Multiple Databases in Django

Based on your question about separating patient information from diagnostic details into different databases, I need to point out some important limitations in Django's multi-database support.

Limitations with Your Proposed Design

Django doesn't support foreign key or many-to-many relationships that span multiple databases. In your model structure:

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class Visit(models.Model):
    patient = models.ForeignKey(Patient, on_delete=models.CASCADE)
    date = models.DateField()
    diagnostic_a = models.CharField(max_length=200)
    diagnostic_b = models.CharField(max_length=200, blank=True)

You have a foreign key relationship between Visit and Patient. If you try to store these models in different databases, you'll encounter referential integrity issues. This is because Django needs to validate that the primary key of the related object is valid, which isn't possible if it's stored in a separate database.

Alternative Approaches

Instead of splitting the models across databases, consider these alternatives:

1. Keep related models in the same database: Store both Patient and Visit in one database to maintain the relationship.

2. Create separate models without relationships: If you must use separate databases, you would need to manually handle the relationships in your application code.

Using Multiple Databases in Django Admin

If you still want to use multiple databases, you'll need custom ModelAdmin classes to direct the admin interface to use specific databases:

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class MultiDBModelAdmin(admin.ModelAdmin):
    # Specify the name of the alternate database
    using = "diagnostics_db"

    def save_model(self, request, obj, form, change):
        # Save objects to the specified database
        obj.save(using=self.using)

    def delete_model(self, request, obj):
        # Delete objects from the specified database
        obj.delete(using=self.using)

    def get_queryset(self, request):
        # Query objects from the specified database
        return super().get_queryset(request).using(self.using)

    def formfield_for_foreignkey(self, db_field, request, **kwargs):
        # Populate ForeignKey widgets using the specified database
        return super().formfield_for_foreignkey(
            db_field, request, using=self.using, **kwargs
        )

    def formfield_for_manytomany(self, db_field, request, **kwargs):
        # Populate ManyToMany widgets using the specified database
        return super().formfield_for_manytomany(
            db_field, request, using=self.using, **kwargs
        )

Recommendation

For your specific use case with patient data and diagnostics, I recommend:

1. Keep both models in the same database to maintain the relationship integrity
2. Use database-level security features to protect sensitive patient information
3. If you need separation for compliance reasons, consider using Django's database routing to direct certain operations to specific databases, but be aware of the relationship limitations

Django's documentation on multiple databases provides comprehensive information on setting up and configuring multiple databases, but remember the limitations regarding cross-database relationships.