Terraform: We should be able to query if an attribute was created or not for a resource.

[djsly]

Hi there,

Terraform Version

Terraform v0.10.0

Hello fellow terraformer! I have been trying for a while to update a variable based on the allocation of a resource. and I can't figure out how to tell Terraform to ignore attribute reference when they don't exist.
For example, I have a resource which could expose the attribute ipv4_address_private; for this attribute to exist it seems that I need to set the config private_networking to true when creating the resource. (digitalocean_droplet resource). Now I want to basically use the private IPv4 when it exists or else fall back to the attribute ipv4_address which is the public IP.
The problem is that I haven't been able to find a way to detect that the following attribute doesn't exist.
digitalocean_droplet.registry.ipv4_address_private

I was thinking of doing something like
value = coalesce(digitalocean_droplet.registry.ipv4_address_private, digitalocean_droplet.registry.ipv4_address) expecting an empty string for the first entry
but terraform throws the error : Resource 'digitalocean_droplet.registry' does not have attribute 'ipv4_address_private' for variable 'digitalocean_droplet.registry.ipv4_address_private'

Is there any way of querying the attribute to know in advance if terraform created them or not ?

I'm using this in a template file to set variables

data "template_file" "postinstall" {
  template = "${file("${path.module}/../../postinstall/postinstall.sh")}"

  vars {
ips = "${join(",", concat(coalesce(digitalocean_droplet.saltmaster.ipv4_address_private, digitalocean_droplet.saltmaster.ipv4_address),
                              coalesce(digitalocean_droplet.registry.ipv4_address_private, digitalocean_droplet.registry.ipv4_address) ))}"


}

[djsly]

after digging in the provider code, I saw that the property doesn't get created when the private_network is false.
https://github.com/terraform-providers/terraform-provider-digitalocean/blob/master/digitalocean/resource_digitalocean_droplet.go#L300-L303

I'm not sure though if the ReadLogic is executed within the same execution of the create command. Or if the prpoerty would be read from an intermediate step/caching Since I'm creating the template within the same execution of terraform apply where the droplet is also created.

[djsly]

Note that I'm keeping this issue under Core until I know more of the expected default behavior for Terraform with property/attribute.

In other works should all attribute be queryable even if not set or present ?

[FrenchBen]

Ran into this recently, where using a resource means that you can't always output its value, if the attribute doesn't exist (ie when the resource didn't get created).

At a higher level, we would need for a way to check, via interpolation, if the attribute exists, then use it if it does. It would be great if methods like coalesce could do this natively.

[apparentlymart]

The new configuration language implementation we're currently working on integrating will expose ipv4_address_private as null when it isn't set, so if we update coalesce to treat null the same as the empty string (and I don't see any reason why we couldn't do that) then I think we'd get the desired behavior here.

[mildwonkey]

To follow up on @apparentlymart's comment: in Terraform 0.12, coalesce treats null the same way it treats empty strings. Here's a silly example to show how this works now:

$ cat main.tf
variable "thing1" {
  type = string
  default = ""
}

variable "thing2" {
  type = string
  default = null
}

variable "thing3" {
  type = string
  default = "string"
}


output "coalesce" {
  value = coalesce(var.thing1, var.thing2, var.thing3)
}

$ terraform output
coalesce = string

I am going to close this issue now. Thank you!

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