omitempty tag并且工作正常.很好,直到我意识到我仍然需要构建一个UPDATE SQL查询.
我的结构看起来像这样：
type Resource struct {
Name *string `json:"name,omitempty" sql:"resource_id"`
Description *string `json:"description,omitempty" sql:"description"`
}
我期待包含这样一个请求主体的PATCH / resources / {resource-id}请求：
{"description":"Some new description"}
在我的处理程序中,我将以这种方式构建Resource对象(忽略导入,忽略错误处理)：
var resource Resource resourceID,_ := mux.Vars(r)["resource-id"] d := json.NewDecoder(r.Body) d.Decode(&resource) // at this point our resource object should only contain // the Description field with the value from JSON in request body
现在,对于正常的UPDATE(PUT请求),我会这样做(简化)：
stmt,_ := db.Prepare(`UPDATE resources SET description = ?,name = ? WHERE resource_id = ?`) res,_ := stmt.Exec(resource.Description,resource.Name,resourceID)
PATCH和omitempty标记的问题是该对象可能缺少多个属性,因此我不能只使用硬编码字段和占位符来准备语句…我将不得不动态地构建它.
这里有我的问题：我如何动态构建这样的UPDATE查询？在最好的情况下,我需要一些解决方案来识别设置属性,获取他们的sql字段名称(可能来自标签)然后我应该能够构建UPDATE查询.我知道我可以使用反射来获取对象属性,但不知道如何获取他们的sql标签名称,当然我希望尽可能避免在这里使用反射…或者我可以简单地检查每个属性它不是没有,但在现实生活中,结构比这里提供的示例大得多……
有人可以帮我这个吗？有人已经有必要解决相同/类似的情况吗？
解：
基于这里的答案,我能够提出这个抽象的解决方案. sqlPatches方法从给定的struct构建sqlPatch结构(因此没有特定的具体结构)：
import (
"fmt"
"encoding/json"
"reflect"
"strings"
)
const tagname = "sql"
type sqlPatch struct {
Fields []string
Args []interface{}
}
func sqlPatches(resource interface{}) sqlPatch {
var sqlPatch sqlPatch
rType := reflect.TypeOf(resource)
rVal := reflect.ValueOf(resource)
n := rType.NumField()
sqlPatch.Fields = make([]string,n)
sqlPatch.Args = make([]interface{},n)
for i := 0; i < n; i++ {
fType := rType.Field(i)
fVal := rVal.Field(i)
tag := fType.Tag.Get(tagname)
// skip nil properties (not going to be patched),skip unexported fields,skip fields to be skipped for sql
if fVal.IsNil() || fType.PkgPath != "" || tag == "-" {
continue
}
// if no tag is set,use the field name
if tag == "" {
tag = fType.Name
}
// and make the tag lowercase in the end
tag = strings.ToLower(tag)
sqlPatch.Fields = append(sqlPatch.Fields,tag+" = ?")
var val reflect.Value
if fVal.Kind() == reflect.Ptr {
val = fVal.Elem()
} else {
val = fVal
}
switch val.Kind() {
case reflect.Int,reflect.Int8,reflect.Int16,reflect.Int32,reflect.Int64:
sqlPatch.Args = append(sqlPatch.Args,val.Int())
case reflect.String:
sqlPatch.Args = append(sqlPatch.Args,val.String())
case reflect.Bool:
if val.Bool() {
sqlPatch.Args = append(sqlPatch.Args,1)
} else {
sqlPatch.Args = append(sqlPatch.Args,0)
}
}
}
return sqlPatch
}
然后我可以简单地这样称呼它：
type Resource struct {
Description *string `json:"description,omitempty"`
Name *string `json:"name,omitempty"`
}
func main() {
var r Resource
json.Unmarshal([]byte(`{"description": "new description"}`),&r)
sqlPatch := sqlPatches(r)
data,_ := json.Marshal(sqlPatch)
fmt.Printf("%s\n",data)
}
你可以在Go Playground检查它.我看到的唯一问题是我分配了传递结构中字段数量的两个切片,可能是10,即使我可能只想在最后修补一个属性导致分配比需要更多的内存……任何想法如何避免这种情况？
解决方法
The difference between the PUT and PATCH requests is reflected in the way the server processes the enclosed entity to modify the resource identified by the Request-URI. In a PUT request,the enclosed entity is considered to be a modified version of the resource stored on the origin server,and the client is requesting that the stored version be replaced. With PATCH,however,the enclosed entity contains a set of instructions describing how a resource currently residing on the origin server should be modified to produce a new version. The PATCH method affects the resource identified by the Request-URI,and it also MAY have side effects on other resources; i.e.,new resources may be created,or existing ones modified,by the application of a PATCH.
例如：
[
{ "op": "test","path": "/a/b/c","value": "foo" },{ "op": "remove","path": "/a/b/c" },{ "op": "add","value": [ "foo","bar" ] },{ "op": "replace","value": 42 },{ "op": "move","from": "/a/b/c","path": "/a/b/d" },{ "op": "copy","from": "/a/b/d","path": "/a/b/e" }
]
这组指令应该可以更轻松地构建更新查询.
编辑
这就是obtain sql tags的方法,但你必须使用反射：
type Resource struct {
Name *string `json:"name,omitempty" sql:"resource_id"`
Description *string `json:"description,omitempty" sql:"description"`
}
sp := "sort of string"
r := Resource{Description: &sp}
rt := reflect.TypeOf(r) // reflect.Type
rv := reflect.ValueOf(r) // reflect.Value
for i := 0; i < rv.NumField(); i++ { // Iterate over all the fields
if !rv.Field(i).IsNil() { // Check it is not nil
// Here you would do what you want to having the sql tag.
// Creating the query would be easy,however
// not sure you would execute the statement
fmt.Println(rt.Field(i).Tag.Get("sql")) // Output: description
}
}
我知道你不想使用反射,但是当你评论状态时,这可能是比前一个更好的答案.
编辑2：
关于分配 – 阅读Effective Go about Data structures and Allocation的本指南：
// Here you are allocating an slice of 0 length with a capacity of n sqlPatch.Fields = make([]string,n) sqlPatch.Args = make([]interface{},n)
使用make(类型,长度,容量(可选))
请考虑以下示例：
// newly allocated zeroed value with Composite Literal // length: 0 // capacity: 0 testSlice := []int{} fmt.Println(len(testSlice),cap(testSlice)) // 0 0 fmt.Println(testSlice) // [] // newly allocated non zeroed value with make // length: 0 // capacity: 10 testSlice = make([]int,10) fmt.Println(len(testSlice),cap(testSlice)) // 0 10 fmt.Println(testSlice) // [] // newly allocated non zeroed value with make // length: 2 // capacity: 4 testSlice = make([]int,2,4) fmt.Println(len(testSlice),cap(testSlice)) // 2 4 fmt.Println(testSlice) // [0 0]
在您的情况下,可能需要以下内容：
// Replace this sqlPatch.Fields = make([]string,n) // With this or simple omit the capacity in make above sqlPatch.Fields = []string{} sqlPatch.Args = []interface{}{} // The allocation will go as follow: length - capacity testSlice := []int{} // 0 - 0 testSlice = append(testSlice,1) // 1 - 2 testSlice = append(testSlice,1) // 2 - 2 testSlice = append(testSlice,1) // 3 - 4 testSlice = append(testSlice,1) // 4 - 4 testSlice = append(testSlice,1) // 5 - 8
