Django : loaddata to update data

Fixtures are only for initial data for completely new database instances, e g when running tests. To modify existing data use migrations.

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 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.