Django Multi-Table Inheritance and Foreign Key Relationships

When using multi-table inheritance in Django, the child models (like TestsReport and MetricsReport) get a one-to-one link to the parent model (Report) through a field called report_ptr_id. This can cause issues with foreign key relationships as you've discovered.

The Problem

With your current setup, Django creates:

• A report_ptr_id in the TestsReport table that links to the Report table
• A tests_report_id in the TestRuns table that doesn't properly link to the inherited ID

Solution

The issue is that Django's multi-table inheritance creates an implicit one-to-one relationship with a field named report_ptr_id, but your foreign key is looking for the id field directly.

Here are the adjustments needed:

Option 1: Use to_field parameter

Specify the exact field to link to in your foreign key:

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class TestRuns(models.Model):
    tests_report = models.ForeignKey(
        'TestsReport', 
        on_delete=models.CASCADE,
        to_field='report_ptr_id'  # Explicitly link to the inherited ID
    )
    # some other fields

Option 2: Use primary_key=True in the parent model

This is the more standard approach. Since you're already defining a custom primary key in the Report model, make sure it's properly set up:

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class Report(models.Model):
    id = HashField(primary_key=True)  # 'unique=True' is redundant with primary_key=True
    timestamp = models.DateTimeField()
    # some other fields

Then your foreign key should work as expected:

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class TestRuns(models.Model):
    tests_report = models.ForeignKey('TestsReport', on_delete=models.CASCADE)
    # some other fields

Option 3: Use a related_name in the parent-child relationship

You can also modify how Django creates the parent-child relationship:

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class TestsReport(Report):
    report = models.OneToOneField(
        Report, 
        on_delete=models.CASCADE, 
        parent_link=True,
        primary_key=True,
        related_name='tests_report'
    )
    # some fields

This approach gives you more control over the inheritance relationship.

Additional Considerations

1. Make sure to run migrations after making these changes
2. You might need to adjust existing data if you're modifying an existing database
3. The same approach would apply to your MetricsReport and its relationships

With these adjustments, you should be able to properly use the inherited ID in your foreign key relationships while maintaining the benefits of multi-table inheritance.