I've just started studying Golang and don't understand how to write only specific JSON fields to an output file.
For example I have this struct:
type example struct {
Ifindex int `json:"ifindex"`
HostID int `json:"host_id"`
Hostname string `json:"hostname"`
Name string `json:"name"`
}
My output file should be in the following format:
[{"Ifindex": int, "Hostname": string}, {...}]
How can I do it?
If I understood correctly, you'd like to omit some of the fields when marshalling to JSON. Then use json:"-" as a field tag.
Per the json.Marshal(...) documentation:
As a special case, if the field tag is "-", the field is always omitted.
So you just need to use the tag "-" for any public field that you do not want serialized, for example (Go Playground):
type Example struct {
Ifindex int `json:"ifindex"`
HostID int `json:"-"`
Hostname string `json:"hostname"`
Name string `json:"-"`
}
func main() {
eg := Example{Ifindex: 1, HostID: 2, Hostname: "foo", Name: "bar"}
bs, err := json.Marshal(&eg)
if err != nil {
panic(err)
}
fmt.Println(string(bs))
// {"ifindex":1,"hostname":"foo"}
}
Related
I'm building a web server with Go and I don't know how to process JSON with Go.
Saying that I have a struct as below:
type User struct{
Id int
Name string
Password string
Status int
}
and now I have had an object of the struct User:
user := User{1, "Test", "password", 1}
Now I need to convert user to a JSON object. Here is what I've found:
b, err := json.Marshal(user)
fmt.Println(string(b))
It works well.
Now I want to do two things:
1) remove the Password from the JSON object
2) add a new filed: "code": 200 into the JSON object
What should I do?
If you want to keep the Password property accessible to outer packages, you can set a tag: json:"-" on it. As specified in the docs:
The encoding of each struct field can be customized by the format
string stored under the "json" key in the struct field's tag. The
format string gives the name of the field, possibly followed by a
comma-separated list of options. The name may be empty in order to
specify options without overriding the default field name.
The "omitempty" option specifies that the field should be omitted from
the encoding if the field has an empty value, defined as false, 0, a
nil pointer, a nil interface value, and any empty array, slice, map,
or string.
As a special case, if the field tag is "-", the field is always
omitted. Note that a field with name "-" can still be generated using
the tag "-,".
type User struct {
Id int
Name string
Password string `json:"-"`
Status int
Code int `json:"code"`
}
make Password lower case (and add Code int to your struct):
Try this:
package main
import (
"encoding/json"
"fmt"
)
func main() {
user := User{1, "Test", "password", 1, 200}
b, err := json.Marshal(user)
if err != nil {
panic(err)
}
fmt.Println(string(b))
}
type User struct {
Id int
Name string
password string
Status int
Code int
}
output:
{"Id":1,"Name":"Test","Status":1,"Code":200}
I have a struct which I convert to JSON in the old fashioned way:
type Output struct {
Name string `json:"name"`
Command string `json:"command"`
Status int `json:"status"`
Output string `json:"output"`
Ttl int `json:"ttl,omitempty"`
Source string `json:"source,omitempty"`
Handlers []string `json:"handlers,omitempty"`
}
sensu_values := &Output{
Name: name,
Command: command,
Status: status,
Output: output,
Ttl: ttl,
Source: source,
Handlers: [handlers],
}
I want to read an arbitrary JSON file from the filesystem, which can be defined as anything by the user, and then add it to the existing JSON string, taking the duplicates from the original.
How can I do this?
Input JSON :
{
"environment": "production",
"runbook": "http://url",
"message": "there is a problem"
}
It's better to unmarshal the input JSON and combine the two structures before marshaling Output struct.
Sample Code
inputJSON := `{"environment": "production", "runbook":"http://url","message":"there is a problem"}`
out := map[string]interface{}{}
json.Unmarshal([]byte(inputJSON), &out)
out["name"] = sensu_values.Name
out["command"] = sensu_values.Command
out["status"] = sensu_values.Status
outputJSON, _ := json.Marshal(out)
Play Link
I'm using this library to access CouchDB (cloudant to be specific) "github.com/mikebell-org/go-couchdb" and I've noticed a problem.
When I go to add a file to the database and pass in a struct, only the fields of the struct which started with a capital letter get added.
For example
type Person struct {
name string
Age int
}
func main() {
db, _ := couchdb.Database(host, database, username, password)
joe := Person{
name: "mike",
Age: 190,
}
m, _ := db.PostDocument(joe)
}
In this case, only the "age" field got updated and inserted into my database.
I've noticed this problem in another case also - when I'm doing something like this :
type Sample struct {
Name string
age int
}
joe := Sample{
Name: "xx",
age: 23,
}
byt, _ := json.Marshal(joe)
post_data := strings.NewReader(string(byt))
fmt.Println(post_data)
in this case, only Name would be printed out :
output : &{{"Name":"xx"} 0 -1}
Why is this? and If I would like to have a field with a lowercase and be inside the database, is that possible?
This is because only fields starting with a capital letter are exported, or in other words visible outside the curent package (and in the json package in this case).
Here is the part of the specifications refering to this: http://golang.org/ref/spec#Exported_identifiers
Still, you can unmarshall json fields that do no start with a capital letters using what is called "tags". With the json package, this is the syntax to use:
type Sample struct {
Name string `json:"name"`
Age int `json:"age"`
}
Refer to the documentation for more information about this.
json package only stringfiy fields start with capital letter.
see http://golang.org/pkg/encoding/json/
Struct values encode as JSON objects. Each exported struct field becomes a member of the object, using the field name as the object key, unless the field is omitted for one of the reasons given below.
You need define the struct like this:
type Sample struct{
Name string `json:"name"`
Age int `json:"age"`
}
json.Marshal method struct-in field-i only accepts fields that start with a capital letter
The json package only accesses the exported fields of struct types (those that begin with an uppercase letter). Therefore only the exported fields of a struct will be present in the JSON output.
type Sample struct {
Name string
Age int
}
I can't decode the json code below ... any ideas why it doesn't work ? See also play.golang
package main
import (
"encoding/json"
)
type LocationReadable struct {
District string
City string
State string
}
type Locale struct {
Location string
CountryCode string
CurrencyId string
CurrencySymbol string
LocationReadable LocationReadable
}
type Media struct {
Image string
Video string
}
type Variations struct {
FixedPrice float64
Media Media
Quantity int
}
type PaymentData struct {
PaymentName string
PaymentService string
}
type Payment struct {
Online PaymentData
Offline PaymentData
}
type Shipping struct {
ShippingService string
ShippingName string
ShippingCost float64
HandlingTimeMax int
DispatchTimeMin int
DispatchTimeMax int
ShippingAdditionalCost int
}
type Item []struct {
_version string
CategoryId string
Title string
Media Media
SellerId string
Locale Locale
ListingType string
Payment Payment
StartTime string
EndTime string
Shipping Shipping
TitleSlug string
Variations Variations
_fpaiStatus string
}
func main() {
itemInfoR := `{"locale":{"location":"51.51121389999999,-0.11982439999997041","countryCode":"GB","currencyId":"GBP","currencySymbol":"£","locationReadable":{"district":"City of Westminster","city":"London","state":"Greater London"}},"_version":"serving","categoryId":["Root","Cameras \u0026 Photo","Digital Cameras"],"title":"many pictures","media":{"image":["//lh5.ggpht.com/O_o_N6CFkClY5AV0-LqntpyFjor7Of4u23ZcK7lYwc2uY1ea7GWi61VDJZCB7UCb79svkjKPHIenqwEUhjHi0jdIQnnl6z_p03yktPUB1FBHezIQ","//lh6.ggpht.com/ih3q2d7CenGLPyupH9FpfsoJQWQpw1i8wWA2Kd26bFnSF2fbnKyGU9WePIhCgEeqw5p6YMVmFi1c9oS0Ag93aF_oZ3ZiwK7fQuSYIrZ9VhgXbrTHkw","//lh6.ggpht.com/7RJRsapsnwWL3_KiLIjMz4QojDzUvsztXtvKTFvIfde_AHccDnOibAvXRN73tTB4SeHzlj8S1LWxbYwwWFGn9elfCKdSb8BUIU5QJY1LO791HutQ","//lh6.ggpht.com/qAtjgyHAB734Ox_4NC_fa-ZRqrCjCmJu0Tp8bo-HMO88duv8l4hhuv2REBkB--yneFzOL7annecVlGty-YsKouondiOFVnAZWzjpdrfsGfbL6wh2","//lh3.ggpht.com/dWUbASepwHF4lHaXIPnpv4BNm2pCml9MlJt7s86s1cpu-PsYNmS0yQmKFKTM38q_oMLW_YJMJ19civ2gVViKAGYcZylRW7jN3w77AJvhzS6JE2g","//lh6.ggpht.com/9aXLmPRVeZnxkwvNb3mWTF8kvfEY_lho_lOVVc9AbNqLb8GQmiS_XXVZ3OKqMv2pxgYSayMYPPRh6ACYyh0H8KtS8mPD6MKUkEajwxkTtp5Q4Lo","//lh3.ggpht.com/FG_QXZPHJ2tTYwI_t5Fg1KqivglVg9RlJn0JRsu9Ox8vJ7IcBirb2IV_I1LL_WVOMxfTuBBSDLMlrw9v0MCAdmnPCR29sCbRGjhm6zEfIH-3q2QSdw","//lh4.ggpht.com/Y23DqORrVkM2m55f-rq5_BBrlkvQg4uX7AsAt-ixhMobjK_SFgFaDfktgLhkNsyKwSr9HcF8iiGY3Nw0xOKXG1sn6wyAWg_qsolmKjVOrM5V5mIR","//lh6.ggpht.com/mQ62Ly-DjMKPMzU1OcSPJ7SLBqym0uBjawlkTHfmb-HOKaD56dnitk1duwPFJVdbi0GUpd63RQvr2VMpHp6S1OQ3di-hq4-JPeRoS5FJzksXSvW_","//lh3.ggpht.com/dqWjWPcNsvlR1tMC_agizX19f9MDiNGWFYTYVn4kjJxzIIkEe0mLzNcvS62zVJxAOaitT-IgaUfZ-Ze23BgzbqYY-l600i_LbVe35Uinz6sXIyoB","//lh6.ggpht.com/xhSdFc9uHgghs_6gf3seUWYM-PG2oLmjTrpF7ptEEMqaIrQIa8VPfC6tXE7f3M13eZvDXYqMW_k0AHO5vwCEPNp-iObixskd_lBaKNfz3MH3SNQ","//lh5.ggpht.com/kYeoKPoZGJCow-G1FhnD8kzVjNjbQA8-Kyj8eAh0HL-fMZX9tTeFPQikTZdSU0kks4-5Ui54cZF2CjGut9vfMJAVDKIq3T-bAQewCxvfl2120tH5zQ","//lh5.ggpht.com/4qUl3d-G9EPBzcYKrimNsWhQw7CmONV0jgfVhxFgB9mEU_QLRCyNJTWs2A3xf6wc7AUF2DXrKEkoX-SNLMZ6s-O4aXXV9WOjOPcWdAYreMRBld0E","//lh5.ggpht.com/z-0C4G6EWYkelAF1LjPfl_UQcsp92H4joIPt8NfsOl0nPJ2VpzZYahWadKqTLfl6kq3C6aDBcwfGQyMWSozYoZIAOAW0yRvZrwxia321PlsKTxbZ","//lh4.ggpht.com/U7I12JrDYmMC_pUXpw8DVBjBilU67BvbM8qT8gJE0bQfkhHo7FOdMttiz3syP5IR-LyO4J1WBlfmZjvMjRr4GIBt4o3Vqp-hKz7q2_OGwGtsN5s","//lh3.ggpht.com/fF2XWEtqG23ybhzClhC_p8gvKJalf1vg7k3H7UkuAaIVubil7EgOvJUCwAZk2KiCtlPYp1E5Ep2xaxZjJRmg5EFSEAjqlMHJS_Wd1Bcje6xre4s","//lh3.ggpht.com/jgOebMihBoIZvHE4EOklJvZ_k-9egjNIlUKfKFcLkvXJs8g2FXjPvdFUbwqGrkHrMtyis8uOvgt-E51Vm11hq4bieh7h0cegca0VI4vFtFaAemU","//lh3.ggpht.com/MOrI-zKNMNrQE_aHj5hzbojP3T0hEMJKK6K8UO3e1NBC-nkcQeIM1QnvtJdT_G-W4e7-qv4BiqwdWcNHBpZXOmmX3tcuYEV8u_ANEoa9_aUIfeyg","//lh6.ggpht.com/SyIS5sGOkTG7k_jFF14wzH9Evrblv6o4pHBI6z6X070-xhAeyut_kRO6xHtDID4KLcWFvItjQy-plPcJ6K1T9tlFOrtaryEPvuAYdMVx8e0TTw","//lh6.ggpht.com/2Pp9kLYFhDT3USwHinU5OxnzcWWOLI0nOWe29gOD5KMzyEcXoHkTN-AutJV9M8F_9eqAP379XB9O1d0BWPanhr-MguzKxfHeUvYTs6yHzDkxyfe0NA","//lh4.ggpht.com/7aofqklSkF3AMDfF19yqsA9J3EfEiKy1NdOelEGKNnW0Cv5tGEpq2PF_jZO1MVoBbrrmVVRv0Tdq7I8KyZbIlyHdbTs1jMl7dEFqVMvsPcyaORyHlQ","//lh4.ggpht.com/anYJHqkMCkuhmIHQTBspLtWcDTyx1ZRe84_q5pAgVEOVmsKkaKhS725N4YFoj2zpJrBP7iTC2vf1GUtrp6H7kkm8c1k6zkW6I_Gf5f9A3re_I8Ex","//lh3.ggpht.com/OtSw0rU-DvfoXgoWrQdkln6Kz7O14TF9qrPNJSGJnZLeDqUEctOn1DT09pdwwVpNQV-cXmVYQL-PX4XPhpZLWH1ciSkVT6WHNmTz1D9pHphBwJUv","//lh3.ggpht.com/cTCZnXPIjI-EO2bvQdLgeoSLOSlMFcv805n347Zyci9XDYUdcVDC_5H7SFVYDr4pC5HtQDYnrOHL6AinLW7hWtfSCLlvVhVUNQ-DlDn0NwZ-1iCO-g","//lh4.ggpht.com/i-mL_JcF9rwjQq6HnuKzuAHU41_UGxQ62IOPZvaDrATXaPFbhe-EbT7ZIpboyNA5PXRCsxNsZ9hu58edRvNs5ScgKN8Lg-00J2LhlwMAbdEsv7b0nw","//lh6.ggpht.com/D_YV2BG1WWwl67xNloP3sxzRkqhcVTgJi58L-A8nLrOcMR_tBqLz4fHEGQ-qiNcG_-32MNy3dlSPWrTBKzBcweJxgMnRVet5yuGfelUlwehDtXX_3w"],"video":[]},"sellerId":"mihai","listingType":"fixedPrice","payment":{"online":[{"paymentName":"PayPal","paymentService":"paypal"}],"offline":[{"paymentName":"Pay on Pick-up","paymentService":"payOnPickup"}]},"startTime":"2014-01-04T10:02:18+00:00","endTime":"2014-04-04T10:02:18+00:00","shipping":[{"shippingService":"economy","shippingName":"Economy","shippingCost":1.0,"handlingTimeMax":4,"dispatchTimeMin":1,"dispatchTimeMax":10,"shippingAdditionalCost":"2"},{"shippingService":"localPickup","shippingName":"Local Pick-Up","shippingCost":0.0,"handlingTimeMax":2,"dispatchTimeMin":0,"dispatchTimeMax":0,"shippingAdditionalCost":"0"}],"titleSlug":"many-pictures","variations":[{"fixedPrice":222999.0,"media":{"image":["//lh6.ggpht.com/ih3q2d7CenGLPyupH9FpfsoJQWQpw1i8wWA2Kd26bFnSF2fbnKyGU9WePIhCgEeqw5p6YMVmFi1c9oS0Ag93aF_oZ3ZiwK7fQuSYIrZ9VhgXbrTHkw","//lh6.ggpht.com/9aXLmPRVeZnxkwvNb3mWTF8kvfEY_lho_lOVVc9AbNqLb8GQmiS_XXVZ3OKqMv2pxgYSayMYPPRh6ACYyh0H8KtS8mPD6MKUkEajwxkTtp5Q4Lo","//lh3.ggpht.com/FG_QXZPHJ2tTYwI_t5Fg1KqivglVg9RlJn0JRsu9Ox8vJ7IcBirb2IV_I1LL_WVOMxfTuBBSDLMlrw9v0MCAdmnPCR29sCbRGjhm6zEfIH-3q2QSdw"],"video":[]},"quantity":1121,"Brand":"Bell \u0026 Howell"},{"fixedPrice":211.0,"media":{"image":["//lh6.ggpht.com/qAtjgyHAB734Ox_4NC_fa-ZRqrCjCmJu0Tp8bo-HMO88duv8l4hhuv2REBkB--yneFzOL7annecVlGty-YsKouondiOFVnAZWzjpdrfsGfbL6wh2","//lh3.ggpht.com/FG_QXZPHJ2tTYwI_t5Fg1KqivglVg9RlJn0JRsu9Ox8vJ7IcBirb2IV_I1LL_WVOMxfTuBBSDLMlrw9v0MCAdmnPCR29sCbRGjhm6zEfIH-3q2QSdw","//lh6.ggpht.com/9aXLmPRVeZnxkwvNb3mWTF8kvfEY_lho_lOVVc9AbNqLb8GQmiS_XXVZ3OKqMv2pxgYSayMYPPRh6ACYyh0H8KtS8mPD6MKUkEajwxkTtp5Q4Lo","//lh3.ggpht.com/MOrI-zKNMNrQE_aHj5hzbojP3T0hEMJKK6K8UO3e1NBC-nkcQeIM1QnvtJdT_G-W4e7-qv4BiqwdWcNHBpZXOmmX3tcuYEV8u_ANEoa9_aUIfeyg"],"video":[]},"quantity":2,"Brand":"Fujifilm"},{"fixedPrice":22.0,"media":{"image":["//lh3.ggpht.com/jgOebMihBoIZvHE4EOklJvZ_k-9egjNIlUKfKFcLkvXJs8g2FXjPvdFUbwqGrkHrMtyis8uOvgt-E51Vm11hq4bieh7h0cegca0VI4vFtFaAemU","//lh3.ggpht.com/MOrI-zKNMNrQE_aHj5hzbojP3T0hEMJKK6K8UO3e1NBC-nkcQeIM1QnvtJdT_G-W4e7-qv4BiqwdWcNHBpZXOmmX3tcuYEV8u_ANEoa9_aUIfeyg","//lh4.ggpht.com/anYJHqkMCkuhmIHQTBspLtWcDTyx1ZRe84_q5pAgVEOVmsKkaKhS725N4YFoj2zpJrBP7iTC2vf1GUtrp6H7kkm8c1k6zkW6I_Gf5f9A3re_I8Ex"],"video":[]},"quantity":12,"Brand":"Gateway"}],"_fpaiStatus":"published"}`
itemInfoBytes := []byte(itemInfoR)
var ItemInfo Item
er := json.Unmarshal(itemInfoBytes, &ItemInfo)
if er != nil {
panic(er)
}
}
Here's a fixed version of it: http://play.golang.org/p/w2ZcOzGHKR
The biggest fix that was needed is when Unmarshalling an array, that property needs to be an array/slice in the struct as well.
For example:
{ "things": ["a", "b", "c"] }
Would Unmarshal into a:
type Item struct {
Things []string
}
And not into:
type Item struct {
Things string
}
The other thing to watch out for when Unmarshaling is that the types line up exactly. It will fail when Unmarshalling a JSON string representation of a number into an int or float field -- "1" needs to Unmarshal into a string, not into an int like we saw with ShippingAdditionalCost int
Determining of root cause is not an issue since Go 1.8; field name now is shown in the error message:
json: cannot unmarshal object into Go struct field Comment.author of type string
You JSON doesn't match your struct fields: E.g. "district" in JSON and "District" as the field.
Also: Your Item is a slice type but your JSON is a dict value. Do not mix this up. Slices decode from arrays.
Just in case you're looking for an answer to this from an AWS perspective, particularly if you're trying to use UnmarshalListOfMaps() on the items coming back from a DynamoDB query(), then watch out for a simple typo I made.
Given that this function UnmarshalListOfMaps() takes in...a list of maps :-) then it needs to unmarshal into a List of whatever struct you're trying to build, and not just the plain struct itself. This was throwing me off because I was trying to start by querying for something that should only return one row.
movie := Movie{}
// Run the DynamoDB query
resp, err := session.Query(input) // type QueryInput
if err != nil {
log.WithError(err).Error("Error running DynamoDB query")
}
// Unmarshal all the attribute values into a Movie struct
err = dynamodbattribute.UnmarshalListOfMaps(resp.Items, &movie); if err != nil {
log.WithError(err).Error("Error marshaling DynamoDB result into Movie")
return link, err
}
The problem is in the first line. It should be movies := []Movie{} and then the reference &movie needs to change to &movies as well. If you leave it just as it is above, then the AWS Go SDK will throw this error:
cannot unmarshal list into Go value of type (etc).
If you ever face this while typing yml file not copy-pasting, it might be a problem with your yml file - mine was wrong indent.
This happened while making pipeline on concourse.
This question already has answers here:
json.Marshal(struct) returns "{}"
(3 answers)
Closed 3 years ago.
I am using Go 1.0.3 on Mac OS X 10.8.2, and I am experimenting with the json package, trying to marshal a struct to json, but I keep getting an empty {} json object.
The err value is nil, so nothing is wrong according to the json.Marshal function, and the struct is correct. Why is this happening?
package main
import (
"encoding/json"
"fmt"
)
type Address struct {
street string
extended string
city string
state string
zip string
}
type Name struct {
first string
middle string
last string
}
type Person struct {
name Name
age int
address Address
phone string
}
func main() {
myname := Name{"Alfred", "H", "Eigenface"}
myaddr := Address{"42 Place Rd", "Unit 2i", "Placeton", "ST", "00921"}
me := Person{myname, 24, myaddr, "000 555-0001"}
b, err := json.Marshal(me)
if err != nil {
fmt.Println(err)
}
fmt.Println(string(b)) // err is nil, but b is empty, why?
fmt.Println("\n")
fmt.Println(me) // me is as expected, full of data
}
You have to make the fields that you want to marshal public.
Like this:
type Address struct {
Street string
Extended string
City string
State string
Zip string
}
err is nil because all the exported fields, in this case there are none, were marshalled correctly.
Working example: https://play.golang.org/p/9NH9Bog8_C6
Check out the docs http://godoc.org/encoding/json/#Marshal
Note that you can also manipulate what the name of the fields in the generated JSON are by doing the following:
type Name struct {
First string `json:"firstname"`
Middle string `json:"middlename"`
Last string `json:"lastname"`
}
JSON library cannot view the fields in a struct unless they are public. In your case,
type Person struct {
name Name
age int
address Address
phone string
}
the fields name, age, address and phone are not public (start with a small letter). In golang, variables/functions are public, when they start with a capital letter. So for this to work, your struct needs to look something like this:
type Person struct {
Name Name
Age int
Address Address
Phone string
}