consider these models:
class Author(models.Model):
name = models.CharField(max_length=200)
class Book(models.Model):
author = models.ForigenKey(Author)
name = models.CharField(max_length=200)
created_at = models.DateTimeField(auto_now_add=True)
active = models.BooleanField(default=True)
I want to query all the Authors, with the latest(e.g Max('created_at') ) book of them in the same row, but only when the book is active=True.
I need to whole related book object - not just the max date.
If there is no such a book - all are active=False, or they simply do not exist - the query should output NULL where needed instead of the a book .
I've tried to write something like this:
Author.objects.annotate(max_book_date=Max('book'))
which does work, but it's missing additional fields on the book, and I'm not sure how it works in case of null (i.e no books), and the active=True condition is not there..
First of all, you should define a related_name param in the author fk field on the Book model:
author = models.ForeignKey(Author, related_name='libro')
Then, you should be able to do something like:
Author.objects.filter(libro__active=True).annotate(max_book_date=Max('libro__created_at')).all()
Related
I'm trying to access the grandchildren records in a list to avoid duplicate records. In this example, a tag can only be used once across articles for a given author. I will use the resulting list of grandchildren records in my clean function to return validation errors.
class Article(models.Model):
tag = models.ManyToManyField(Tag)
author = models.ForeignKey(Author, on_delete=models.CASCADE)
class Tag(models.Model):
class Author(models.Model):
Right now I can do this:
print(author.articles.first().tag.first())
Travel
I'd like to be able to use something like author.articles.tags.all() to return the list and check the submitted form against it to raise a ValidationError message to the user.
How can this be done efficiently with the basic Many-to-Many setup without creating an intermediate table for the tag relationships? This is solely in the Admin interface, in case that matters at all.
i come from the link you posted on upwork,
the way i understand your question,
what you want to achieve seems to be impossible ,
what i think can work , is to fetch articles related to the author, with their corresponding tags,
after that they are retrieved you do filtering and remove duplicates.
otherwise the tag has nothing to connect it with the author,,
I'm so jealous that's a great answer #since9teen94
Be aware, I will not base my answer in the easiest solution but how we model reality (or how we should do it).
We put tags after things exists right?
In that order I will make your life horrible with something like this:
from django.db import models
class Author(models.Model):
name = models.CharField(max_length=30, null=False)
class Article(models.Model):
name = models.CharField(max_length=30, null=False)
author = models.ForeignKey(Author, on_delete=models.CASCADE)
class Tag(models.Model):
name = models.CharField(max_length=30, null=False, unique=True)
articles = models.ManyToManyField(Article)
but believe me you don't want to follow this approach it will make your work harder.
You can search for Articles based on Tags directly
Tag.objects.filter(name='Scify').first().articles.all() # name is unique
The real issue with this is that the reverse lookup is really complex (I mean.. get ready for debug)
Article.objects.filter(
id__in=list(
Tag.objects.filter(name='Scify').first().articles.values_list('id', flat=True)
)
)
I am sure this does not solve your problem and I don't like complex code for no reason but if you're open to suggestions I don't mind to think different and add some options
Edit:
About the author and clean repeated tags.. well you don't have to deal with that and if you want to find all Tag your author has you could loop the tags
for tag in Tag.objects.all():
if tag.articles.filter(author__name='StackoverflowContributor'):
print(tag.name)
# > Scify
I am just saying that there are options not that this is the best for you but don't be afraid of the terminal, it's really cool
The Django ORM is pretty cool when you get used to it, but regular SQL is pretty cool too
# created_at, updated_at, name and tag_val are variables I
# added due to my slight ocd lol
class Author(models.Model):
name = models.CharField(max_length=255)
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(auto_now=True)
class Tag(models.Model):
tag_val = models.CharField(max_length=255)
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(auto_now=True)
class Article(models.Model):
author = models.ForeignKey(Author,related_name='articles', on_delete=models.CASCADE)
tags = models.ManyToManyField(Tag, related_name='articles')
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(auto_now=True)
I can write my query like this, assuming the variable 'author' has been assigned an Author object instance, and get a list of dictionaries [{'tags':1},{'tags':2}] where the value is the auto generated primary key id of the Tag object instance
author.articles.values('tags').distinct()
Good day.
I wish to annotate my model with information from a different table.
class CompetitionTeam(models.Model):
competition_id = models.ForeignKey('Competition', on_delete=models.CASCADE, to_field='id', db_column='competition_id')
team_id = models.ForeignKey('Team', on_delete=models.CASCADE, to_field='id', null=True, db_column='team_id')
...
class Team(models.Model):
id = models.AutoField(primary_key=True)
name = models.CharField(max_length=30)
teamleader_id = models.ForeignKey('User', on_delete=models.CASCADE, to_field='id', db_column='teamleader_id')
...
class Competition(models.Model):
id = models.AutoField(primary_key=True)
name = models.CharField(max_length=30)
...
Looping through my competitions, I wish to retrieve the list of competitionteam objects to be displayed with the relevant team's name. I tried:
CompetitionTeam.objects.filter(competition_id=_competition.id).filter(team_id__in=joined_team_ids).annotate(name=...)
-where instead of the ellipses I put Subquery expressions in. However, I'm unsure of how to match the team_id variable. eg.
*.anotate(name=Subquery(Team.objects.filter(id=competitionteam.team_id)).values('name'))
Related is the question: Django annotate field value from another model but I am unsure of how to implement that in this case. In that case, in place of mymodel_id, I used team_id but it only had parameters from the Team object, not my competition team object. I didn't really understand OuterRef but here is my attempt that failed:
CompetitionTeam.objects.filter(competition_id=_competition.id).filter(team_id__in=joined_team_ids).annotate(name=Subquery(Team.objects.get(id=OuterRef('team_id'))))
"Error: This queryset contains a reference to an outer query and may only be used in a subquery."
The solution for my question was:
CompetitionTeam.objects.filter(
competition_id=_competition.id,
team_id__in=joined_team_ids
).annotate(
name=Subquery(
Team.objects.filter(
id=OuterRef('team_id')
).values('name')
))
Thanks.
I have created a model called Department, Course. Models are as follow
This is the model for departments and course
class Departments(models.Model):
Department_Id = models.IntegerField(primary_key=True)
Department_Name = models.CharField(max_length=200)
Department_Code = models.CharField(max_length=200)
class Course(models.Model):
Course_Id = models.IntegerField(primary_key=True)
Department_Id = models.ForeignKey(Departments, on_delete=models.CASCADE)
Course_Name = models.CharField(max_length=200)
Course_Code = models.CharField(max_length=200)
I want to create a model called view which can be later on called for search. I want a view model in a such a way that it consit of the data in concat form i.e. name= Department_name+ Course_Name
class View (models.model):
view_id= models.IntegerField(primary_key=True)
Name= Department_name(I want this from Departments table)
+ Course_Name(I want this from Course table)
I try using one to one relation . I would really appricate the help
It's not clear why you'd want to do that. It's never a good idea to duplicate data from one model into another one, as it can lead to inconsistencies.
You can add a ForeignKey in View to your Course model and then when you do f"{view.course.name} {view.course.department.name}" you already have your string:
class View(models.Model):
course = models.ForeignKey(Course, on_delete=models.CASCADE)
def name(self):
return f"{self.course.name} {self.course.department.name}"
Notes:
Don't call your foreign key Department_id because it's not referring to the id but to the object itself in the Django ORM: department = models.ForeignKey(Department, on_delete=models.CASCADE). As you can see, this makes reading the code much simpler: self.course.Department_id is a Department object not an integer, so self.course.department makes more sense.
Don't prefix your field names with the class, it just makes the code so much less readable: Do you prefer department.name or department.Department_name?
The View model is still a mystery to me, as you can search without it. You can search for example for courses with a matching department name like this:
Course.objects.filter(department__name__icontains="maths")
which will return all courses with "maths" in their department name.
Remove all the ids from your models, they are created automatically by Django anyway (and called id). Again, department.id is much easier to read than department.Department_id. Also in your code, you have to generate the ids yourself since you don't set them to auto-populate.
If having different books stored in django database, each book has a date in which it was added to the database. Is their a way of filtering books written by a certain author that was within a date range only using django views?
Not sure what you mean by only django views, I assume you want to use querysets. Your question is poorly written - read this.
class Book(models.Model):
title = models.CharField(max_length=200)
date = models.DateTimeField()
author = models.ForeignKey(Author)
class Author(models.Model):
name = models.CharField(max_length=200)
And Queryset would be something like this.
books = Book.objects.filter(author__name=authors_name,
date__range=["2011-01-01", "2011-01-31"])
Lets say I have following models:
class Book(models.Model):
name = models.CharField(max_length=70)
lang = models.CharField(max_length=70)
author = models.FK(Author)
class Author(models.Model):
name = models.CharField(max_length=70)
And I want to write something to get a list of authors with annotated field which shows amount of books in each language. Can't imagine an annotation for it :(, e.g. {'en': 10, 'ru': 1...etc}
e.g. just counts all, Author.objects.annotate(languages=Count(book__lang))
Simple annotation should help you:
Book.objects.values('lang').annotate(lang=Count('author')).order_by('lang')