RestKit: Could not find an object mapping for keyPath: ' '

RestKit is an excellent Objective-C framework to make dealing with downloading and uploading data to and from a REST-based server considerably simpler than coding it all yourself.

When I started out I found it pretty straightforward until I hit a REST command that returned a JSON array at the root object, where I ran into the dreaded Could not find an object mapping for keyPath: ' ' error.

Here's an example REST response that caused this error:

[ { "name":"foo"}, { "name":"bar"} ]

It seems restkit expects there to be a root level object e.g. 

{ "data": [ { "name":"foo"}, { "name":"bar"} ] }

If your server doesn't do this, and you can't change it, then the automatic mapping system doesn't seem to work.

The solution is pretty simple (manually tell the system which mapping to use) but I took me a while to figure out, so I thought it might help others if I posted it here.