datastore/load.go
func setVal(v reflect.Value, pValue interface{}) string {
	switch v.Kind() {
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		x, ok := pValue.(int64)
		if !ok && pValue != nil {
			return typeMismatchReason(pValue, v)
		}
		if v.OverflowInt(x) {
			return fmt.Sprintf("value %v overflows struct field of type %v", x, v.Type())
		}
		v.SetInt(x)
	case reflect.Bool:
		x, ok := pValue.(bool)
		if !ok && pValue != nil {
			return typeMismatchReason(pValue, v)
		}
		v.SetBool(x)
	case reflect.String:
		switch x := pValue.(type) {
		case appengine.BlobKey:
			v.SetString(string(x))
		case ByteString:
			v.SetString(string(x))
		case string:
			v.SetString(x)
		default:
			if pValue != nil {
				return typeMismatchReason(pValue, v)
			}
		}
	case reflect.Float32, reflect.Float64:
		x, ok := pValue.(float64)
		if !ok && pValue != nil {
			return typeMismatchReason(pValue, v)
		}
		if v.OverflowFloat(x) {
			return fmt.Sprintf("value %v overflows struct field of type %v", x, v.Type())
		}
		v.SetFloat(x)
	case reflect.Ptr:
		x, ok := pValue.(*Key)
		if !ok && pValue != nil {
			return typeMismatchReason(pValue, v)
		}
		if _, ok := v.Interface().(*Key); !ok {
			return typeMismatchReason(pValue, v)
		}
		v.Set(reflect.ValueOf(x))
	case reflect.Struct:
		switch v.Type() {
		case typeOfTime:
			x, ok := pValue.(time.Time)
			if !ok && pValue != nil {
				return typeMismatchReason(pValue, v)
			}
			v.Set(reflect.ValueOf(x))
		case typeOfGeoPoint:
			x, ok := pValue.(appengine.GeoPoint)
			if !ok && pValue != nil {
				return typeMismatchReason(pValue, v)
			}
			v.Set(reflect.ValueOf(x))
		default:
			ent, ok := pValue.(*Entity)
			if !ok {
				return typeMismatchReason(pValue, v)
			}

			// Recursively load nested struct
			pls, err := newStructPLS(v.Addr().Interface())
			if err != nil {
				return err.Error()
			}

			// if ent has a Key value and our struct has a Key field,
			// load the Entity's Key value into the Key field on the struct.
			if ent.Key != nil && pls.codec.keyField != -1 {

				pls.v.Field(pls.codec.keyField).Set(reflect.ValueOf(ent.Key))
			}

			err = pls.Load(ent.Properties)
			if err != nil {
				return err.Error()
			}
		}
	case reflect.Slice:
		x, ok := pValue.([]byte)
		if !ok {
			if y, yok := pValue.(ByteString); yok {
				x, ok = []byte(y), true
			}
		}
		if !ok && pValue != nil {
			return typeMismatchReason(pValue, v)
		}
		if v.Type().Elem().Kind() != reflect.Uint8 {
			return typeMismatchReason(pValue, v)
		}
		v.SetBytes(x)
	default:
		return typeMismatchReason(pValue, v)
	}
	return ""
}
func setVal(v reflect.Value, pValue interface{}) string {
	switch v.Kind() {
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		x, ok := pValue.(int64)
		if !ok && pValue != nil {
			return typeMismatchReason(pValue, v)
		}
		if v.OverflowInt(x) {
			return fmt.Sprintf("value %v overflows struct field of type %v", x, v.Type())
		}
		v.SetInt(x)
	case reflect.Bool:
		x, ok := pValue.(bool)
		if !ok && pValue != nil {
			return typeMismatchReason(pValue, v)
		}
		v.SetBool(x)
	case reflect.String:
		switch x := pValue.(type) {
		case appengine.BlobKey:
			v.SetString(string(x))
		case ByteString:
			v.SetString(string(x))
		case string:
			v.SetString(x)
		default:
			if pValue != nil {
				return typeMismatchReason(pValue, v)
			}
		}
	case reflect.Float32, reflect.Float64:
		x, ok := pValue.(float64)
		if !ok && pValue != nil {
			return typeMismatchReason(pValue, v)
		}
		if v.OverflowFloat(x) {
			return fmt.Sprintf("value %v overflows struct field of type %v", x, v.Type())
		}
		v.SetFloat(x)
	case reflect.Ptr:
		x, ok := pValue.(*Key)
		if !ok && pValue != nil {
			return typeMismatchReason(pValue, v)
		}
		if _, ok := v.Interface().(*Key); !ok {
			return typeMismatchReason(pValue, v)
		}
		v.Set(reflect.ValueOf(x))
	case reflect.Struct:
		switch v.Type() {
		case typeOfTime:
			x, ok := pValue.(time.Time)
			if !ok && pValue != nil {
				return typeMismatchReason(pValue, v)
			}
			v.Set(reflect.ValueOf(x))
		case typeOfGeoPoint:
			x, ok := pValue.(appengine.GeoPoint)
			if !ok && pValue != nil {
				return typeMismatchReason(pValue, v)
			}
			v.Set(reflect.ValueOf(x))
		default:
			ent, ok := pValue.(*Entity)
			if !ok {
				return typeMismatchReason(pValue, v)
			}

			// Recursively load nested struct
			pls, err := newStructPLS(v.Addr().Interface())
			if err != nil {
				return err.Error()
			}

			// if ent has a Key value and our struct has a Key field,
			// load the Entity's Key value into the Key field on the struct.
			if ent.Key != nil && pls.codec.keyField != -1 {

				pls.v.Field(pls.codec.keyField).Set(reflect.ValueOf(ent.Key))
			}

			err = pls.Load(ent.Properties)
			if err != nil {
				return err.Error()
			}
		}
	case reflect.Slice:
		x, ok := pValue.([]byte)
		if !ok {
			if y, yok := pValue.(ByteString); yok {
				x, ok = []byte(y), true
			}
		}
		if !ok && pValue != nil {
			return typeMismatchReason(pValue, v)
		}
		if v.Type().Elem().Kind() != reflect.Uint8 {
			return typeMismatchReason(pValue, v)
		}
		v.SetBytes(x)
	default:
		return typeMismatchReason(pValue, v)
	}
	return ""
}
// setVal sets v to the value pValue.