my problem is the following. I have two models in the database, which I link together using a ManyToMany relationship. For the admin page I currently use "admin.TabularInline" to bind different objects to one via the graphic. I still want to specify an order in the connections, preferably numbers which represent an order for processing. A bit more figuratively described I have the model "Survey" and the model "SurveyQuestion". So I connect many SurveyQuestions with the Survey. But I can't specify an order, because I don't have an additional field for it. It is not known before how many questions will be in a survey. Nor is it known which questions will be inserted. Usually they are built during the compilation of the survey and may be used later for another survey. I am grateful for every tip!
This can be achieved by defining a custom relationship table between the Survey and SurveyQuestion using through argument. For example you can define a relationship model:
class Question(models.Model):
question = models.CharField(max_length=256)
class Survey(models.Model):
name = models.CharField(max_length=256)
questions = models.ManyToManyField(Questions, through='Questionnaire')
class Questionnaire(models.Model):
survey = models.ForeignKey(Survey, on_delete=models.CASCADE)
question = models.ForeignKey(Question, on_delete=models.CASCADE)
question_order = models.IntegerField()
The details and example can be found here:
https://docs.djangoproject.com/en/3.1/topics/db/models/#extra-fields-on-many-to-many-relationships. If you do not want to mess up with the models, then you have to find out some hack like was proposed by Ronak Muhta.
Related
I've been reading through the Django documentation and looking over some of the other answers on the site for a couple of hours now, yet I still can't get it to sink in. I know this isn't Django specific, but the examples I use will be from a Django project.
My question boils down to when is it appropriate to use each:
Many-to-many relationships
Many-to-one relationships
One-to-one relationships
One-to-one, more or less makes sense to me.
Now for the other two. While I understand the differences between them in isolation, when it comes to using them practically in a project, I get confused. Here is an example:
class User(AbstractUser):
pass
class Listing(models.Model):
title = models.CharField(max_length=64)
description = models.TextField()
class Watchlist(models.Model):
user = models.ForeignKey(User, related_name='watchlist', on_delete=models.CASCADE)
item = models.ManyToManyField(Listing)
class Comment(models.Model):
user = models.ForeignKey(User, related_name='comments', on_delete=models.SET_NULL)
comment = models.TextField()
Would this be the correct use of Many-to-one(ForeignKey) and Many-to-many?
Should Watchlist.item be a ForeignKey? Or is M2M correct?
Wouldn't it simplify to make the 'Watchlist' part of the User class? (give them an empty list to populate with listing ID's)
Why is Watchlist.user not a One-to-one relationship, if each watchlist belongs to a single user, and a user can only have one list?
Apologies for my stupidity, I just can't get this to sink in!
Thank you.
edit: Context, the models are from a 'learning' project I was working on intended to be an auction site, similar to eBay. The watchlist is sort of a 'wish' list... for the user to watch an item, not for site to watch a user!
To explain it simply these django-models or objects represents tables in your database and the fields are like the columns in them. So with a one-to-one relation you can only have one row in one table relating to one row in another table. For example one user in the user table (represented by one row) can only relate to one row in a profile table. But your user can have many comments, so this would be a one-to-many/foreignkey relation (if you set unique=true on a fk, it will in practice function as 1:1). If the users can collaborate on writing comments, or for example as here on stackoverflow where users can edit other users comments, that would be a many-to-many relation.
Database design can be complicated/complex, especially using an ORM without basic knowledge of SQL and how it all works beneath. In general it requires a bit of planning even for a simple application.
A version of this question has been asked here several times but none of the answers provided solve my exact problem.
I'm trying to bulk_create a batch of objects of a model with a ManyToMany field.
In this case, the ManyToMany field refers to the same model, though I'd also be interested in the general case.
Let's say this is my model:
class Person(models.Model):
name = models.CharField(max_length=20, blank=True, null=True)
friends = models.ManyToMany("self", related_name="friends_with", null=True)
After bulk_creating a large number of Person objects, I want to add the information who's friends with whom within this group.
Is there a more efficient way to go about this than looping through each new Person and calling .set(friend_pks) or .add(*friend_pks)?
I.e., an analogue of bulk_update.
I've achieved some speed-up by wrapping the loop into with transaction.atomic() (from this answer) but it's still quite slow.
Okay, my post was premature -- it seems that this answers the question.
The key is to bulk_create the through models. In this example:
friends_relation_1 = Person.friends.through(from_person_id=1, to_person_id=2)
friends_relation_2 = Person.friends.through(from_person_id=2, to_person_id=8)
Person.friends.through.objects.bulk_create([friends_relation_1, friends_relation_2, ...])
I know the foreign key concept,Djongo Array field , however is there an alternative?
The issue with the foreign key concept is that I would need to make multiple hits to the database and the issue with Array field is the lack of information and the errors emerging without a known solution.
What I would like to do basically is in fact add multiple instances of a model say comments to a single field in another model but I would like to embed it rather than creating a new table for comments which I tried using an abstract model however it did not work out.
I would like to know any alternative solution.
You can use foreign keys, and to avoid making separate query for every related record you can extract them all at once using prefetch_related - https://docs.djangoproject.com/en/3.1/ref/models/querysets/#prefetch-related :
Returns a QuerySet that will automatically retrieve, in a single batch, related objects for each of the specified lookups.
Code example:
# models.py
class Post(models.Model):
title = models.CharField(max_length=100)
content = models.TextField()
class Comment(models.Model):
post = models.ForeignKey(Post, models.CASCADE)
text = models.TextField()
# Somewhere else, fetch posts together with related comments:
# It will take 2 requests in total
# - one to fetch posts, another to fetch all related comments
for post in Post.objects.all().prefetch_related('comment_set'):
print([c.text for c in post.comment_set.all()])
I want to create an Instruction that has multiple Steps. A simplified model would look like this:
class Step(models.Model):
description = models.CharField(max_length=255)
class Instruction(models.Model):
steps = models.ForeignKey(Step)
The problem is that I want to create a new Instruction with multiple steps, but when I create one in the admin, I should have a repeatable form field. For each step I can add a field and create a new Step. I do not need to be able to select an already existing step. I'm not sure if there is something OOTB of a package that already does that... Any ideas how to tackle this?
To give an example of what I'm trying to accomplish: the ACF repeater field in WP:
In my case I would only need a description field with the description of the step
You have things a bit backwards. The ForeignKey relationship should be the other way around (since an instruction can have many steps, but each step only has one related instruction...a Many-to-One relationship).
class Step(models.Model):
description = models.CharField(max_length=255)
instruction = models.ForeignKey(Instruction, related_name='steps')
class Instruction(models.Model):
# some fields
Now, in your Admin, you can use inlines to display these fields in a "repeatable" manner, similar to ACF.
I have a fairly simple model in Django, let's say Book:
class Book(models.Model):
name = models.CharField(max_length=255, default="")
description = models.TextField(null=True, blank=True)
creation_date = models.DateTimeField()
Because the structure and data basically comes from an external API, I have contained it in its own app and I rather not want to modify it.
Now I want to add another source of different books to the project and I'm not really sure what the best solution is here.
Let's say the second model is:
class NextBook(models.Model):
title = models.CharField(max_length=255, default="")
long_description = models.TextField(null=True, blank=True)
created = models.DateTimeField()
So the basic fields are there, but have different names. To get the two together, I can probably use another model with a GenericForeignKey:
class BaseBook(models.Model):
book_type = models.ForeignKey(ContentType)
book_id = models.PositiveIntegerField()
book_object = generic.GenericForeignKey('book_type', 'book_id')
But then I cannot query for all the book as e.g. BaseBook.objects.all().order_by('created') wouldn't work. Surely, I could go on and duplicate a datetime field in the base model and then the title and so on, but I think that would be the wrong direction. Also inheritance seems not a choice if I don't want to touch the specific models.
I am looking for some design pattern or something that let's me efficiently 'plug in' more providers and query for all objects, while not making a huge mess of model structure and database queries.
There’s a fundamental contradiction in your question. Do you assume that, for a given book, the created reported by different providers will be the same?
If yes, why can’t you make a created field on your BaseBook and order by that? You can then drop the creation_date from your Book (or just ignore it, if you don’t want to touch it).
If not, how do you want to order books (not provider entries) by created in the first place?
Also, what you’re trying to do sounds like a good case for a free-schema database like MongoDB. There, you can embed provider entries directly in your book documents, then order by something like “created from the first provider entry for the book”. Thus, you maintain self-contained provider documents without denormalization. You still need created to mean the same thing for all providers, though.