Bug in validate uniqueness of VM name

[xcdr]

Environment

• Python version: 3.8.6
• NetBox version: 2.9.10

Cannot add VM without Tenant to different clusters.

Steps to Reproduce

1. Add cluster A, B
2. Add VM named "Example VM" to cluster A without Tenant
3. Try to add VM named "Example VM" to cluster B without Tenant

Expected Behavior

VM should be added.

Observed Behavior

I see validation error: A virtual machine with this name already exists.

I see that validate_unique(self, exclude=None) in VirtualMachine model doesn't filter by cluster!

[jeremystretch]

The ability to have multiple VirtualMachine instances with duplicate names was introduced in #2669, however looking back it's not entirely clear what the constraints were intended to be. The current code looks like this:

def validate_unique(self, exclude=None):
    if self.tenant is None and VirtualMachine.objects.exclude(pk=self.pk).filter(
        name=self.name, tenant__isnull=True
    ):

This allows duplicate names only for VMs which do not have a tenant assigned, but makes no reference to Cluster. Meta.unique_together sets ['cluster', 'tenant', 'name'], which leads me to believe the intent was to constrain scope to individual cluster-tenant pairs.

We can "fix" this by extending validate_unique() to check against assigned cluster as well, which I think makes sense.