I have a catalog and the price is a one to many relationship, so that I can track the price of a catalog item over time. The model looks like:
class CatalogItem(db.Model)
id = db.Column(db.Integer(), primary_key=True)
name = db.Column(db.String(250))
price = db.relationship("Price", back_populates="catalogitem")
class Price(db.Model):
id = db.Column(db.Integer(), primary_key=True)
price = db.Column(db.Float())
timestamp = db.Column(db.DateTime(), server_default=func.now(), nullable=False)
catalogitem_id = db.Column(db.Integer(), db.ForeignKey("catalogitem.id"))
catalogitem = db.relationship("CatalogItem", back_populates="material_einheitspreis_verkauf")
And this is my View. At least I managed to only show the price.
class CatalogItemView(ModelView):
inline_models = [(Price, dict(form_columns=["id","price"]))]
There are two issues:
When I render a catalog item and set price as inline model, I can do that just fine, but the default behavior would allow me to add multiple prices. What I would actually like to do is to have just a price field. I'm looking for a way to limit the form to just one entity (and also leaving away the button "add price".
When editing a catalogitem, it shouldn't edit the price, but actually create a new relationship -> basically when I edit the price it will create a new Price entity.
For 1 I have not idea on how to achieve this. For 2 I guess I could maybe do this by adding some kind of additional form field outside of the model and then create the "magic" with some kind of listeners.
Any other ideas?
Related
I want to inquire data field from model which contains another model in ManyToMany relationship.
For instance, I want to calculate total cost from Dinner model, which has ManyToMany relationship with Menu model. Here goes simplified code.
class Menu(models.Model):
cost = models.IntegerField()
class Dinner(models.Model):
menus = models.ManyToManyField(Menu)
objects = DinnerManager()
class DinnerManager(models.Manager):
def get_total_cost(self):
total_cost = 0
for each_menu in self.menus.all():
total_cost += each_menu.cost
return total_cost
So my question is this: how should I set
for each_menu in self.menus.all():
total_cost += each_menu.cost
these two lines to get my class method work?
(or if other parts is wrong, please let me know. I'm fairly new to django..)
This code would work fine, except that it belongs on the Dinner model, not the Manager. It's relating to a specific dinner, not dinners generally, and needs to calculate the value of the menu items related to that individual dinner.
Say I have a model like
class Product(models.Model):
name = models.CharField(max_length=64)
[...]
and another like
class Price(models.Model):
product = models.ForeignKey('Product')
date = models.DateField()
value = models.FloatField()
[...]
and I want to display products in a modelAdmin, with the last price registered in a column.
So far, the only way I have found is to add the following method to the product object:
#property
def last_price(self):
price = Price.objects.filter(product=self.pk).order_by("-date")
return price[0].value if price else None
and then adding last_price to the list_display in Product's ModelAdmin. I don't like this approach because it ends up doing a query for each row displayed.
Is there a better way to do this in code?. Or do I have to create a column in the Product table and cache the last price there?.
Thanks!
to reduce the the queries for each entry use the following:
Price.objects.filter(product=self.pk).order_by("-date").select_related('product')
this will decrease the product query at each object, hope it is helpful, vote up please
A cleaner version of what you have would be:
def last_price(self):
latest_price = self.price_set.latest('date')
return latest_price.value if latest_price else None
But this still involves queries for each item.
You if you want to avoid this I would suggest adding a latest_price column to Product. Then you could set up a post_save signal for Price that then updates the related Product latest_price (This could be a ForiegnKey or the value itself.)
Update
Here is a receiver that would update the products latest price value when you save a Price. Obviously this assumes that you are saving Price models in chronological order so the lastest one saved is the latest_value
#receiver(post_save, sender=Price)
def update_product_latest_value(sender, instance, created, **kwargs):
if created:
instance.product.latest_value = instance.value
instance.product.save()
I am reading Excel using xlrd. One of the columns has the Bank name, which is linked to vehicle model via Foreign Key. When xlrd finishes reading a row, it should save that record to vehicle table. However getting the actual pk value and error that Vehicles.bank must a Banks instance.
After checking dozens of questions related to this issue, I found this one the most similar one, but still I am not getting the expected result.
The relevant Vehicle model section is as follows:
class Vehicles(models.Model):
stock = models.CharField(max_length=10, blank=False, db_index=True)
vin = models.CharField(max_length=17, blank=False, db_index=True)
sold = models.DateField(blank=True, null=True, db_index=True)
origin = models.CharField(max_length=10, blank=False, db_index=True)
bank = models.ForeignKey('banks.Banks', db_column='bank', null=True)
I am using python 2.7, django 1.5.4 and Postgresql 9.2.5. Dbshell utility does show that banks table has a Foreign contraint referring to vehicles table, via banks(id).
Since I am not using a form for this particular part, I think it does not matter whether I use a ModelForm or not.
Current scenario: Excel file has FBANK as the cell value. There is an existing record in banks table that contains FBANK in its name column, id=2. The python line is:
def bank(value):
return Banks.objects.get(name=value).id
With the above line, error is:
Cannot assign "2": "Vehicles.bank" must be a "Banks" instance.
If I remove the ".id" at the end, error is then:
Banks matching query does not exist.
Appreciate your help.
Ricardo
When saving Vehicle you need to pass Banks instance with corresponding bank name. See example, I suppose that you have all data in corresponding cells from 0 to 4, replace with your own cells numbers:
def get_bank_instance(bank_name):
try:
bank = Banks.objects.get(name=bank_name)
except Banks.DoesNotExist:
return None
return bank
# reading excel file here, we have list of cells in a row
for cell in cells:
bank = get_bank_instance(cell[4])
if bank:
# get other cells values to be saved in Vehicles
stock, vin, sold, origin = cell[0], cell[1], cell[2], cell[3]
Vehicles.create(bank=bank, stock=stock, vin=vin, sold=sold, origin=origin)
You also can create save instance of Vehicles passing bank id directly:
b_id = Banks.objects.get(name=bank_name).id
Vehicles.create(bank_id=b_id, stock=stock, vin=vin, sold=sold, origin=origin)
Update:
create() is a built-in model method to create and save into database model instance. If you are asking about "Add a classmethod on the model class" in Django docs, this is not the case, because you are just using built-in method for the model. For some cases you can use custom method for creating new models, but I would do so if I had to pass a lot of default attributes for the new instance.
Also, it's possible to create and save new model instance by using save():
bank_instance = Banks.objects.get(name=bank_name)
vehicle = Vehicles()
vehicle.bank = bank_instance
vehicle.stock = stock
vehicle.vin = vin
vehicle.sold = sold
vehicle.origin = origin
# without save() data will not be saved to db!
vehicle.save()
It's quite long and you always need to remember to call .save(), so it's a good idea to use .create()
You should be returning a Banks instance when you want to assign it to a Vehicle model instance; so you should not have the .id part at the end of the return value for your bank() method.
Secondly, if it says that it isn't finding the Banks instance, then you should check the value of your value parameter to see what it is and try to manually do a Banks.objects.get from your database. If it can't be found then there is probably another reason for this other than using the Django ORM incorrectly.
When you are assigning instances to other instances in Django, for example setting the Bank for a Vehicle it must be an instance of the model and not the id or pk value of model; this is stated in the other StackOverflow question that you reference in your question.
i have three classes
Product have many Descriptions and each model have many stores
what i want to do
select all products but store.qty value > 0
I've tried
pr = Product.objects.all().exclude(Product__Product_description__qty > 0)
how can i do that ?
class Product
id = models.AutoField(primary_key=True)
name = models.CharField(max_length=255)
class Product_description
id = models.AutoField(primary_key=True)
name = models.CharField(max_length=255)
product = models.ForeignKey(Product)
class Store
id = models.AutoField(primary_key=True)
name = models.CharField(max_length=255)
desc = models.ForeignKey(Product_description)
qty = models.IntegerField()
pr = Product.objects.filter(Product_description__qty__lte = 0)
Or if you really must use exclude:
pr = Product.objects.exclude(Product_description__qty__gt = 0)
all() is not necessary in either case; you just end up building an untriggered proxy that goes into building the filter/exclude queryset afterward. It wastes memory and CPU, but otherwise does nothing. Only the .delete() operator requires a working all() queryset, but it's a special case designed explicitly to avoid the accidental destruction of datasets.
The Django Queryset API documentation is very readable.
Django convention is to name your class ProductDescription.
This seems like a backward hierarchy. Why would stores have "product descriptions?" Isn't that metadata on the product itself, and what you care about is that the stores have a certain quantity of product? Or are these product variants, i.e you want to find all the products for which stores have at least one green or blue or orange one? Something tells me that your project needs a careful re-think.
Previously had a go at asking a more specific version of this question, but had trouble articulating what my question was. On reflection that made me doubt if my chosen solution was correct for the problem, so this time I will explain the problem and ask if a) I am on the right track and b) if there is a way around my current brick wall.
I am currently building a web interface to enable an existing database to be interrogated by (a small number of) users. Sticking with the analogy from the docs, I have models that look something like this:
class Musician(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
dob = models.DateField()
class Album(models.Model):
artist = models.ForeignKey(Musician)
name = models.CharField(max_length=100)
class Instrument(models.Model):
artist = models.ForeignKey(Musician)
name = models.CharField(max_length=100)
Where I have one central table (Musician) and several tables of associated data that are related by either ForeignKey or OneToOneFields. Users interact with the database by creating filtering criteria to select a subset of Musicians based on data the data on the main or related tables. Likewise, the users can then select what piece of data is used to rank results that are presented to them. The results are then viewed initially as a 2 dimensional table with a single row per Musician with selected data fields (or aggregates) in each column.
To give you some idea of scale, the database has ~5,000 Musicians with around 20 fields of related data.
Up to here is fine and I have a working implementation. However, it is important that I have the ability for a given user to upload there own annotation data sets (more than one) and then filter and order on these in the same way they can with the existing data.
The way I had tried to do this was to add the models:
class UserDataSets(models.Model):
user = models.ForeignKey(User)
name = models.CharField(max_length=100)
description = models.CharField(max_length=64)
results = models.ManyToManyField(Musician, through='UserData')
class UserData(models.Model):
artist = models.ForeignKey(Musician)
dataset = models.ForeignKey(UserDataSets)
score = models.IntegerField()
class Meta:
unique_together = (("artist", "dataset"),)
I have a simple upload mechanism enabling users to upload a data set file that consists of 1 to 1 relationship between a Musician and their "score". Within a given user dataset each artist will be unique, but different datasets are independent from each other and will often contain entries for the same musician.
This worked fine for displaying the data, starting from a given artist I can do something like this:
artist = Musician.objects.get(pk=1)
dataset = UserDataSets.objects.get(pk=5)
print artist.userdata_set.get(dataset=dataset.pk)
However, this approach fell over when I came to implement the filtering and ordering of query set of musicians based on the data contained in a single user data set. For example, I could easily order the query set based on all of the data in the UserData table like this:
artists = Musician.objects.all().order_by(userdata__score)
But that does not help me order by the results of a given single user dataset. Likewise I need to be able to filter the query set based on the "scores" from different user data sets (eg find all musicians with a score > 5 in dataset1 and < 2 in dataset2).
Is there a way of doing this, or am I going about the whole thing wrong?
edit: nevermind, it's wrong. I'll keep it so you can read, but then I'll delete afterward.
Hi,
If I understand correctly, you can try something like this:
artists = Musician.objects.select_related('UserDataSets').filter( Q(userdata__score_gt=5, userdata__id=1) | Q(userdata__sorce_lt=2, userdata__id=2 )
For more info on how to use Q, check this: Complex lookups with Q objects.