英文:
Omitting JSON for Empty Custom Type
问题
我正在尝试为可能为nil
的数据库类型编写自定义的编组器(marshaler)。它的结构与sql.NullFloat64
类型完全相同:
type NullFloat64 sql.NullFloat64
func (ni *NullFloat64) MarshalJSON() ([]byte, error) {
if !ni.Valid {
return []byte("null"), nil
}
return json.Marshal(ni.Float64)
}
这是要编组的较大struct
的一部分:
type Data struct {
X time.Time `json:"x"`
Y float32 `json:"y"`
Stderr NullFloat64 `json:"stderr,omitempty"`
}
如果我尝试对这个struct
进行json.Marshal()
,它会正确地创建如下结果:
{"x":"2017-01-12T23:36:12-05:00","y":4,"stderr":null}
我希望如果值为null
,完全省略JSON键。我在Data
中添加了json:"stderr,omitempty"
。
根据这里的建议,我尝试从MarshalJSON
中返回nil
值,但是得到了以下错误:
json: error calling MarshalJSON for type common.NullFloat64: unexpected end of JSON input
我还尝试将Data
更新为:
type Data struct {
X time.Time `json:"x"`
Y float32 `json:"y"`
Stderr *NullFloat64 `json:"stderr,omitempty"`
}
并进行编组:
Data {
X: datetime,
Y: value,
Stderr: &stderr,
}
但是当从MarshalJSON
中返回nil
时,得到了与之前相同的错误。
那么,我该如何实现自定义类型的MarshalJSON
并在编组时省略键呢?谢谢帮助!
英文:
I am trying to write a custom marshaler for a possibly nil
database type. It is structured in the exact same way as the sql.NullFloat64
type:
type NullFloat64 sql.NullFloat64
func (ni *NullFloat64) MarshalJSON() ([]byte, error) {
if !ni.Valid {
return []byte("null"), nil
}
return json.Marshal(ni.Float64)
}
Which is a part of larger struct
to be marshaled:
type Data struct {
X time.Time `json:"x"`
Y float32 `json:"y"`
Stderr NullFloat64 `json:"stderr"`
}
If I try to json.Marshal()
this struct
, it works correctly, creating:
{"x":"2017-01-12T23:36:12-05:00","y":4,"stderr":null}
I would like to omit the JSON key entirely if the value is null
. I added json:"stderr,omitempty"
to Data
.
Per the suggestion here, I tried just returning a nil
value from MarshalJSON
, but got:
json: error calling MarshalJSON for type common.NullFloat64: unexpected end of JSON input
I also tried updating Data
as:
type Data struct {
X time.Time `json:"x"`
Y float32 `json:"y"`
Stderr *NullFloat64 `json:"stderr,omitempty"`
}
And marshaling:
Data {
X: datetime,
Y: value,
Stderr: &stderr,
}
But got the same error when returning nil
from MarshalJSON
as before.
So, how can I implement MarshalJSON
for a custom type and omit the key when marshaling?
Thanks for the help!
答案1
得分: 2
如果你按照以下方式创建类型:
Data {
X: datetime,
Y: value,
Stderr: nil,
}
omitempty
会起作用并且"做正确的事情"。不幸的是,我相当确定这对你没有帮助。
如果你真的想要根据内部状态省略一个字段,你需要在结构体上实现json.Marshaller
,而不是在它的子结构体上实现。最简单的方法如下:
func (d Data) MarshalJSON() ([]byte, error) {
if !d.Stderr.Valid {
return json.Marshal(Data{d.X, d.Y, nil})
}
return json.Marshal(d)
}
英文:
If you create you type like so:
Data {
X: datetime,
Y: value,
Stderr: nil,
}
omitempty
will kick in and "do the right thing". Sadly, I'm pretty sure this won't help you.
If you really want to omit a field based and internal state, you need to implement json.Marshaller
on your structure, not its children. The easiest way to do this would be as follows:
func (d Data) MarshalJSON() ([]byte, error) {
if !d.Stderr.Valid {
return json.Marshal(Data{d.X, d.Y, nil})
}
return json.Marshal(d)
}
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