Django - accessing values in multiple databases

Question

I need to make sure that an object (Device) is only saved once and only to one database.

I have several PostGre SQL databases as so:

                                       List of databases
             Name              | Owner | Encoding |  Collate   |   Ctype    | Access privileges 
-------------------------------+-------+----------+------------+------------+-------------------
 admin                         | admin | UTF8     | en_US.utf8 | en_US.utf8 | 
 postgres                      | admin | UTF8     | en_US.utf8 | en_US.utf8 | 
 reference                     | admin | UTF8     | en_US.utf8 | en_US.utf8 | 
 template0                     | admin | UTF8     | en_US.utf8 | en_US.utf8 | =c/admin         +
                               |       |          |            |            | admin=CTc/admin
 template1                     | admin | UTF8     | en_US.utf8 | en_US.utf8 | =c/admin         +
                               |       |          |            |            | admin=CTc/admin
 workspace_A                   | admin | UTF8     | en_US.utf8 | en_US.utf8 | 
 workspace_B                   | admin | UTF8     | en_US.utf8 | en_US.utf8 | 
 workspace_C                   | admin | UTF8     | en_US.utf8 | en_US.utf8 | 
 workspace_D                   | admin | UTF8     | en_US.utf8 | en_US.utf8 |

Workspaces A,B,C and D all have a table called devices_device which contains an ID, a name and some other fields.

What function(s) do I need to call when saving a Device (over-writing the Django save() function) to make sure that a Device with the same parameters is not already present?

This is what I currently have outlined, with question marks where I don't know which function to use

def save(self, *args, **kwargs):
    for tab in ?.objects.all():
        if tab.object.using('devices_device')? == self.device_reference
        and ?.device_name == self.device_name
        and  ?.device_address == self.device_address
        and  ?.device_position== self.device_position
        and  ?.device_desciption == self.device_desciption:
            raise ValidationError(
                "This device already exists in another workspace!"
            )
    super().save(*args, **kwargs)

Endre Both · Accepted Answer · 2019-03-19 12:30:45Z

1

The obvious disclaimer first: Imposing unique constraints in code is a recipe for chaos and low performance. That's what RDBMS's are for (and you have several of them :).

That said, you can of course loop over every one of the databases where devices are stored and check for the presence of a matching device. For that to work, you have to include each database in your settings.DATABASES.

from django.conf import settings

for db in settings.DATABASES:
    if YourModel.objects.using(db).filter(
        # add your filter conditions here
    ).exists():
        raise ValidationError(
            "This device already exists in another workspace!"
        )

I assume you have already read the helpful intro to multiple databases in the Django documentation.

edited Mar 19, 2019 at 12:30

answered Mar 19, 2019 at 12:04

Endre Both

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