Django multi table inheritance: move instance between child mode

Django multi table inheritance: move instance between child models

Assume I have in models.pysomething like:

Class ModelA(models.Model):
    # many fields, including
    relatives = models.ManyToManyField(Person)
)

# also, A is foreign key to other models:
Class SomeOtherModel(models.Model):
    mya = models.ForeignKey(A)

# now we produce two classes with multi-table inheritance
# WITHOUT any additional fileds
Class InhertA1(ModelA):
    pass

Class InhertA2(ModelA):
    pass

So as as I understand, this will create Tables for ModelA, InheritA1 and InheritA1; each instance of ModelA will get a row in the ModelA-table only, each instance of InheritA1 will have a row both in the ModelA-table (basically containing all the data) and another in the InheritA1-table (only containing a PK and a OneToOne key pointing to the ModelA-table row), etc. Django queries for ModelA-objects will give all objects, queries for InheritA1-objects only the InheritA1 ones etc.

So now I have an InheritA1-object a1, and want to make it into a InheritA2-object, without changing the according ModelA-object. So previously the parent-IbeToOne Key of a1 points to the ModelA-row with key 3, say (and the ForeignKey of some SomeOtherModel-object is set to 3). In the end, I want a InheritA1-object a2 pointing to the same, unchanged ModelA-row (and the object a1removed).

It seems that django doesn't offer any such move-Class-functionality?

Can I safely implement the according SQL operations myself?

Or will things go horribly wrong? I.e., can I just execute the SQL commands that

Create a new row in the InheritA2-table, setting the parent-OneToOne key to the one of a1,
Remove the a1 row in the InheritA2-table?

It seems I cannot do this from non-SQL-django without automatically creating a ModelA-row. Well, for 1., maybe I can create a temporary object x that way, then let p be the parent of x, then change the parent-OneToOne key of x to point to the one of a1, then delete the obect p? But for 2, I do not think that it is possible in non-SQL-django to remove an instance of a child while keeping the parent object?

Alternatively, is there a good django way to copy instances in django and change references to them?

I.e., I could create a new InheritA2 object y, and copy all the properties of a1 into the new object, and then go through the database and find all ManyToMany and ForeignKey entries that point to the parent of a1 and change it to point to the parent of y instead, and then delete a1.

That could be done with non-SQL-django, the downside is that it seems wasteful performance-wise (which would be of no great concern for my project), and that it might also not be so "stable" I.e., if I change ModelA or other models in relation to it, the code that copies everything might break? (Or is there a good way to do something like

Forall models M in my project:
    Forall keys k used in M:
        If k is a descendant of a ManyToMany or Foreign or  ... key:
            If k points to ModelA:
                Forall instances x of M:
                    If x.k=a1:
                        x.k=y

The first four lines seem rather dubious.

Answer

You're absolutely correct that Django's ORM doesn't provide a built-in, straightforward method for moving an instance of a child class (InheritA1) into another child class (InheritA2) while keeping the parent (ModelA) intact. This type of operation requires careful manipulation of both the parent and child tables, which isn't typically handled directly by Django.

Here's a breakdown of how you could handle this problem and the potential risks involved:

1. Understanding Multi-table Inheritance:

In Django's multi-table inheritance, each subclass gets its own table, and the child class only stores the primary key as a foreign key pointing to the parent table. When querying the child model (InheritA1 or InheritA2), Django performs a join between the child table and the parent table, so you get access to the data from both the parent (ModelA) and the child (InheritA1 or InheritA2) at the same time.

The typical setup for multi-table inheritance looks like this:

ModelA has its table with its own fields.
InheritA1 and InheritA2 will have tables that only store the primary_key (PK) and a one-to-one relationship to the ModelA record (through the parent PK).

For your use case, if you want to "move" an instance from InheritA1 to InheritA2 without changing the ModelA row, you would need to:

Create a new row in InheritA2 that points to the same ModelA row as the InheritA1 object.
Delete the original InheritA1 row.

2. SQL Operation Feasibility:

Yes, you can manually execute the necessary SQL commands to achieve this, but there are risks involved. Here's a step-by-step explanation of what would need to happen, along with some considerations:

Steps for Manual SQL Operation:

Insert into InheritA2 Table: You can insert a new row into the InheritA2 table, setting the foreign key (OneToOne relation) to the same ModelA record as the InheritA1 instance. This can be done by executing raw SQL like:
```
INSERT INTO InheritA2 (modela_ptr_id)
SELECT modela_ptr_id FROM InheritA1 WHERE id = a1_id;
```
This assumes a1_id is the primary key of your InheritA1 instance, and modela_ptr_id is the foreign key that links to ModelA.
Delete InheritA1 Row: After that, you can delete the InheritA1 row that you want to "move":
```
DELETE FROM InheritA1 WHERE id = a1_id;
```

However, manually executing SQL has a few concerns:

If there are any signals or cascade deletions associated with the deletion of an InheritA1 object, these would not be handled automatically unless you ensure that you manage these triggers properly in the raw SQL.
If a1_id is being referenced by other records in your database (via ForeignKey or ManyToMany relationships), those references will not be automatically updated. This means you'd need to manually ensure that any other references to InheritA1 (or ModelA) are updated accordingly.

Alternative: Copying Instances and Changing References:

This method involves creating a new InheritA2 object, copying the fields from the InheritA1 object, and updating references throughout your project.

Here’s how to do it step by step:

Create a New InheritA2 Object:
- First, create a new InheritA2 object and set its ModelA reference (primary key) to the same as a1.
```
a2 = InheritA2.objects.create(mya=a1.mya)  # assuming mya is the foreign key
```
Copy Attributes from a1 to a2:
- If InheritA1 and InheritA2 have fields that differ, you'll need to copy the values from a1 to a2. You can do this by manually copying each field.
```
for field in a1._meta.fields:
    if field.name != 'id':  # don't copy the primary key
        setattr(a2, field.name, getattr(a1, field.name))
a2.save()
```
Update References in Other Models:
- For any other models that have a ForeignKey or ManyToManyField pointing to a1, you will need to update them to point to a2. This can be done by finding all references to the a1 instance and updating them:
```
SomeOtherModel.objects.filter(mya=a1).update(mya=a2)
```
Delete a1: After copying and updating all references, you can safely delete the original InheritA1 object.
```
a1.delete()
```

3. Performance Considerations:

Efficiency: The copy method could be slower since you're copying field by field and updating multiple related models. However, it's safer and avoids SQL issues.
Stability: The approach of copying objects and updating references manually is stable, as long as you account for every related model (ForeignKey, ManyToMany) that references the parent ModelA object.
Automatic Handling: As you noted, there isn’t a simple built-in Django function for copying objects across child classes with automatic reference updates, but you can implement this manually as described.

4. Potential Alternative: Using Signals:

Another possible approach is to use Django signals (e.g., pre_save, post_save) to automate parts of this process when moving objects between classes. However, this can add complexity and still requires careful management of the references.

5. Conclusion:

Direct SQL Operations: While possible, manually managing the raw SQL has risks (especially with cascading updates and signals).
Copying Objects and Changing References: This method is safer and aligns better with Django's ORM, but could be less efficient for large datasets.

The copying and updating references method is generally safer and more "Django-friendly." It avoids messing with the underlying database schema and handles the integrity of foreign key and many-to-many relationships automatically. If performance is not a critical concern, this approach would be the preferred method.