How to assign floating ip address using terraform?

I’m unable to join floating ip and droplet creation.

# I create a resource:
resource "digitalocean_droplet" "mail" {
  image = "centos-7-x64"
  name = "mail"
  region = "${var.region}"
  size = "2gb"
  private_networking = true
}

#I create a floating ip:
resource "digitalocean_floating_ip" "mail" {
  droplet_id = "${digitalocean_droplet.mail.id}"
  region = "${var.region}"
}

The IDs are the same, but the IP’s are not.

$ doctl compute droplet list
ID		Name	Public IPv4	Public IPv6	Memory	VCPUs	Disk	Region	Image			Status	Tags
38296000	mail	188.166.174.104			2048	2	40	lon1	CentOS 7.3.1611 x64	active	
$ doctl compute fip list
IP		Region	Droplet ID	Droplet Name
67.207.68.145	lon1	38296000	mail

How do I get the droplet to take the floating IP address?

Thanks,