I have a model "Item", which has a 1:n to "Location". Means, there is a location-history for items.
Location has a FK to "Room", "Room" to "Floor" and "Floor" to "Building".
Now, I want to select all Items which are currently located an a specific Floor.
I could solve it with a list comprehension, but is there any nicer way to solve it with one query?
-- Update --
Thanks for your answers, unfortunately, they do not match the requirement. To clarify the issue, here some code snippets of the models:
class Item(models.Model):
[..]
class Location(models.Model):
item = models.ForeignKey(Item)
room = models.ForeignKey(Room)
created_at = models.DateTimeField(auto_now_add=True)
[..]
class Room(models.Model):
floor = models.ForeignKey(Floor)
[..]
class Floor(models.Model):
building = models.ForeignKey(Building)
[..]
class Building(models.Model):
[..]
I want to get all items that are currently located on a specific floor, specified by a floor id (as you can see in the models, an item can be relocated). Thanks again.
I don't know your models exactly, but something like that should do it:
items_on_floor_one = Item.objects.filter(\
location__in=Location.objects.filter(room__floor__number=1,\
room__floor__building__name='my_building'))
items = Item.objects.filter(location__room__floor__name='building 1', location__room__floor__num=1)
Related
Can somebody please explain me how to represent a weak entity relationship in django?
i searched for it in django's documentation, but couldn't find. I even searched on google and stackoverflow before posting a question here.
In case its not possible to represent a weak entity relationship, then please let me know what is the most appropriate alternative to it in django
Update:
I was developing an e-learning site, so there's a Course class with attributes like title, description, creator, etc. and I want to add a weak entity called "Week" (to store week-wise content for the course) which would be attached to this strong entity called Course, and I wanted course_id from Course class and week_no from Week class to act as a primary key for each entry in Week class
Well, let's take this example from wikipedia
You have a class Order and a class Product.
You'd then have a class OrderItem which would be the weak entity.
class Order(models.Model):
some_attributes
class Product(models.Model):
some_other_attributes
class OrderItem(models.Model)
order = models.ForeignKey(Order)
product = models.ForeignKey(Product)
other_attributes
class Meta:
unique_together = (order, product)
the unique_together meta property would make sure each OrderItem's won't have more than a database entry where both these values are repeated.
I reckon this may not match exactly what it you're looking for, but it may be a start. If you provide more details on what's you're trying to accomplish, perhabs I can help with some table tweaking or even query examples for getting data using this approach.
edit:
You are correct, there is no such field as weaker entity field. My sugestion is that you treat the week model as you would any other. And link it to the Course model, like so:
class Course(models.Model):
title = models.CharField()
description = models.CharField()
etc..
class CourseWeek(models.Model):
course = models.ForeignKey(Course)
week_number = models.IntegerField()
topic = models.CharField()
slideshow = models.FileField()
class Meta:
unique_together = ('course' , 'week_number')
Hope this helps :)
I have models like this:
class Vendor(models.Model):
title = models.CharField()
class Product(models.Model):
...
vendor = models.ForeignKey(Vendor, null=True, blank=True)
stock = models.ManyToManyField(Supplier, through='Stock')
class Stock(models.Model):
in_stock = models.BooleanField(default=True)
supplier = models.ForeignKey('catalog.Supplier', related_name='supplier_stock')
product = models.ForeignKey('catalog.Product', related_name='product_stock')
priority = models.IntegerField(default=0)
I designed models like this, because one Product can be supplied by different suppliers, and I need to know, what supplier exactly has this Product in stock.
So, in my view I want to get all results in values, to reduce number of queries and some specific logic. Also it duplicates me Product row with different Stock, by in python I group them up.
In my view I use:
Product.objects.all().values(
'id', 'title', 'vendor_code', 'vendor__title', 'price',
'product_stock__in_stock', 'stock__title', 'stock__id', 'stock__priority')
Because of INNER JOIN and null=True for Vendor related model, it returns me not all records for Product model. It just returns values where Vendor reference is set.
If I use 'vendor' instead of 'vendor__title' it returns me more results, than previous one, because in vendor field I can get {...'vendor': *id goes here*...} or {...'vendor': None...}, but I need the vendor__title value there. So any suggestions, how to achieve this?
Thanks in advance
Changed from vendor__title to product_stock__product__vendor__title helped me to fix my problem.
Given the following models
class Category(models.Model):
name = models.CharField(max_length=50)
class Business(models.Model):
name = models.CharField(max_length=50)
category = models.ForeignKey(Category, related_name="businesses")
class Package(models.Model):
business_id = models.ForeignKey(Business)
status = models.CharField(max_length=50)
I have 2 following queries get list of business and categories which the packages are live:
filter_businesses = Business.objects.filter(package__status = 'live')
filter_categories = Category.objects.filter(businesses__package__status = 'live')
Now the questions is, given the related name "businesses" should be equals to category.business_set, why shouldn't the filter in first query be package_set?
Suppose you have two related models: SomeModel and SomeOtherModel, and SomeOtherModel.somemodel is a ForeignKey to SomeModel.
Given any SomeModel instance, the someothermodel_set property is a manager for the related model already filtered. For example:
>>> your_some_model_instance = SomeModel.objects.all()[0]
In this case your_some_model_instance.shomeothermodel_set is equivalent to:
>>> SomeOtherModel.objects.filter(somemodel=your_some_model_instance)
[ update ]
sorry perhaps I didn't explain my questions more clearer, it's complicated to explain... I understand that XX_set and related_name refer to the manager, what I want to ask is in the first query why not use (package_set_status = 'live') given the second working query (businesses_package__status = 'live'), it's confusing because the second query references to the manager(by related_name), but the first query is not...
The filter interface uses the convention relatedmodelname__relatedmodelfield; In your example, related_name was used to give a fancier name to the backreference, but this is not its main purpose; the purpose of the related_name parameter in ForeignKey fields is solving the ambiguity in cases where relatedmodelname clashes with an already existing field at the ForeignKey.
To describe the system quickly, I have a list of Orders. Each Order can have 1 to n Items associated with it. Each Item has a list of ItemSizes. Given the following models, which have been abbreviated in terms of fields for this question, my goal is to get a distinct list of ItemSize objects for a given Order object.
class ItemSize(models.Model):
name = models.CharField(max_length=10, choices=SIZE_CHOICES)
class Item(models.Model):
name = models.CharField(max_length=100)
sizes = models.ManyToManyField(ItemSize)
class OrderItem(models.Model):
order = models.ForeignKey(Order)
item = models.ForeignKey(Item)
class Order(models.Model):
some_field = models.CharField(max_length=100, unique=True)
So... if I have:
o = Order.objects.get(id=1)
#how do I use the ORM to do this complex query?
#i need o.orderitem_set.items.sizes (pseudo-code)
In your current set up, the answer by #radious is correct. However, OrderItems really shouldn't exist. Orders should have a direct M2M relationship with Items. An intermediary table will be created much like OrderItems to achieve the relationship, but with an M2M you get much simpler and more logical relations
class Order(models.Model):
some_field = models.CharField(max_length=100, unique=True)
items = models.ManyToManyField(Items, related_name='orders')
You can then do: Order.items.all() and Item.orders.all(). The query you need for this issue would be simplified to:
ItemSize.objects.filter(item__orders=some_order)
If you need additional data on the Order-Item relationship, you can keep OrderItem, but use it as a through table like:
class Order(models.Model):
some_field = models.CharField(max_length=100, unique=True)
items = models.ManyToManyField(Items, related_name='orders', through=OrderItem)
And you still get your simpler relationships.
ItemSize.objects.filter(items__orderitems__order=some_order)
Assuming you have reverse keys like:
ItemSize.items - reverse fk for all items with such size
Item.orderitems - reverse for all orderitems connected to item
Item.orders - you can guess ;)
(AFAIR that names would be choose by default, but I'm not sure, you have to test it)
More informations about reverse key queries are available in documentation.
I have 2 models Category and Item. An Item has a reference to a Category.
class Category(models.Model):
name = models.CharField(max_length=32)
class Item(model.Models):
name = models.CharField(max_length=32)
category = models.ForeignKey(Category)
sequence = models.IntegerField()
The sequence field is supposed to capture the sequence of the Item within a category.
My question is:
What Meta Options do i need to set on category and/or item such that when i do:
category.item_set.all()
that I get the Items sorted by their sequence number.
PS: I am now aware of a meta option called ordering_with_respect_to .. but it is still unclear how it works, and also i have legacy data in the sequence columns. I am open to data migration, if the right approach requires that.
What you're looking for is:
class Item(model.Models):
name = models.CharField(max_length=32)
category = models.ForeignKey(Category)
sequence = models.IntegerField()
class Meta:
ordering = ['sequence',]
That will ensure that Items are always ordered by sequence.
category.item_set.all().order_by('sequence')
Kinda late, and the previous answers don't solve my specific question, but they led me to an answer, so I'm gonna throw this in:
I need to sort my prefetch_related objects specifically for only one view, so changing the default ordering is no good (maybe a model_manager would do it, idk). But I found this in the docs.
I have the following models:
class Event(models.Model):
foo = models.CharField(max_length=256)
....
class Session(models.Model):
bar = models.CharField(max_length=256)
event = models.ForeignKey(Event)
start = models.DateTimeField()
....
class Meta:
ordering = ['start']
Now in a particular view, I want to see all the Events, but I want their Sessions in reverse order, i.e., ordering = ['-start']
So I'm doing this in the view's get_queryset():
from django.db.models import Prefetch
session_sort = Session.objects.all().order_by('-start')
prefetch = Prefetch('sessions', queryset=session_sort)
events = Event.objects.all().prefetch_related(prefetch)
Hope it helps somebody!
*BTW, this is just a simplified version, there are a bunch of filters and other prefetch_related parameters in my actual use case.