How to inject an appengine.Context in martini

When I try to inject an appengine.Context from a middleware doing this:

//Share Context
m.Use(func(r *http.Request) {
    c := appengine.NewContext(r)
	c, err := appengine.Namespace(c, namespace)
	if err != nil {
		c.Debugf("[Namespace] %s", err)
	}
	m.Map(c)
})

I get this Panic saying that apparently there is no appengine.Context to be injected:

<h1>PANIC</h1>
<pre style="font-weight: bold;">Value not found for type appengine.Context</pre>

<pre>github.com/go-martini/martini/router.go:320 (0xc3731d)
	(*routeContext).run: panic(err)
github.com/go-martini/martini/router.go:221 (0xc36729)
	(*route).Handle: context.run()
github.com/go-martini/martini/router.go:112 (0xc35628)
	(*router).Handle: route.Handle(context, res)
app/nc_backend.go:37 (0xc30fe0)
	Router.Handle.fm: m.Action(r.Handle)
go/src/pkg/runtime/asm_amd64.s:340 (0xc23b82)
go/src/pkg/reflect/value.go:474 (0xd41bd3)
go/src/pkg/reflect/value.go:345 (0xd40c65)
github.com/codegangsta/inject/inject.go:102 (0xd8449a)
	(*injector).Invoke: return reflect.ValueOf(f).Call(in), nil
github.com/go-martini/martini/martini.go:165 (0xc33607)
	(*context).run: _, err := c.Invoke(c.handler())
github.com/go-martini/martini/martini.go:156 (0xc33500)
	(*context).Next: c.run()
github.com/go-martini/martini/recovery.go:140 (0xc37a4b)
	func.004: c.Next()
go/src/pkg/runtime/asm_amd64.s:339 (0xc23b22)
go/src/pkg/reflect/value.go:474 (0xd41bd3)
go/src/pkg/reflect/value.go:345 (0xd40c65)
github.com/codegangsta/inject/inject.go:102 (0xd8449a)
	(*injector).Invoke: return reflect.ValueOf(f).Call(in), nil
github.com/go-martini/martini/martini.go:165 (0xc33607)
	(*context).run: _, err := c.Invoke(c.handler())
github.com/go-martini/martini/martini.go:69 (0xc32b08)
	(*Martini).ServeHTTP: m.createContext(res, req).run()
go/src/pkg/net/http/server.go:1496 (0xc98dda)
go/src/pkg/appengine_internal/api_prod.go:246 (0xc26e3f)
go/src/pkg/appengine_internal/api_prod.go:212 (0xc268c5)
go/src/pkg/runtime/asm_amd64.s:340 (0xc23b82)
go/src/pkg/reflect/value.go:474 (0xd41bd3)
go/src/pkg/reflect/value.go:345 (0xd40c65)
_:410 (0xcf6255)
go/src/pkg/runtime/proc.c:279 (0xc170a0)

</pre>

What am I doing wrong?

appengine.Context is an interface, so you have to use the alternative MapTo. Also, according to the docs, mapping should be performed on the martini Context, not on the Martini object itself.

So your code should be:

m.Use(func(c martini.Context, req *http.Request) {
	ctx := appengine.NewContext(req)
    ctx, err := appengine.Namespace(ctx, namespace)
    if err != nil {
        ctx.Debugf("[Namespace] %s", err)
    }
	c.MapTo(ctx, (*appengine.Context)(nil))
})