing.
So if i have an 'Animal' class with come basic info. like, 'breed', 'animal name', 'DOB', 'DOD'. But then, in the front end have a form that will allow the users to add all the other columns they would like.
Is this possible? hope I've explained it well enough.
#WillemVanOnsem already mentioned some good options in the comments. I'm going to chime in to say that modifying your schema's structure based on user input is an extremely bad idea and opens another avenue for abuse... for Django in particular, it means you either can't use the ORM's migration facilities for some of your models, or you probably have to do some really awful automation.
If your animal types are well-defined and consistent, you can consider (carefully) making them subclasses of the Animal model. Otherwise, this would be the simplest way to handle it (note that the following isn't valid code, it needs required arguments for the field types):
class AnimalAttribute(models.Model):
animal = models.ForeignKey(Animal)
name = models.CharField()
value = models.CharField()
This works best if attributes aren't shared, e.g. users are directly inputting their animals' names and attributes, not picking from an existing list.
If you need to provide a normalized list of attributes users can pick from (actual EAV, which is something you should avoid if possible, since it moves some of your data structure from code into the data persistence layer), doing that in your models is a little more complex. For example:
class Species(models.Model):
name = models.CharField()
class SpeciesAttribute(models.Model):
species = models.ForeignKey(Species)
name = models.CharField()
class Animal(models.Model):
name = models.CharField()
species = models.ForeignKey(Species)
class AnimalAttributeValue(models.Model):
animal = models.ForeignKey(Animal)
attribute = models.ForeignKey(SpeciesAttribute)
value = models.CharField()
I have the following models:
class Profile(models.Model):
user = models.ForeignKey(User)# User can have more than one profile
class Order(models.Model):
ship_to = models.ForeignKey(Profile)
class Shipping(models.Model):
order = models.ForeignKey(Order)# one order can have more than one shipping
shipping_company = models.ForeignKey(Shipping_company)
class Shipping_company(models.Model):
name = ...
So now i have the following structure:
User > Receiver > Order > Shipping > Shipping_company
The question is: How can i get all User models, who ordered with specific Shipping company?
If i make a query like this
User.objects.filter(receiver__order__shipping__shipping_company__pk=1)
i get
FieldError: Relation fields do not support nested lookups
if i make something like this
sh_comp = items.objects.get(pk=1) # __unicode__ returns "FedEx"
User.objects.filter(receiver__order__shipping__shipping_company=sh_comp)
the result is
ValueError: Cannot query "FedEx": Must be "Receiver" instance.
This seemed to be a simple and trivial task, but i can't make it work.
One approach that can be taken is as following(I am only considering the four models you have presented in your question),
You have foreign key of Shipping company in Shipping model. So you can make use of model function here on Shipping_company model.
Take a look at this model function
class Shipping_company(models.Model):
fields...
def get_profiles(self):
shippings = Shipping.objects.filter(shipping_company=self)
users = list(set([x.order.ship_to for x in shippings]))
Explanation:
shippings = Shipping.objects.filter(shipping_company=self)
will return all the shippings for one Shipping company(FedEx in your case). Further loop through the shippings to get ship_to from order field.
PS: You can take it as reference and design your own solution.
Walkthrough:
Lets say there is shipping company 'FedEx'. So we do,
fedex = Shipping_company.objects.get(name='FedEx')
Now, when you call get_profiles on fedex, like
fedex.get_profiles()
what will happen is this.
fedex instance refers to self in get_profiles() function now.
Using self(fedex), we filter out shippings by fedex.
Then we loop through those shippings to get order per shipping and each of that order has a ship_to(profile) foreign key.
I guess, you are getting confused because of the return statement.
In elaborate fashion the whole function will look something like this
def get_profiles(self):
users = list()
shippings = Shipping.objects.filter(shipping_company=self)
for shipping in shippings:
order = shipping.order
#Now you have an order per shipping, so you do
if not order.ship_to in users:
users.append(order.ship_to)
return users
I'm trying to figure out how to design my model. I've been going over the documentation, and it ultimately seems like I should be using the "through" attribute, but I just can't figure out how to get it to work how I want.
If someone could take a look and point out what I'm missing, that would be really helpful. I have pasted my model below.
This is what I am trying to do:
1) Have a list of server types
2) Each server type will need to have different parts available to that specific server type
3) The asset has a FK to the servermodel, which has a M2M to the parts specific to that server type.
My question is, how can each "Asset" store meta data for each "Part" specific to that "Asset"? For example, each "Asset" should have it's own last_used data for the part that's assigned to it.
Thanks! :)
class Part(models.Model):
part_description = models.CharField(max_length=30,unique=1)
last_used = models.CharField(max_length=30)
def __unicode__(self):
return self.part_description
class ServerModel(models.Model):
server_model = models.CharField(max_length=30,unique=1)
parts = models.ManyToManyField(Part)
def __unicode__(self):
return self.server_model
class Asset(models.Model):
server_model = models.ForeignKey(ServerModel)
serial_number = models.CharField(max_length=10,unique=1)
def __unicode__(self):
return self.server_model.server_model
EDIT:
Thank you for the help!
I may have not explained myself clearly, though. It's probably my confusing model names.
Example:
ServerModel stores the type of server being used, say "Dell Server 2000".
The "Dell Server 2000" should be assigned specific parts:
"RAM"
"HARD DISK"
"CDROM"
Then, I should be able to create 10x Assets with a FK to the ServerModel. Now, each of these assets should be able to mark when the "RAM" part was last used for this specific asset.
I'm not sure I exactly understand what you want to do, but basically you can solve that with a "through" model, as you expected:
import datetime
class Part(models.Model):
name = models.CharField(max_length=30,unique=1)
class ServerModel(models.Model):
server_model = models.CharField(max_length=30,unique=1)
parts = models.ManyToManyField(Part,through='Asset')
class Asset(models.Model):
server_model = models.ForeignKey(ServerModel)
part = models.ForeignKey(Part)
serial_number = models.CharField(max_length=10,unique=1)
used = models.DateTimeField(default=datetime.datetime.now())
First thing to notice is the relation of the parts to the servermodel using the "through"-model: that way for each Part instance assigned to the "parts"-property of a ServerModel instance a new Asset instance is created (Phew - hope that doesn't sound too complicated). At the time of creation the "used"-property of the Asset instance is set to the current date and time (thats what default=datetime.datetime.now() does).
If you do that, you can then just query the database for the last asset containing your part. That queryset can then be sorted by the "used" property of the Asset model, which is the date when the Asset instance has been created.
ServerModel.objects.filter(parts__name='ThePartYouAreLookingFor').order_by('asset__used')
I'm not absolutely sure if the queryset is correct, so if someone finds huge nonsense in it, feel free to edit ;)
edit:
The models above do not exactly that. But you do not even need a through model for what you want:
class ServerModel(models.Model):
server_model = models.CharField(max_length=30,unique=1)
parts = models.ManyToManyField(Part)
class Asset(models.Model):
server_model = models.ForeignKey(ServerModel)
parts = models.ForeignKey(Part)
serial_number = models.CharField(max_length=10,unique=1)
used = models.DateTimeField(default=datetime.datetime.now())
Basically you can just add assets and then query all assets that have a RAM in parts.
Asset.objects.filter(parts__contains='RAM').order_by('used')
Get the date of the first (or last) result of that queryset and you have the date of the last usage of your 'RAM'-part.
I'm new to django and I think this is a simple question -
I have an intermediate class which is coded as follows -
class Link_Book_Course(models.Model):
book = models.ForeignKey(Book)
course = models.ForeignKey(Course)
image = models.CharField(max_length = 200, null=True)
rating = models.CharField(max_length = 200,null=True)
def __unicode__(self):
return self.title
def save(self):
self.date_created = datetime.now()
super(Link_Book_Course,self).save()
I'm making this call as I'd like to have to have all of the authors of the books (Book is another model with author as a CharField)
storeOfAuthorNames = Link_Book_Course.objects.filter(book__author)
However, it doesn't return a querySet of all of the authors, in fact, it throws an error.
I think it's because book__author has multiple values- how can I get all of them?
Thanks!
I don't think you're using the right queryset method. filter() filters by its arguments - so the expected usage is:
poe = Author.objects.get(name='Edgar Allen Poe')
course_books_by_poe = Link_Book_Course.objects.filter(book__author=poe)
It looks like you're trying to pull a list of the names all the authors of books used in a particular course (or maybe all courses?). Maybe you're looking for .values() or values_list()?
all_authors_in_courses = Link_Book_Course.objects.values_list(
'book__author', flat=True
).distinct()
(Edit: Updated per #ftartaggia's suggestion)
As others already explained, the use of filter method is to get a subset of the whole set of objects and does not return instances of other models (no matter if related objects or so)
If you want to have Author models instances back from django ORM and you can use aggregation APIs then you might want to do something like this:
from django.db.models import Count
Author.objects.annotate(num_books=Count('book')).filter(num_books__gt=1)
the filter method you are trying to use translates more or less into SQL like this:
SELECT * FROM Link_Book_Course INNER JOIN Book ON (...) WHERE Book.author = ;
So as you see your query has an incomplete where clause.
Anyway, it's not the query you are looking for.
What about something like (assuming author is a simple text field of Book and you want only authors of books referred from Link_Book_Course instances):
Book.objects.filter(pk__in=Link_Book_Course.objects.all().values_list("book", flat=True)).values_list("author", flat=True)
To start with, a filter statement filters on a field matching some pattern. So if Book has a simple ForeignKey to Author, you could have
storeOfAuthorNames = Link_Book_Course.objects.filter(book__author="Stephen King"), but not just
storeOfAuthorNames = Link_Book_Course.objects.filter(book__author).
Once you get past that, I am guessing Book has Author as a ManyToManyField, not a ForeignKey (because a book can have multiple authors, and an author can publish multiple books?) In that case, just filter(book__author="Stephen King") will still not be enough. Try Link_Book_Course.objects.filter(book_author__in=myBookObject.author.all())
I have a base LoggedEvent model and a number of subclass models like follows:
class LoggedEvent(models.Model):
user = models.ForeignKey(User, blank=True, null=True)
timestamp = models.DateTimeField(auto_now_add=True)
class AuthEvent(LoggedEvent):
good = models.BooleanField()
username = models.CharField(max_length=12)
class LDAPSearchEvent(LoggedEvent):
type = models.CharField(max_length=12)
query = models.CharField(max_length=24)
class PRISearchEvent(LoggedEvent):
type = models.CharField(max_length=12)
query = models.CharField(max_length=24)
Users generate these events as they do the related actions. I am attempting to generate a usage-report of how many of each event-type each user has caused in the last month. I am struggling with Django's ORM and while I am close I am running into a problem. Here is the query code:
def usage(request):
# Calculate date range
today = datetime.date.today()
month_start = datetime.date(year=today.year, month=today.month - 1, day=1)
month_end = datetime.date(year=today.year, month=today.month, day=1) - datetime.timedelta(days=1)
# Search for how many LDAP events were generated per user, last month
baseusage = User.objects.filter(loggedevent__timestamp__gte=month_start, loggedevent__timestamp__lte=month_end)
ldapusage = baseusage.exclude(loggedevent__ldapsearchevent__id__lt=1).annotate(count=Count('loggedevent__pk'))
authusage = baseusage.exclude(loggedevent__authevent__id__lt=1).annotate(count=Count('loggedevent__pk'))
return render_to_response('usage.html', {
'ldapusage' : ldapusage,
'authusage' : authusage,
}, context_instance=RequestContext(request))
Both ldapusage and authusage are both a list of users, each user annotated with a .count attribute which is supposed to represent how many particular events that user generated. However in both lists, the .count attributes are the same value. Infact the annotated 'count' is equal to how many events that user generated, regardless of type. So it would seem that my specific
authusage = baseusage.exclude(loggedevent__authevent__id__lt=1)
isn't excluding by subclass. I have tried id__lt=1, id__isnull=True, and others. Halp.
The key to Django model inheritance is remembering that with a non-abstract base class everything is really an instance of the base class which might happen to have some extra data strapped on the side from a separate table. This means that when you do searches on the base table you get back instances of the base class and there's no way to tell which subclass it is without doing repeated database queries on the subclass tables to see if they contain a record with a matching key ("I have an event. Does it have a record in AuthEvent? No. What about LDAP Event?…"). Among other things this means that you can't easily filter on them in normal queries on the base class without doing a join on every subclass table.
You have a couple of choices: one would simply be to do your queries on the subclass and tally the results (ldap_event_count = LDAPEvent.objects.filter(user=foo).count(), …), which might be sufficient for a single report. I usually recommend adding a content type field to the base class so you can efficiently tell which particular subclass an instance is without having to do another query:
content_type = models.ForeignKey("contenttypes.ContentType")
That allows two major improvements: the most common one is that you can deal with many Events generically without having to do something like hit the subclass-specific accessors (e.g. event.authevent or event.ldapevent) and handling DoesNotExist. In this case it would also make it trivial to rewrite your query since you could just do something like Event.objects.aggregate(Count("content_type")) to get the report values, which becomes particularly handy if your logic gets more complicated ("Event is Auth or LDAP and …").