为什么facebookgo / inject为对象字段提供默认值？

Question

I'd like to use facebookgo/inject, but I have a problem. I wrote this code:

package main

import (
    "github.com/facebookgo/inject"
)

type BookService struct {
    Database  Database  `inject:""`
}

type Database struct {
    ConnectionString string `inject:""`
}

func main() {
    var graph inject.Graph
    var service BookService
    _ = graph.Provide(&inject.Object{Value: &service}, &inject.Object{Value: Database{ConnectionString: "uri"}})
    _ = graph.Populate()

    println(service.Database.ConnectionString)
}

So, I expect that after injection the value of the field ConnectionString will be uri, but I get an empty string. What am I doing wrong?