unmarshal nested json without knowing structure

I am using a key value store as the backend for my golang application, with the date serving as the key (to keep entries sorted) and json documents as the values. The top level namespace of the json (foo) and the type and date are present in each json document that I'm storing but otherwise there are some differences (especially with respect to some nested json data), so when keyI'm pulling from the database, I don't really know what I'm pulling out at any time that I'm looping through . Here is a sample of the json data

{
  "foo": {
    "id": "124",
    "type": "baz",
    "rawdata": [
      123,
      345,
      345
    ],
    "epoch": "1433120656704"
  }
}

{
  "foo": {
    "id": "234",
    "type": "bar",
    "rawdata": [
      {
        "key": "dog",
        "values": [
          123,
          234
        ]
      },
      {
        "key": "cat",
        "values": [
          23,
          45
        ]
      }
    ],
    "epoch": "1433120656705"
  }
}

when I'm pulling from the database, the first thing I do is unmarshal each entry into a map[string]*json.RawMessage to deal with the foo namespace

//as I'm looping through the entries in the database
   var objmap map[string]*json.RawMessage
   if err := json.Unmarshal(dbvalue, &objmap); err !=nil{
       return err
   }

which I do thanks to this SO answer

However, unlike in that SO answer, when I have to unmarshal again whatever is contained under the foo namespace I don't know which struct to unmarshal into

   if err :=json.Unmarshal(*objmap["foo"], &bazorbar; err != nil{
         return err
   }

 type Baz struct{
  Id string `json:"id"`
  Type string `json:"type"`
  RawData []int `json:"rawdata"`
  Epoch string  `json:"epoch"`
}

type Bar struct{
  Id string `json:"id"`
  Type string `json:"type"`
  RawData []*Qux `json:"rawdata"`
  Epoch string  `json:"epoch"`
}
//nested inside Bar
type Qux struct{
  Key string `json:"key"`
  Values []int `json:"values`
}

Two part Question:

1. Is there a way to avoid repeated unmarshals (or is that something I shouldn't even care about)
2. how can I figure out which struct to unmarshal the json.RawMessage into (which also allows for nested json data)

Update: the initial answer provided by @chendesheng enables me to find out the type but not to unmarshal again into a struct once that type has been determined (which I need to do), so based on a conversation in the comments to his/her answer, I would be interested in either of these possibilities

a) make a copy of the json.RawMessage, unmarshal into the interface as you shown (by chendesheng's answer), and then unmarshal the copy itno the struct once you know the type (from having unmarshaled into the interface)?

b) use a regular expression to determine the type and then unmarshal into a struct of that type once it's known

Two ways to check struct type:

1. Unmarshal json.RawMessage to a map[string]interface{}
2. Use a regular expression to extract type string

http://play.golang.org/p/gfP6P4SmaC