Working with Joins in Django ORM Without Foreign Keys

When working with databases that lack foreign key constraints, Django's ORM still provides several options for performing joins. Let's address your questions:

Understanding Your Situation

Both select_related() and prefetch_related() are designed to work with Django model relationships (foreign keys, one-to-one, or many-to-many). Since your database tables don't have these relationships defined, these methods won't work as expected without some additional setup.

Solutions for Your Case

1. Using Raw SQL Queries

For direct control over the join operation, you can use Django's raw SQL capabilities:

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from django.db import connection

def perform_join():
    with connection.cursor() as cursor:
        cursor.execute("""
            SELECT t1.id as history_id, t1.machine_number, t1.client_id, t2.other_field
            FROM some_table1 t1
            JOIN some_table2 t2 
            ON t1.machine_number = t2.machine_number AND t1.client_id = t2.client_id
        """)
        return cursor.fetchall()

2. Creating Virtual Relationships

You can define relationships in your Django models even if they don't exist in the database:

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class SomeTable1(models.Model):
    machine_number = models.CharField(max_length=100)
    client_id = models.CharField(max_length=100)
    
    class Meta:
        db_table = 'some_table1'
        # If needed, specify that this doesn't manage the table
        managed = False

class SomeTable2(models.Model):
    machine_number = models.CharField(max_length=100)
    client_id = models.CharField(max_length=100)
    other_field = models.CharField(max_length=100)
    
    class Meta:
        db_table = 'some_table2'
        managed = False

Then use Django's Q objects for the join:

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from django.db.models import Q

# This performs a join on the two fields
results = SomeTable1.objects.filter(
    Q(machine_number__in=SomeTable2.objects.values('machine_number')) &
    Q(client_id__in=SomeTable2.objects.values('client_id'))
)

3. Using Subqueries

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from django.db.models import OuterRef, Subquery

subquery = SomeTable2.objects.filter(
    machine_number=OuterRef('machine_number'),
    client_id=OuterRef('client_id')
).values('other_field')[:1]

results = SomeTable1.objects.annotate(
    other_field=Subquery(subquery)
)

Answers to Your Specific Questions

1. Using prefetch_related: This isn't the right approach for your case since it's designed for model relationships that don't exist in your database.

2. Chaining vs. parameters: When using prefetch_related() with actual relationships, both approaches work, but including all relationships as parameters is generally cleaner.

3. Better ways to perform joins: The methods above (raw SQL, Q objects, or subqueries) are your best options when working with databases without foreign keys.

Important Limitations

From the Django documentation, it's important to note:

Django doesn't currently provide any support for foreign key or many-to-many relationships spanning multiple databases. If you have used a router to partition models to different databases, any foreign key and many-to-many relationships defined by those models must be internal to a single database.

This means that even with the workarounds above, you'll need to ensure all tables involved in your joins are in the same database.