Setting Up Foreign Key Relationships with Unmanaged Models

When working with unmanaged models generated from database views in Django, setting up foreign key relationships requires some special considerations.

The Issue with Your Current Code

Your current code has two issues:

1. You have two cost_bearer fields defined in the PentaSystem model (one is commented out)
2. The foreign key definition doesn't specify which field it's referencing in the parent model

Correct Implementation

Here's how you should modify your code to establish the foreign key relationship:

PYTHONCopied
from django.db import models


class PentaProjectMasterData(models.Model):
    project = models.CharField(max_length=10, blank=False, null=False)
    project_name = models.CharField(max_length=8000, blank=True, null=True)
    cost_bearer = models.CharField(max_length=6, blank=False, null=False, primary_key=True)

    class Meta:
        managed = False  # Created from a view. Don't remove.
        db_table = 'penta_project_master_data'


class PentaSystem(models.Model):
    system = models.CharField(max_length=4, blank=False, null=False, primary_key=True)
    system_name = models.CharField(max_length=30, blank=True, null=True)
    # Remove the CharField definition and use only the ForeignKey
    cost_bearer = models.ForeignKey(
        PentaProjectMasterData, 
        on_delete=models.CASCADE,
        db_column='cost_bearer'  # This is important - it tells Django which column to use
    )

    class Meta:
        managed = False  # Created from a view. Don't remove.
        db_table = 'penta_system'

Key Points to Note:

1. Remove the CharField definition for cost_bearer in the PentaSystem model
2. Add the db_column='cost_bearer' parameter to the ForeignKey field to explicitly tell Django which database column to use
3. The on_delete=models.CASCADE parameter specifies what happens when the referenced object is deleted

Since these are unmanaged models based on views, Django won't create or modify the database schema. This relationship will only exist in your Django application layer, allowing you to use the relationship in your queries and templates.