[2.x, API] Creating a subnet with fabric specified gets the default fabric anyway

[Данило Шеган]

In noticing a problem with how defaults are handled with Django 1.11 (see https://code.launchpad.net/~danilo/maas/+git/maas/+merge/326514), I've wanted to ensure API works correctly and added a test from http://paste.ubuntu.com/24981036/.

However, that fails even with master: it still picks up the default fabric and default vlan from it.

http://paste.ubuntu.com/24981040/

[Adam Collard (adam-collard)]

This bug has not seen any activity in the last 6 months, so it is being automatically closed.

If you are still experiencing this issue, please feel free to re-open.

MAAS Team

[jjqq (jjqq)]

Let me leave a memo for anyone who encounter this issue.

To create a subnet in A SPECIFIC fabric, you should use the `vlan=...` option instead of `fabric=...` option.

```
maas <USER> subnets create ...options... vlan=<VLAN_ID_OF_UNTAGGED_VLAN_OF_THE_FABRIC>
```

I have confirmed this in MAAS 2.5.1 ( 2.2.2 also should be fine, please see the reason followed).

Having checked source code(maasserver/forms/subnet.py), I found that the fabric=... is not used at all, the table which store the subnets information does not have any column named fabric or fabric_id, it just has vlan_id column.

```
    class Meta:
        model = Subnet
        fields = (
            'name',
            'description',
            'vlan',
            'cidr',
            'gateway_ip',
            'dns_servers',
            'rdns_mode',
            'active_discovery',
            'allow_dns',
            'allow_proxy',
            'managed',
            )

```

I manually added subnets from MAAS Subnets UI, then inspected the db (also source code):
```
maasdb=# select * from maasserver_subnet;
 id | created | updated | name | cidr | gateway_ip | dns_servers | vlan_id | rdns_mode | allow_proxy | description | active_discovery | managed | allow_dns
----+-------------------------------+-------------------------------+--------------------------+-------------------+----------------+-------------+---------+-----------+-------------+-------------+------------------+---------+-----------
  4 | 2022-01-19 00:05:12.577246+00 | 2022-01-19 00:05:12.577246+00 | subnet-100.79.223.64/26 | 100.79.223.64/26 | | {} | 5001 | 2 | t | | f | t | t
  5 | 2022-01-19 04:20:30.429739+00 | 2022-01-19 04:20:30.432391+00 | 100.79.223.128/26 | 100.79.223.128/26 | 100.79.223.190 | {} | 5001 | 2 | t | | f | t | t
  6 | 2022-01-20 13:40:17.55268+00 | 2022-01-20 13:40:17.55268+00 | subnet-100.79.222.128/26 | 100.79.222.128/26 | | {} | 5001 | 2 | t | | f | t | t
  8 | 2022-10-17 15:40:03.724161+00 | 2022-10-17 15:40:03.724161+00 | subnet-100.79.223.0/26 | 100.79.223.0/26 | | {} | 5001 | 2 | t | | f | t | t
  9 | 2022-10-21 07:34:00.3236+00 | 2022-10-21 07:34:00.3236+00 | subnet-100.95.3.128/25 | 100.95.3.128/25 | | {} | 5001 | 2 | t | | f | t | t
  7 | 2022-10-14 18:59:25.204786+00 | 2023-03-08 15:13:48.06542+00 | subnet-100.95.2.0/25 | 100.95.2.0/25 | | {} | 5002 | 2 | t | | f | t | t
 11 | 2023-03-08 15:48:23.449369+00 | 2023-03-08 15:56:12.629415+00 | subnet-100.95.4.0/24 | 100.95.4.0/24 | | {} | 5002 | 2 | t | | f | t | t
(7 rows)

```

See the vlan_id col...