droplets: refactor how to assign resources (#1500)

This refactors how we move created resources to projects to make it
generic-ish. As long as the resource implements the `URN()` method, we
can use a helper that performs the assign resources call.

This is not quite ideal due to how the commands are architected. Because
we double-wrap the raw godo resources (first in a wrapper struct, then
in the type alias for a slice of those wrapped structs), Go's type
system isn't able to recognize those slices as implementing a slice of
`urner` interfaces. What it can do though, is recognize each individual
element of those slices as implementing the `urner` interface, so the
method I created will only accept a single `urner`, despite the call to
assign resources accepting an slice of URNs.

I think the tradeoff is worth it, since each command that can support
the projects functionality can use the same `moveToProject`, and what we
lose in efficiency by having multiple calls to assign resources in some
instances, we gain in consistency.

commands/command_config.go

@@ -225,3 +225,19 @@ func (c *CmdConfig) Display(d displayers.Displayable) error {
 
 	return dc.Display()
 }
+
+// An urner implements the URN method, wihich returns a valid uniform resource
+// name.
+type urner interface {
+	URN() string
+}
+
+// moveToProject moves the given resource to the project with the given
+// project UUID.
+func (c *CmdConfig) moveToProject(projectUUID string, u urner) error {
+	if projectUUID == "" {
+		return nil
+	}
+	_, err := c.Projects().AssignResources(projectUUID, []string{u.URN()})
+	return err
+}

commands/droplets.go

@@ -335,12 +335,8 @@ func RunDropletCreate(c *CmdConfig) error {
 		}
 	}
 
-	if projectUUID != "" {
-		dropletURNs := make([]string, 0, len(createdList))
-		for _, createdDroplet := range createdList {
-			dropletURNs = append(dropletURNs, createdDroplet.URN())
-		}
-		if _, err := c.Projects().AssignResources(projectUUID, dropletURNs); err != nil {
+	for _, createdDroplet := range createdList {
+		if err := c.moveToProject(projectUUID, createdDroplet); err != nil {
 			return err
 		}
 	}