Querying across database tables - django

I've got Django tables like the following (I've removed non-essential fields):
class Person(models.Model):
nameidx = models.IntegerField(primary_key=True)
name = models.CharField(max_length=300, verbose_name="Name")
class Owner(models.Model):
id = models.IntegerField(primary_key=True)
nameidx = models.IntegerField(null=True, blank=True) # is Person.nameidx
structidx = models.IntegerField() # is PlaceRef.structidx
class PlaceRef(models.Model):
id = models.IntegerField(primary_key=True)
structidx = models.IntegerField() # used for many things and not equivalent to placeidx
placeidx = models.IntegerField(null=True, blank=True) # is Place.placeidx
class Place(models.Model):
placeidx = models.IntegerField(primary_key=True)
county = models.CharField(max_length=36, null=True, blank=True)
name = models.CharField(max_length=300)
My question is as follows. If in my views.py file, I have a Person referenced by name and I want to find out all the Places they own as a QuerySet, what should I do?
I can get this far:
person = Person.objects.get(name=name)
owned_relations = Owner.objects.filter(nameidx=nameidx)
How do I get from here to Place? Should I use database methods?
I'm also not sure if I should be using ForeignKey for e.g. Owner.nameidx.
Thanks and apologies for this extremely basic question. I'm not sure how to learn the basics of database queries except by trying, failing, asking SO, trying again... :)

The whole point of foreign keys is for uses like yours. If you already know that Owner.nameidx refers to a Person, why not make it a foreign key (or a OneToOne field) to the Person table? Not only do you get the advantage of referential integrity - it makes it impossible to enter a value for nameidx that isn't a valid Person - the Django ORM will give you the ability to 'follow' the relationships easily:
owned_places = Place.objects.filter(placeref__owner__person=my_person)
will give you all the places owned by my_person.
Incidentally, you don't need to define the separate primary key fields - Django will do it for you, and make them autoincrement fields, which is almost always what you want.

If u could redesign.Then
In owner nameidx can be a foreign key to Person(nameidx)
Placeref(structidx) could be a foreign key to Owner(structidx) and
Place(placeidx) could be a foreign key Place ref(placeidx)
Then u could deduce the place value easily..

Related

Django: Annotate with field from another table (one-to-many)

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.

Getting a queryset using a foreign key field in the "other side" of a foreign key relation

Forgive me if the question does not make sense, trying to teach myself django. I've been trying to search how to do this but i'm not sure if i'm using the right words in my search.
I have the following models.
class Category(models.Model):
code = models.CharField(max_length=10, unique=True)
description = models.CharField(max_length=50)
class UserGroupHeader(models.Model):
code = models.CharField(max_length=10, unique=True)
description = models.CharField(max_length=50)
class UserGroupDetail(models.Model):
usergroupheader = models.ForeignKey(UserGroupHeader, on_delete=models.CASCADE)
category = models.ForeignKey(Category, on_delete=models.PROTECT)
How do i get a query set from the Category model using the UserGroupHeader? so far what i've got is something like this UserGroupHeader.objects.get(pk=9).usergroupdetail_set.all(), now from the result of this how do i get the Category model?
I'm not sure if I understood exactly what you are trying to do, but in general, while querying, you can follow relations using double underscores. Below are a couple of possible queries:
my_group_header = UserGroupHeader.objects.get(...)
Category.objects.filter(usergroupdetail__usergroupheader=my_group_header) # Gets Category objects related to my_group_header through UserGroupDetail model
Category.objects.filter(usergroupdetail__usergroupheader__code='abc') # Gets Category objects related to UserGroupHeader object with code 'abc' through UserGroupDetail model
UserGroupHeader.objects.filter(usergroupdetail__category__code='abc') # Gets UserGroupHeader objects related to Category object with code 'abc' through UserGroupDetail model
Your query UserGroupHeader.objects.get(pk=9).usergroupdetail_set.all() would return a QuerySet of UserGroupDetail objects. In order to get the category of each UserGroupDetail, you can:
for user_group_detail in UserGroupHeader.objects.get(pk=9).usergroupdetail_set.all():
category = user_group_detail.category
print(category. description)
Or something similar according to your needs

m2m for existing table (through/unique_together)

I have found in internet different examples on how to handle m2m relations with existing DB models, such as ex1 or here ex2, however I'm still not able to solve the error I get.
My models are depicted below. Basically, all the tables where created manually.
I got the following error message:
OperationalError: (1054, "Unknown column 'supervisor_project.id' in 'field list'").
I'm still a bit confused on when to use unique_together with through. Do you see any errors in the model below? The table supervisor_project has no id field and its PK is composed actually of two FK's, i.e. surrogate PK.
class Supervisor(models.Model):
name = models.CharField(max_length=45, blank=True, null=True, help_text="Name, e.g. John Smith")
class Meta:
managed = False
db_table = 'supervisor'
def __unicode__(self):
return self.name
class Project(models.Model):
title = models.CharField(max_length=45, blank=True, null=True)
supervisors = models.ManyToManyField(Supervisor, through='SupervisorProject', through_fields=('project', 'supervisor'))
class SupervisorProject(models.Model):
supervisor = models.ForeignKey('Supervisor', on_delete=models.CASCADE)
project = models.ForeignKey('Project', on_delete=models.CASCADE)
class Meta:
managed = False
db_table = 'supervisor_project'
unique_together = (('supervisor', 'project'),)
Django requires each model to have exactly one primary key field. It doesn't support multiple-column primary keys yet.
Since you haven't explicitly defined a primary key on the SupervisorProject model, Django assumes that there is an automatic primary key field id. When it includes the id field in a query, you get the error because it doesn't exist.
If possible, I would add an auto-incrementing id column to each intermediate table. There isn't a way to get Django to add the column to the tables automatically. You have set managed=False, so Django expects you to manage the database table.

Django Spanning multi-valued relationships

I have the following models:
class Sked(models.Model):
pass
class Class(models.Model):
class_key = models.CharField(max_length=20, null=True)
name = models.CharField(max_length=150)
class Sked_class(models.Model):
class_room = models.CharField(max_length=100, null=True)
sked = models.ForeignKey(Sked)
class_took = models.ForeignKey(Class)
class User(djmodels.User):
sked = models.ForeignKey(Sked, null=True)
I want to make a query that selects all the classes that a specific user is taking, but i still can't get the idea of how to do it without using SQL, I already read this document https://docs.djangoproject.com/en/dev/topics/db/queries/ , but I still don't get it, How can I span multi-valued relationships through this models?
You're looking for two things - how to span foreign keys, and how to only return a unique set of classes that doesn't include duplicates.
You span foreign keys using __ to separate the relationships, and use distinct() on the query set to filter out duplicates. Remember that foreign key relationships work both ways with Django syntax, as the ORM will recognize reverse relationships. This should work:
user = User.objects.get(id=1)
Class.objects.filter(sked_class__user=user).distinct()
It's unclear to me if you data model makes sense, however. I think this makes more sense:
class Class(models.Model):
key = models.CharField(max_length=20, null=True)
name = models.CharField(max_length=150)
class Schedule(models.Model):
student = models.ForeignKey(User)
classes = models.ManyToManyField(Class)
Then you'd say:
user = User.objects.get(id=1)
Class.objects.filter(schedule__student=user)

Django: foreign key queries

I'm learning Django and trying to get the hang of querying foreign keys across a bridging table. Apologies if this is a duplicate, I haven't been able to find the answer by searching. I've got models defined as follows
class Place(models.Model):
id = models.IntegerField(primary_key=True)
name = models.CharField(max_length=100)
class PlaceRef(models.Model):
place = models.ForeignKey(Place) # many-to-one field
entry = models.ForeignKey(Entry) # many-to-one field
class Entry(models.Model):
id = models.IntegerField(primary_key=True)
name = models.CharField(max_length=10)
If I want to retrieve all the Entries associated with a particular Place, how do I do it?
place = get_object_or_404(Place, id=id)
placerefs = PlaceRef.objects.filter(place=place)
entries = Entry.objects.filter(id.....)
Also, if there is a more sensible way for me to define (or get rid of) PlaceRefs in Django, please feel free to suggest alternatives.
Thanks for helping out a beginner!
First, I'd suggest rewriting the models to:
class Place(models.Model):
id = models.IntegerField(primary_key=True)
name = models.CharField(max_length=100)
class Entry(models.Model):
id = models.IntegerField(primary_key=True)
name = models.CharField(max_length=10)
places = models.ManyToManyField(Place, related_name='places')
So you can query:
Entry.objects.filter(places__id=id)
In your current model:
Entry.objects.filter(placeref_set__place__id=id)
Note that the double underscore __ is used to jump from one model to the next. Also, django creates some fields on the model that help you navigate to related objects. In this example: Entry.placeref_set. You can read more about it here:
http://docs.djangoproject.com/en/dev/topics/db/queries/#following-relationships-backward