Django: How do I move part of the data from one instance to another

Question

We have 2 identical Django instances and one is trial the other is for production. Recently one client on trial purchased full product and I need to move ONLY their data from trial to production. I don't know if there's a convenient way to do that, since:

1. If I use Django fixtures then it might overwrite the existing data in production system because of the default id that Django assigned to each entry(I might be wrong but I think fixtures are only good for initialization).

2. Using sql to dump the DB might not help either because of the similar problem with the first approach, and it's also complex because there are other customers in trial but I only need to move this client's data.

Please give me some advice if you have similar experience.

Answer 1

The issue is you cannot transfer the primary keys (IDs) from the trial to the production DB, isn't it ? So 2 solutions:

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1) A tank to kill an ant

You do a SQL export of your trial database, and you increase every primary key and foreign key by a number (for ex: 10000). This number needs to high enough to avoid unicity constraint violation when you will import it in the DB

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2) The smart solution

If, and only if, your model is well designed: for every model you can find a set of its columns that could make a substitute primary key to the ID. If you have attributes with unique = True, or models with unique_together = (...) it's perfect: you can use natural keys !

In every model of your source code, you add the method get_by_natural_key:

class Person(models.Model):
    firstname = models.CharField...
    last_name = models.CharField...

    class Meta:
        unique_together = ("first_name", "last_name")

    def get_by_natural_key(self, first_name, last_name):
        return self.get(first_name=first_name, last_name=last_name)

Then you can use Django dumpdata command to export the trial database with the IDs replaced by the natural keys ! Then with the same code, you use the loaddata command to import these data files.