Google App Engine の Go データストアに動的プロパティを実装するにはどうすればよいですか?

package main

import (
    "context"
    "time"

    "google.golang.org/appengine/datastore"
)

func main() {
    ctx := context.Background()

    props := datastore.PropertyList{
        datastore.Property{Name: "time", Value: time.Now()},
        datastore.Property{Name: "email", Value: "example@email.com"},
    }

    k := datastore.NewIncompleteKey(ctx, "DynEntity", nil)
    key, err := datastore.Put(ctx, k, &props)
    if err != nil {
        // Handle error
    }

    c.Infof("%v", key)
}

type DynEnt map[string]interface{}

func (d *DynEnt) Load(props []datastore.Property) error {
    for _, p := range props {
        (*d)[p.Name] = p.Value
    }
    return nil
}

func (d *DynEnt) Save() (props []datastore.Property, err error) {
    for k, v := range *d {
        props = append(props, datastore.Property{Name: k, Value: v})
    }
    return
}

d := DynEnt{"email": "example@email.com", "time": time.Now()}

k := datastore.NewIncompleteKey(ctx, "DynEntity", nil)
key, err := datastore.Put(ctx, k, &d)
if err != nil {
    // Handle error
}

c.Infof("%v", key)