Strongly-typed ResourceID

[stan-sz]

Is your feature request related to a problem? Please describe.
Given a full resource ID to a child resource have the ability to create a resource ID of the parent.

Excerpted from #3173: some RPs provide a full resource ID to some other (child) resources. For example: Microsoft.Network/loadBalancers provides the full ip configuration resource ID in its properties 'backendAddressPools[0].properties.backendIPConfigurations[0].id' as:

{
  "id": "/subscriptions/<sub ID>/resourceGroups/<rg name>/providers/Microsoft.Compute/virtualMachineScaleSets/<vmss name>/virtualMachines/0/networkInterfaces/<nic name>/ipConfigurations/<ipconfig name>"
},

Describe the solution you'd like

• Idea 1: have a type resourceID that can be initiated using a string (or the output of *resourceId() function). The type could allow .parent to obtain the parent resource ID.

• Idea 2: similar to above but using a new parentResourceId(<child resource ID>) function

[anthony-c-martin]

Great suggestion!

[ishepherd]

I'm interested in using these ResourceID objects in the resource existing scenario

resource foo 'foo@date' existing = {
  id = ResourceID
}

This would be a convenient way for modules to reference things created by other modules.
Only one module param required, instead of 3.

[alex-frankel]

I'm interested in using these ResourceID objects in the resource existing scenario

This should be resolved with #2245 and #2246

[jeskew]

I don't know if we have a great way to identify ID fields returned by resources. The specific field mentioned by @stan-sz (backendAddressPools[0].properties.backendIPConfigurations[0].id on a Microsoft.Network/loadBalancers resource) is a property named id on an object with the x-ms-azure-resource extension, but I'm not sure if that pattern is followed reliably enough by resource providers to be a good heuristic.