Django : loaddata to update data

I have a fixture seed_data.json on which I have my initial data.

Sometimes I add new data into this fixtures and reload it, which updates my data correctly.

However, I now want to remove some data from it. So I modified my seed_data.json, for instance, I had something like that :

{"fields": {"name": "Field 0"},"model": "catalog.product","pk": 1},
{"fields": {"name": "Field 1"},"model": "catalog.product","pk": 2},
{"fields": {"name": "Field 2"},"model": "catalog.product","pk": 3},
# ...

That became :

{"fields": {"name": "Field 1"},"model": "catalog.product","pk": 1},
{"fields": {"name": "Field 2"},"model": "catalog.product","pk": 2},
# ...

But I'm getting :

django.db.utils.IntegrityError: Problem installing fixture .....\seed_data.json

Could not load catalog.Product(pk=2): column name is not unique

So there's no problem when adding some data, but when trying to remove some, there are conflicts with the primary keys.

How can I achieve what I'm trying to do ?

Solution

Getting there after all those years and with an unresolved question makes me want to add something because I did struggle with fixtures too.

For modifying existing data, as @Thomas said the best it to use Django data migrations.

When new to Django though, you might be tempted to use fixtures for updating some basic data. Again, this is not recommended but it doesn't hurt to explain what can be done. So, using the example:

{"fields": {"name": "Field 0"},"model": "catalog.product","pk": 1},
{"fields": {"name": "Field 1"},"model": "catalog.product","pk": 2},
{"fields": {"name": "Field 2"},"model": "catalog.product","pk": 3},
# ...

That became :

{"fields": {"name": "Field 1"},"model": "catalog.product","pk": 1},
{"fields": {"name": "Field 2"},"model": "catalog.product","pk": 2},
# ...

Here, the issue is indeed to try to update an object with a primary key that already exists on another object (object with pk = 3). You cannot delete the object with pk = 3 just with the fixture. If you do it manually though, then the updated fixture above would have worked.

Here are some rules that can help.

What you can do when modifying a fixture and calling loaddata on it:

Update data (as long as it respects unicity constraints). Django will use pk to match object and will replace the previous object without calling any pre_save functions (so that's another reason why it's not recommended to use it)
Create data (with the same constraints as before)

What you cannot do when modifying a fixture and calling loaddata on it:

You cannot explicitly delete an object with a fixture. As the pk of the object you want to delete won't be in the file, it will stay as is in the database. So you have to delete it manually.