Merge structs with some overlapping fields - go

I saw several questions asking on how to merge unique structs and how to merge identical structs.
But how would I merge structs that have some overlap? and which fields get taken & when?
e.g.:
type structOne struct {
id string `json:id`
date string `json:date`
desc string `json:desc`
}
and
type structTwo struct {
id string `json:id`
date string `json:date`
name string `json:name`
}
how would I merge it such that I get
{
id string `json:id`
date string `json:date`
desc string `json:desc`
name string `json:name`
}
also, what happens if in this case the two id's are the same (assuming a join over id's) but the names are different?
In javascript, doing something like Object.assign(structOne, structTwo).

Go is a strongly typed language, unlike javascript you can't merge two struct into one combined struct because all type are determined at compile-time. You have two solution here :
Using embedded struct:
One great solution is to use embedded struct because you don't have to merge anything anymore.
package main
import (
"bytes"
"encoding/json"
"fmt"
"log"
)
// Shared field
type common struct {
ID string `json:id`
Date string `json:date`
}
type merged struct {
// Common field is embedded
common
Name string `json:name`
Desc string `json:desc`
}
func main() {
buf := bytes.Buffer{}
buf.WriteString("{ \"id\": \"1\", \"date\": \"27/07/2020\", \"desc\": \"the decription...\" }")
merged := &merged{}
err := json.Unmarshal(buf.Bytes(), merged)
if err != nil {
log.Fatal(err)
}
// Look how you can easily access field from
// embedded struct
fmt.Println("ID:", merged.ID)
fmt.Println("Date:", merged.Date)
fmt.Println("Name:", merged.Name)
fmt.Println("Desc:", merged.Desc)
// Output:
// ID: 1
// Date: 27/07/2020
// Name:
// Desc: the decription...
}
If you want to read more about struct embedding:
golangbyexample.com
travix.io
Using Maps
Another solution is to use maps but you will loose the benefits of struct and methods. This example is not the simplest but there is some great example in the other responses.
In this example I'm using Mergo. Mergo is library that can merge structs and map. Here it is used for creating maps object in the Map methods but you can totally write your own methods.
package main
import (
"fmt"
"log"
"github.com/imdario/mergo"
)
type tOne struct {
ID string
Date string
Desc string
}
// Map build a map object from the struct tOne
func (t1 tOne) Map() map[string]interface{} {
m := make(map[string]interface{}, 3)
if err := mergo.Map(&m, t1); err != nil {
log.Fatal(err)
}
return m
}
type tTwo struct {
ID string
Date string
Name string
}
// Map build a map object from the struct tTwo
func (t2 tTwo) Map() map[string]interface{} {
m := make(map[string]interface{}, 3)
if err := mergo.Map(&m, t2); err != nil {
log.Fatal(err)
}
return m
}
func main() {
dst := tOne{
ID: "destination",
Date: "26/07/2020",
Desc: "destination object",
}.Map()
src := tTwo{
ID: "src",
Date: "26/07/1010",
Name: "source name",
}.Map()
if err := mergo.Merge(&dst, src); err != nil {
log.Fatal(err)
}
fmt.Printf("Destination:\n%+v", dst)
// Output:
// Destination:
// map[date:26/07/2020 desc:destination object iD:destination name:object name
}

Go structs and JavaScript objects are very different. Go structs do not have dynamic fields.
If you want dynamic key/value sets that you can easily iterate over and merge, and are very JSON friendly, why not a map[string]interface{}?
$ go run t.go
map[a:1 b:4]
map[a:1 b:4 c:3]
$ cat t.go
package main
import(
"fmt"
)
type MyObj map[string]interface{}
func (mo MyObj)Merge(omo MyObj){
for k, v := range omo {
mo[k] = v
}
}
func main() {
a := MyObj{"a": 1, "b": 4}
b := MyObj{"b": 2, "c": 3}
b.Merge(a)
fmt.Printf("%+v\n%+v\n", a, b)
}

you can use github.com/fatih/structs to convert your struct to map. Then iterate over that map and choose which fields need to copy over. I have a snippet of code which illustrates this solution.
func MergeStruct (a structOne,b structTwo) map[string]interface{}{
a1:=structs.Map(a)
b1:=structs.Map(b)
/* values of structTwo over writes values of structOne */
var myMap=make(map[string]interface{})
for val,key:=range(a1){
myMap[key]=val
}
for val,key:=range(b1){
myMap[key]=val
}
return myMap
}

You can use the reflect package to do this. Try iterating through the two structs and then you can either use another struct type to store the values or maybe use a map.
Check out this question to find out how you can iterate over a struct.
Check out this question to find out how to get the name of the fields.
Remember to use exported field names for the reflect package to work.
Here is an example which works.

Related

How to convert string to struct array

I have a json array which is converted into a string. Now I want to map the string to a struct array so that I can modify the string json. Below is my code base
type ProcessdetailsEntity struct {
Source []int64 `json:"source"`
Node string `json:"node"`
Targets []int64 `json:"targets"`
Issave bool `json:"isSave"`
Instate []int64 `json:"inState"`
OutState []int64 `json:"outState"`
}
func main() {
var stringJson = "[{\"source\":[-1],\"node\":\"1_1628008588902\",\"targets\":[],\"isSave\":true,\"inState\":[1],\"outState\":[2]},{\"source\":[\"1_1628008588902\",\"5_1628008613446\"],\"node\":\"2_1628008595757\",\"targets\":[],\"isSave\":true,\"inState\":[2,5],\"outState\":[3,6]}]"
in := []byte(stringJson)
detailsEntity := []ProcessdetailsEntity{}
err := json.Unmarshal(in, &detailsEntity)
if err != nil {
log.Print(err)
}
}
Now when I run this code base I got the error:
json: cannot unmarshal string into Go struct field ProcessdetailsEntity.source of type int64
How to properly map the string to struct so that I can modify the inState and outState value of the json ?
The error you get is already pretty much on the nose:
cannot unmarshal string into Go struct field ProcessdetailsEntity.source of type int64
That tells you that (at least one) of your source fields appears to have the wrong type: a string instead of something that can be represented by a int64.
So let's check your source fields in your stringJson:
"source":[-1]
"source":["1_1628008588902","5_1628008613446"]
As you can see the second source is an array of string. Hence the error.
To solve this you need to make sure that the source is an array of int. Unfortunately, 1_1628008588902 and 5_1628008613446 are not valid integers in Go.
I slightly modified your JSON and fixed your code an then it works:
package main
import (
"encoding/json"
"log"
)
type ProcessdetailsEntity struct {
Source []int64 `json:"source"`
Node string `json:"node"`
Targets []int64 `json:"targets"`
Issave bool `json:"isSave"`
Instate []int64 `json:"inState"`
OutState []int64 `json:"outState"`
}
func main() {
var stringJson = `[
{
"source":[-1],
"node":"1_1628008588902",
"targets":[],
"isSave":true,
"inState":[1],
"outState":[2]
},
{
"source":[11628008588902,51628008613446],
"node":"2_1628008595757",
"targets":[],
"isSave":true,
"inState":[2,5],
"outState":[3,6]
}
]`
in := []byte(stringJson)
detailsEntity := []ProcessdetailsEntity{}
err := json.Unmarshal(in, &detailsEntity)
if err != nil {
log.Print(err)
}
}
See: https://play.golang.org/p/kcrkfRliWJ5

Parsing JSON using struct

I'm trying to parse JSON using Go. Can anyone tell me why my code is not working as expected?
package main
import (
"encoding/json"
"fmt"
)
type Message struct {
Name string
Body string
Time int64
}
type Person struct {
M Message
}
func get_content() {
body := []byte(`{"person":{"Name":"Alice","Body":"Hello","Time":1294706395881547000}}`)
var data Person
err := json.Unmarshal(body, &data)
if err != nil {
panic(err.Error())
}
fmt.Printf("%v",data.M.Name)
}
func main() {
get_content()
}
I'm expecting it to print the Name.
Go playground Code
There are two problems in the code.
The first one is what #umar-hayat mentioned above -> you are unmarshalling into the data object and you should be aiming at the data.M field.
The second problem is that your JSON's structure doesn't match your struct's structure. Your Person has a single field called M. If we want to represent this as JSON it would look like this:
{
"M": {
"Name": "Joe",
"Body": "Hi",
"time": 2600
}
}
Instead, you have a field called person in your JSON which cannot be matched to any field in your struct. The fact that it's similar to the name of the struct's type doesn't help in any way, I'm afraid.
So, you can either change your JSON and your target:
body := []byte(`{"Name":"Alice","Body":"Hello","Time":1294706395881547000}`)
var data Person
err := json.Unmarshal(body, &data.M)
Or just your JSON:
body := []byte(`{"M":{"Name":"Alice","Body":"Hello","Time":1294706395881547000}}`)
var data Person
err := json.Unmarshal(body, &data)
But it's essential that the names of the fields in your JSON match the names of the fields in your struct. Or, as mentioned by Konstantinos, you can use tags in order to specify particular names with which your struct's fields will be represented in the JSON.
You might find this helpful: https://gobyexample.com/json
Here is how to Unmarshel JSON to the struct. you can check it on Go Playground here:
package main
import (
"encoding/json"
"fmt"
)
type Message struct {
Name string
Body string
Time int64
}
type Person struct {
M Message
}
func get_content() {
body := []byte(`{"Name":"Alice","Body":"Hello","Time":1294706395881547000}`)
var data Person
err := json.Unmarshal(body, &data.M)
if err != nil {
panic(err.Error())
}
fmt.Printf(data.M.Name)
}
func main() {
get_content()
}
Replace data with data.M in below line.
err := json.Unmarshal(body, &data)
As long as you intent to map Json keys on structs whose fields have different names you should add tags:
type Message struct {
Name string `json:"Name"`
Body string `json:"Body"`
Time int64 `json:"Time"`
}
type Person struct {
M Message `json:"person"`
}
You can find more information here
In addition this answer explains in an nutshell the purpose of tags in go.

how do I implement custom struct tags AND be able to XML encode a map?

I have the following:
https://play.golang.org/p/ADX6H-bh0CU
package main
import (
"encoding/xml"
"fmt"
)
type xmlMap map[string]string
type xmlMapEntry struct {
XMLName xml.Name
Value string `xml:",chardata"`
}
func (m xmlMap) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
if len(m) == 0 {
return nil
}
err := e.EncodeToken(start)
if err != nil {
return err
}
for k, v := range m {
e.Encode(xmlMapEntry{XMLName: xml.Name{Local: k}, Value: v})
}
return e.EncodeToken(start.End())
}
func main() {
type Family struct {
Siblings map[string]string
}
type MyFamily struct {
Siblings xmlMap `json:"siblings" xml:"siblings"`
}
// In reality, "f" comes from a function in an outside package...e.g. "f := somepackage.Function()"
f := &Family{
Siblings: map[string]string{"bob": "brother", "mary": "sister"},
}
var m = &MyFamily{}
*m = MyFamily(*f)
x, err := xml.Marshal(m)
if err != nil {
panic(err)
}
fmt.Println(string(x))
}
The Family struct comes from an outside package. I created MyFamily struct in order to add json and xml tags. If I try to xml encode MyFamily then I get an error:
xml: unsupported type: map[string]string
This is easily resolved by implementing the custom XML marshaller that I have done.
This creates a separate problem. I have tried to simplify the example but "f" comes from a function in an outside package...e.g. "f := somepackage.Function()". When I try to Alias f to m I get the error:
cannot convert *f (type Family) to type MyFamily
This happens because Family has a map[string]string but MyFamily has xmlMap. Even though they are the same underlying type I get that error.
The question is, how do I implement my custom struct tags AND be able to XML encode a map?
EDIT:
This is a simple example and can just be resolved by
var m = &MyFamily{Siblings: f.Siblings}
Since there are MANY fields in both Family and MyFamily this is just inefficient and just wanted to know if there's a better way.

How to create struct runtime in golang

For example, I have a struct that I take from the outside. I do not know the struct in fields and field values. I want to copy and use the same struct.
With reflection I find the fields and types in it. But how do I create this struct in the runtime?
Edit : I just want to create a struct in the same name as the runtime. Imagine I do not know my person type. I just want to create the same struct by reflection with interface.
I only know one interface. Person struct I just created it for instance. When a person creates a struct and sends it out, I will create it. instead of person, customer, student etc. You can send.
consider the following code as a 3rd party library.
package main
import(
"fmt"
"reflect"
)
type Person struct {
Id int
Name string
Surname string
}
func main(){
person := NewSomething()
newPerson := typeReflection(person)
ChangePerson(newPerson)
fmt.Println("Success")
}
func typeReflection(_person interface{}){
val := reflect.ValueOf(_person)
//How to create same struct
}
The github.com/mitchellh/copystructure library handles this operation, which is known as a deep copy. After performing a deep copy, the original and the copy contain the same data but modifications to either one do not affect the other.
package main
import (
"fmt"
"github.com/mitchellh/copystructure"
)
type Person struct {
Id int
Name string
Surname string
}
func main() {
original := Person{Id: 0, Name: "n", Surname: "s"}
copy := deepCopy(original)
// Change fields of the original Person.
original.Id = 9
fmt.Printf("original: %#v\n", original)
// The copy of the Person has not change, still has Id:0.
fmt.Printf("copy: %#v\n", copy)
}
func deepCopy(original interface{}) interface{} {
copy, err := copystructure.Copy(original)
if err != nil {
panic(err)
}
return copy
}

Golang interface to struct

I have a function that has a parameter with the type interface{}, something like:
func LoadTemplate(templateData interface{}) {
In my case, templateData is a struct, but each time it has a different structure. I used the type "interface{}" because it allows me to send all kind of data.
I'm using this templateData to send the data to the template:
err := tmpl.ExecuteTemplate(w, baseTemplateName, templateData)
But now I want to append some new data and I don't know how to do it because the "interface" type doesn't allow me to add/append anything.
I tried to convert the interface to a struct, but I don't know how to append data to a struct with an unknown structure.
If I use the following function I can see the interface's data:
templateData = appendAssetsToTemplateData(templateData)
func appendAssetsToTemplateData(t interface{}) interface{} {
switch reflect.TypeOf(t).Kind() {
case reflect.Struct:
fmt.Println("struct")
s := reflect.ValueOf(t)
fmt.Println(s)
//create a new struct based on current interface data
}
return t
}
Any idea how can I append a child to the initial interface parameter (templateData)? Or how can I transform it to a struct or something else in order to append the new child/data?
Adrian is correct. To take it a step further, you can only do anything with interfaces if you know the type that implements that interface. The empty interface, interface{} isn't really an "anything" value like is commonly misunderstood; it is just an interface that is immediately satisfied by all types.
Therefore, you can only get values from it or create a new "interface" with added values by knowing the type satisfying the empty interface before and after the addition.
The closest you can come to doing what you want, given the static typing, is by embedding the before type in the after type, so that everything can still be accessed at the root of the after type. The following illustrates this.
https://play.golang.org/p/JdF7Uevlqp
package main
import (
"fmt"
)
type Before struct {
m map[string]string
}
type After struct {
Before
s []string
}
func contrivedAfter(b interface{}) interface{} {
return After{b.(Before), []string{"new value"}}
}
func main() {
b := Before{map[string]string{"some": "value"}}
a := contrivedAfter(b).(After)
fmt.Println(a.m)
fmt.Println(a.s)
}
Additionally, since the data you are passing to the template does not require you to specify the type, you could use an anonymous struct to accomplish something very similar.
https://play.golang.org/p/3KUfHULR84
package main
import (
"fmt"
)
type Before struct {
m map[string]string
}
func contrivedAfter(b interface{}) interface{} {
return struct{
Before
s []string
}{b.(Before), []string{"new value"}}
}
func main() {
b := Before{map[string]string{"some": "value"}}
a := contrivedAfter(b)
fmt.Println(a)
}
You can't append data arbitrarily to a struct; they're statically typed. You can only assign values to the fields defined for that specific struct type. Your best bet is probably to use a map instead of structs for this.
Not recommended, but you can create structs dynamically using the reflect package.
Here is an example:
package main
import (
"encoding/json"
"os"
"reflect"
)
type S struct {
Name string
}
type D struct {
Pants bool
}
func main() {
a := Combine(&S{"Bob"}, &D{true})
json.NewEncoder(os.Stderr).Encode(a)
}
func Combine(v ...interface{}) interface{} {
f := make([]reflect.StructField, len(v))
for i, u := range v {
f[i].Type = reflect.TypeOf(u)
f[i].Anonymous = true
}
r := reflect.New(reflect.StructOf(f)).Elem()
for i, u := range v {
r.Field(i).Set(reflect.ValueOf(u))
}
return r.Addr().Interface()
}
You could use something like the Combine function above to shmush any number of structs together. Unfortunately, from the documentation:
StructOf currently does not generate wrapper methods for embedded fields. This limitation may be lifted in a future version.
So your created struct won't inherit methods from the embedded types. Still, maybe it does what you need.
If you are just looking to convert your interface to struct, use this method.
type Customer struct {
Name string `json:"name"`
}
func main() {
// create a customer, add it to DTO object and marshal it
receivedData := somefunc() //returns interface
//Attempt to unmarshall our customer
receivedCustomer := getCustomerFromDTO(receivedData)
fmt.Println(receivedCustomer)
}
func getCustomerFromDTO(data interface{}) Customer {
m := data.(map[string]interface{})
customer := Customer{}
if name, ok := m["name"].(string); ok {
customer.Name = name
}
return customer
}

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