Django - querying 2 tables with a many to many mapping table - django

Question

If i have two tables people and fruits with a third table that maps a many to many relationship between the two.
PEOPLE TABLE
id person
1 bob
2 alice
FRUITS TABLE
id fruit
1 apple
2 pear
3 orange
4 grapes
PEOPLE_FRUITS_MAP
id person_id fruit_id
1 1 1
2 1 3
3 1 4
4 2 1
How would I get a Django QuerySet containing the names of all the fruits related to bob for example.
I guess in SQL it would be something like:
SELECT
fruits.id AS fid,
fruits.name AS fn,
FROM
people
LEFT JOIN
people_fruits_map
ON people.id = people_fruits_map.person_id
LEFT JOIN
fruits
ON fruits.id = people_fruits_map.fruit_id
WHERE
person.id = 1;
result of query
fid fn
--------------
1 apple
3 orange
4 grapes
DJANGO MODELS
class Fruits(models.Model):
fruit = models.TextField(unique=True, blank=True, null=True)
class Meta:
managed = False
db_table = 'fruits'
class People(models.Model):
person = models.TextField(unique=True, blank=True, null=True)
class Meta:
managed = False
db_table = 'people'
class PeopleFruitsMap(models.Model):
fruit = models.ForeignKey(Fruits, models.DO_NOTHING, blank=True, null=True)
people = models.ForeignKey(People, models.DO_NOTHING, blank=True, null=True)
class Meta:
managed = False
db_table = 'people_fruits_map'

Answer 1

For querying a many to many relation from your given models, you can write an ORM query in Django.
First you can add a reverse relation name in your model to make it more readable. This is not a necessary step as Django adds a default name itself if you don't.
In your PeopleFruitsMap model, add related_name
class PeopleFruitsMap(models.Model):
fruit = models.ForeignKey(Fruits, models.DO_NOTHING, blank=True, null=True, related_name='fruit_to_people_mapping')
people = models.ForeignKey(People, models.DO_NOTHING, blank=True, null=True)
class Meta:
managed = False
db_table = 'people_fruits_map'
Now try running this query in your Django shell. Run python manage.py shell, then run this query, where 1 is the id you require:
from your_app.models import Fruit
fruits = Fruit.objects.filter(fruit_to_people_mapping__people__id=1)
Id can be passed dynamically if you want.
To match the query, you can always use print(fruits.query) to check the equivalent postgres query.
Related name helps you refer the reverse relations in model objects, in your case, it will check the PeopleFruitsMap model mapping and in that mapping, we can query the people relation for matching the id.
References:
Django Querying Documentation