英文:
Getting a datastore entity into an interface in Go
问题
我有几个数据存储种类，它们具有相同的字段Id。是否可能创建一个通用函数来获取这个值？类似于这样的代码？
var i interface{}
err = datastore.Get(c, key, &i)
v := reflect.ValueOf(i)
id := v.FieldByName("Id").String()
上述代码会给我一个"datastore: invalid entity type"错误。
英文:
I have a couple of datastore Kinds that have the same field Id. Is it possible to create one generic function that can get me this value? Something similar to this?
var i interface{}
err = datastore.Get(c, key, &i)
v := reflect.ValueOf(i)
id := v.FieldByName("Id").String()
The above code, as it is, gives me a "datastore: invalid entity type" error.
答案1
得分: 8
var i interface{} 不属于任何具体类型。appengine数据存储需要一个具体类型来反序列化数据，因为它使用反射。从文档中看，缺少字段或字段与存储的数据类型不同也会导致返回错误，因此不能仅定义具有ID字段的结构体。
即使如此，您仍然可以使用实现PropertyLoadSaver接口的自定义类型来解决问题，如下所示：
type IdField struct {
Id string
}
func (f *IdField) Load(ch <-chan Property) error {
for p := range ch {
if p.Name == "Id" {
f.Id = p.Value.(string)
}
}
return nil
}
func (f *IdField) Save(ch chan<- Property) error {
return fmt.Errorf("Not implemented")
}
var i = &IdField{}
err := datastore.Get(c, key, i)
id := i.Id
这可能不如您希望的那样简洁，但它更加类型安全，不需要反射，并且说明了您可以使用的一般方法来从数据存储中获取部分数据。
英文:
var i interface{} isn't of any concrete type. The appengine datastore requires a concrete type to deserialize the data into since it uses reflection. It looks like from the documentation that missing fields or fields of a different type than the data was stored from will cause an error to be returned as well so you can't create a struct with just the ID field defined.
Even so it's possible you could work something out using a custom type that implements the PropertyLoadSaver interface like so:
type IdField struct {
Id string
}
function (f *IdField) Load(ch <-chan Property) error {
for p := range ch {
if p.Name == "Id" {
f.Id = p.Value.(string)
}
}
return nil
}
function (f *IdField) Save(ch chan<- Property) error {
return fmt.Errorf("Not implemented")
}
var i = &IdField{}
err := datastore.Get(c, key, i)
id := i.Id
It's probably not as concise as you were hoping but it's a little more typesafe doesn't require reflection and illustrates the general approach you could use to get partial data out of the datastore.
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