Creating a machine with an existing IPMI IP results in db contraint error

Each IPMI BMC maps to a single machine. Its not possible to have one IP address for multiple machines as this would result in one power command effecting multiple machines.

If there is an edge case I'm missing could you please explain?

Oh, sorry I should have been more explicit, it's not that fact that there's an error, only that we return the db constraint error to the UI. It would be better if we didn't reveal details about the schema etc in the UI, and provided something friendlier.

Have updated the description to make it clearer, apologies Lee.

The error should not be returned to the UI, the issue is largely cosmetic.

Why this unique constraint is introduced in 3.3? It is breaking some users' use case. Like the following, a 3.2 maas had no problem dealing with duplicated IPMI IP, and then when it is migrated to 3.3, db migration crashes.

Applying auth.0006_default_auto_field... OK
Applying maasserver.0277_replace_nullbooleanfield... OK
Applying maasserver.0278_generic_jsonfield... OK
Applying maasserver.0279_store_vpd_metadata_for_nodedevice... OK
Applying maasserver.0280_set_parent_for_existing_vms... OK
Applying maasserver.0281_secret_model... OK
Applying maasserver.0282_rpc_shared_secret_to_secret... OK
Applying maasserver.0283_migrate_tls_secrets... OK
Applying maasserver.0284_migrate_more_global_secrets... OK
Applying maasserver.0285_migrate_external_auth_secrets... OK
Applying maasserver.0286_node_deploy_metadata... OK
Applying maasserver.0287_add_controller_info_vault_flag... OK
Applying maasserver.0288_rootkey_material_secret... OK
Applying maasserver.0289_vault_secret... OK
Applying maasserver.0290_migrate_node_power_parameters...Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/django/db/backends/utils.py", line 84, in _execute
return self.cursor.execute(sql, params)
psycopg2.errors.UniqueViolation: duplicate key value violates unique constraint "maasserver_bmc_power_type_parameters_idx"
DETAIL: Key (power_type, md5(power_parameters::text))=(ipmi, fc8ed3c5711cc12878b8bf8360674799) already exists.

The above error from comment #5 is actually caused by having duplicate IPMI BMCs where the IP+username is the same but the password is different. I have filed a new bug for this case:
https://bugs.launchpad.net/maas/+bug/2025026

I did note the following in that bug relevant to this bug

> While it makes sense to avoid duplicate IPMI IP/Username/Password combinations, the current unique constraint also does not really achieve that properly since any differing config value in power_parameters would cause the Unique constraint to pass. So I also wonder if this particular Unique constraint is the best enforcement mechanism.