I have a nested ActiveResource model (i.e. it's within another model's namespace). Trying to call `save` raises:

```
ActiveResource::MissingPrefixParam: client_id prefix_option is missing

```

How do I supply the needed prefix?

Here's my class:

```
class Foo::Bar < ActiveResource::Base
  self.site = "http://www.example.com"
  self.prefix = "/clients/:client_id"
  self.element_name = "policy"
  self.collection_name = "policies"
end

```

Here's my save attempt:

```
bar = Foo::Bar.new :client_id => 123
bar.valid? # => true
bar.client_id # => 123
bar.save # => ActiveResource::MissingPrefixParam...

```

Time and again I've looked for elucidation on this, but I only find the same instruction:

`When a GET is requested for a nested resource and you don’t provide the prefix_param an ActiveResource::MissingPrefixParam will be raised.`

I'm trying to access our server's API endpoint at `http://www.example.com/clients/[client_id]/policies`, but I'm apparently failing to supply `client_id`, so my app makes its request to `http://www.example.com/clients//policies`

The server log tells me: `ActionController::RoutingError (No route matches "/clients//policies/" with {:method=>:post})`.