How to map JSON objects with dynamic fields to Go structs

I am developing a website using Go and connecting it to Elastic Search. In Elastic Search I can have dynamic fields for index types. When I read a document from Elastic Search it will return a JSON object as the result, which can include fields with dynamic names (or user defined fields).

I can get the JSON result and unmarshal it into a Go struct, but I do not know what is the best way to keep those dynamic fields as part of the Go struct.

This is what I am doing. For example, if I get a document for a Contact from Elastic Search it may look something like this:

{  
   "EmailAddress": "test@test.com",
   "Name": "Test Contact",
   "Phone": "17894785236",
   "City": "San Francisco",
   "State": "California"
}

And the Go struct for Contact is:

type Contact struct {
	EmailAddress            string
	Name                    string
	Phone                   string
    CustomFields            map[string]interface{}
}

And I implement Marshaler and Unmarshaler to override how the object is Marshaled and Unmarshalled.

func (c *Contact) MarshalJSON() ([]byte, error) {
	contactMap := make(map[string]interface{})
	contactMap["EmailAddress"] = c.EmailAddress
	contactMap["Name"] = c.Name
	contactMap["Phone"] = c.Phone

	for k, v := range c.CustomFields {
		contactMap[k] = v
	}

	return json.Marshal(contactMap)
}

func (c *Contact) UnmarshalJSON(data []byte) error {
	var contactMap map[string]interface{}

	if c == nil {
		return errors.New("RawString: UnmarshalJSON on nil pointer")
	}

	if err := json.Unmarshal(data, &contactMap); err != nil {
		return err
	}

	c.EmailAddress = contactMap["EmailAddress"].(string)
	c.Name = contactMap["Name"].(string)
	c.Phone = contactMap["Phone"].(string)

	for key, val := range contactMap {
		if key != "EmailAddress" && key != "Name" && Key != "Phone" {
			c.CustomFields[key] = value
		}
	}

	return nil
}

Is this the best way to do this? What would you recommend?

Just add little cleanup

var contactMap map[string]interface{}

if c == nil {
	return errors.New("RawString: UnmarshalJSON on nil pointer")
}

if err := json.Unmarshal(data, &contactMap); err != nil {
	return err
}

for key, val := range contactMap {
	switch key {
		case "EmailAddress":
			c.EmailAddress = val.(string)
		case "Name":
			c.Name = val.(string)
		case "Phone":
			c.Phone = val.(string)
		default:
			c.CustomFields[key] = val
		}
	}
}

As Simon has pointed out in comment, using one big map[string]interface{} isn't ideal if the structure of the json is fixed. Best way then is to use structure and unmarshall it using http://golang.org/pkg/encoding/json/#Unmarshal. (refer the example: http://play.golang.org/p/cDTe8x4xLk)

But for large json blob, for which structure is not known beforehand, your implementation works perfectly.

edit: added link to example