django howto use select_related not using local id key - django

i have made two models where i would like to get them joined on the two name keys.
models.py
class MODELa (models.Model):
......
nameXX = models.CharField(_("name"), max_length=255, primary_key=True)
class MODELb (models.Model):
......
nameYY = models.CharField(_("nameYY"), max_length=255)
FKxx = models.ForeignKey(to=MODELa, on_delete=models.CASCADE, null=True)
views.py
rows = MODELb.objects.all().select_related('FKxx')
using debug toolbar, i can see the join is using the id field
LEFT OUTER JOIN "MODELa"
ON ("MODELb"."FKxx_id" = "MODELa"."nameXX")
How do i set it use the local key nameYY?

Related

Django tables connection

I have 3 django tables connected like this:
Is there anyway to make a query for table Table that will get id_equip from table equip?
models.py
class Vendor(models.Model):
vendor_name = models.CharField(max_length=50)
def __str__(self):
return self.vendor_name
class Equipment(models.Model):
equipment_name = models.CharField(max_length=50)
id_vendor = models.ForeignKey(Vendor, on_delete=models.CASCADE, default=None)
def __str__(self):
return self.equipment_name
class Table(models.Model):
table_name = models.CharField(max_length=100)
id_vend = models.ForeignKey(Vendor, on_delete=models.CASCADE, default=None)
id_equip = models.ManyToManyField(Equipment)
This part of the django docs is relevant and helpful, I definitely recommend your review at least that section and ideally the whole page.
Your models are already denormalized as evidenced by Table.id_equip which relates to Equipment so you could do:
table = Table.objects.get(SOME_FILTER)
equipment_ids = list(table.id_equip.all().values_list('id', flat=True))
If you wanted to go through the vendor I'd suggest:
table = Table.objects.get(SOME_FILTER)
equipment_ids = list(Equipment.objects.filter(vendor_set__table_set=table).values_list('id', flat=True))
I would recommend that you don't name your relationship fields with id_. With an ORM, these fields should represent the instances of the Model they are mapping to. For example:
class Table(models.Model):
name = models.CharField(max_length=100)
vendor = models.ForeignKey(Vendor, on_delete=models.CASCADE, default=None)
equipment = models.ManyToManyField(Equipment)
If you're trying to create the model on top of an existing table, you can make use of the db_column parameter when defining the field.
vendor = models.ForeignKey(Vendor, on_delete=models.CASCADE, default=None, db_column="id_vend")

Filtering Django query filtering

I'm doing some querying currently and I was wondering if I would be able to query something from these 3 models where the return would give me all the projects the users are working on. I know about the basic filtering however thats not really enough in this case, how would one go about querying through 2 foreign keys.
class User(models.Model):
first_name = models.CharField(max_length=255)
last_name = models.CharField(max_length=255)
email = models.EmailField()
class ProjectUser(models.Model):
project = models.ForeignKey("Project", on_delete=models.CASCADE)
user = models.ForeignKey("User", on_delete=models.CASCADE)
is_lead = models.BooleanField(default=False)
class Meta:
unique_together = (("project", "user"),)
class Project(models.Model):
name = models.CharField(max_length=255)
client = models.CharField(max_length=255)
complete = models.BooleanField(default=False)
You can obtain the Projects a user is working on with:
Project.objects.filter(
projectuser__user=user
)
The double underscores are used to look "through" relations. Furthermore the default related_query_name=… parameter [Django-doc] is, if not specified, the name of the model in lowercase.

Django Models relation with primary key add extra "_id" to the column

These are my two models, when I try to open City page on Django I get an error: "column city.country_id_id does not exist". I don't know why python adds extra _id there.
class Country(models.Model):
country_id = models.CharField(primary_key=True,max_length=3)
country_name = models.CharField(max_length=30, blank=True, null=True)
class Meta:
managed = False
db_table = 'country'
class City(models.Model):
city_id=models.CharField(primary_key=True,max_length=3)
city_name=models.CharField(max_length=30, blank=True, null=True)
country_id = models.ForeignKey(Country, on_delete=models.CASCADE)
class Meta:
managed = False
db_table = 'city'
Because if you construct a foreign key, Django will construct a "twin field" that stores the primary key of the object. The foreign key itself is thus more a "proxy" field that fetches the object.
Therefore you normally do not add an _id suffix to the ForeignKey:
class City(models.Model):
city_id = models.CharField(primary_key=True,max_length=3)
city_name = models.CharField(max_length=30, blank=True, null=True)
country = models.ForeignKey(Country, on_delete=models.CASCADE)
class Meta:
managed = False
db_table = 'city'
It however might be better for unmanaged tables, to specify a db_column=… parameter [Djang-doc] in the ForeignKey:
class City(models.Model):
city_id = models.CharField(primary_key=True,max_length=3)
city_name = models.CharField(max_length=30, blank=True, null=True)
country = models.ForeignKey(Country, db_column='country_id', on_delete=models.CASCADE)
class Meta:
managed = False
db_table = 'city'
With this parameter you make it explicit how the column is named at the database side.
this is due to Django's behind the scenes magic.
The fields documentation is very clear about that and I highly recommend you read the Foreign Key section in the link below:
https://docs.djangoproject.com/en/3.0/ref/models/fields/#django.db.models.ForeignKey
Basically, when you want to access the Country reference in the if a City instance, you would do it like this:
city.country_id
I also recommend another naming convention for your Foreign Key fields. Instead of <modelname>_id = models.ForeignKey... just call it <modelname> = models.ForeignKey...
Hope this helps, happy coding

How to set foreign key to a field of another model?

I want to set a foreign key to a field of another model.
I have tried Foreign Key to_field='field_name'
class Banks(models.Model):
name = models.TextField()
id = models.IntegerField(unique=True)
class Meta:
db_table = 'banks'
class Branches(models.Model):
ifsc = models.CharField(max_length=20, null=False)
bank_id = models.ForeignKey(Banks, to_field='id', on_delete=models.CASCADE)
branch = models.CharField(max_length=50)```
ProgrammingError: column branches.id does not exist
LINE 1: SELECT "branches"."id", "branches"."ifsc", "branches"."bank_...
Just add unique=True in the name column, in the Banks model.
class Banks(models.Model):
name = models.TextField(unique=True) # Just add unique=True
id = models.IntegerField(unique=True, primary_key=True)
class Meta:
db_table = 'banks'
class Branches(models.Model):
ifsc = models.CharField(max_length=20, null=False)
bank_id = models.ForeignKey(Banks, to_field='id', on_delete=models.CASCADE) # Now it will work
branch = models.CharField(max_length=50)
This problem is not caused by the foreign key. The error is happening in the Branches model, which presumably also has a db_table Meta attribute and is based on a legacy table.
You must define a primary key for your models. If you don't, Django will do so automatically and call it id. In the case of your Banks model, you should set that id field as primary_key=True - or indeed remove it completely, since that is the default. You need to find a suitable pk for Branches as well and declare it in the field.
For your actual question, you don't need to do anything; Django will automatically set the FK to point to the PK of the target model.
class Bank(models.Model):
# removed `id` as that is the default PK
name = models.TextField()
class Meta:
db_table = 'banks'
class Branch(models.Model):
ifsc = models.CharField(max_length=20, primary_key=True) # assume this is the PK
bank = models.ForeignKey(Bank, on_delete=models.CASCADE)
branch = models.CharField(max_length=50)
class Meta:
db_table = 'branches'
Note also, since these are legacy tables you probably want to add managed = False to both Meta classes. And as suggested by AKX, it is Django style to make model names singular; you can do that without affecting the table name since that is declared explicitly.

How to fetch translation record in django

I am new in django framework.I have 3 tables in mysql database. I want to fetch data from main table with translation table and images table.
My model.py
class Country(models.Model):
#id = models.IntegerField(primary_key=True)
iso_code = models.CharField(max_length=2, unique=True)
slug = models.CharField(max_length=255, unique=True)
is_featured = models.IntegerField(max_length=1)
class Meta:
db_table = 'rh_countries'
class CountryTranslation(models.Model):
country_id = models.ForeignKey(Country, on_delete=models.CASCADE)
name = models.CharField(max_length=255)
description = models.TextField(blank=True, null=True)
locale = models.CharField(max_length=2)
class Meta:
db_table = 'rh_countries_translations'
class CountryImage(models.Model):
country_id = models.ForeignKey(Country, on_delete=models.CASCADE)
image = models.CharField(max_length=255)
is_main = models.IntegerField(max_length=1)
class Meta:
db_table = 'rh_country_images'
Now I want to fetch all country with translation record by locale and associated image.
Please give a solution if anyone know.
You can do this by using a filtering, and annotate:
from django.db.models import F
Country.objects.filter(
countrytranslation__locale=mylocale
).annotate(
name=F('countrytranslation__name')
)
This will result in a QuerySet with all Countrys (that have a Translation for the given mylocale). These Countrys will have an extra attribute .name that is the translated name of the Country.
So given the translations exist, then for mylocale='en', this will result in a QuerySet, with Country(name='Germany', iso_code='de'), and for mylocale='de', it will result in Country(name='Deutschland', iso_code='de') (here this is a bit an ad hoc format, to demonstrate how it works).
Note: ForeignKeys typically do not end with _id. Django will automatically add an _id suffix to the database column. The foreign key itself is on the Python/Django level represented as a lazy loaded attribute.