I'm working on unmarshaling some nested json data that I have already written a struct for. I've used a tool that will generate a struct based off json data, but am a bit confused how to work with accessing nested json data (and fields can sometimes be emtpy).
Here is an example of struct:
type SomeJson struct {
status string `json:"status"`
message string `json:"message"`
someMoreData []struct {
constant bool `json:"constant,omitempty"`
inputs []struct {
name string `json:"name"`
type string `json:"type"`
} `json:"inputs,omitempty"`
Name string `json:"name,omitempty"`
Outputs []struct {
Name string `json:"name"`
Type string `json:"type"`
} `json:"outputs,omitempty"`
I'm able to unmarshal the json data and access the top level fields (such as status and message), but am having trouble accessing any data under the someMoreData field. I understand this field is a (I assume an unknown) map of structs, but only have experience working with basic single level json blobs.
For reference this is the code I have to unmarshal the json and am able to access the top level fields.
someData := someJson{}
json.Unmarshal(body, &someData)
So what is exactly the best to access some nested fields such as inputs.name or outputs.name?
to iterate over your particular struct you can use:
for _, md := range someData.someMoreData {
println(md.Name)
for _, out := range md.Outputs {
println(out.Name, out.Type)
}
}
to access specific field:
someData.someMoreData[0].Outputs[0].Name
Couple of things to note:
The struct definition is syntactically incorrect. There are couple of closing braces missing.
type is a keyword.
The status and message and other fields with lower case first letter fields are unexported. So, the Json parser will not throw error, but you will get zero values as output. Not sure that's what you observed.
someMoreData is an array of structs not map.
Background
I'm working with JSON data that must be non-repudiable.
The API that grants me this data also has a service to verify that the data originally came from them.
As best as I can tell, they do this by requiring that the complete JSON they originally sent needs to be supplied to them inside another JSON request, with no byte changes.
Issue
I can't seem to preserve the original JSON!
Because I cannot modify the original JSON, I have carefully preserved it as a json.RawMessage when unmarshalling:
// struct I unmarshal my original data into
type SignedResult struct {
Raw json.RawMessage `json:"random"`
Signature string `json:"signature"`
...
}
// struct I marshal my data back into
type VerifiedSignatureReq {
Raw json.RawMessage `json:"random"`
Signature string `json:"signature"`
}
// ... getData is placeholder for function that gets my data
response := SignedResult{}
x, _ := json.Unmarshal(getData(), &response)
// do some post-processing with SignedResult that does not alter `Raw` or `Signature`
// trouble begins here - x.Raw started off as json.RawMessage...
y := json.Marshal(VerifiedSignatureReq{Raw: x.Raw, Signature: x.Signature}
// but now y.Raw is base64-encoded.
The problem is that []bytes / RawMessages are base64-encoded when marshaled. So I can't use this method, because it completely alters the string.
I'm unsure how to ensure this string is correctly preserved. I had assumed that the json.RawMessage specification in my struct would survive the perils of marshaling an already marshaled instance because it implements the Marshaler interface, but I appear mistaken.
Things I've Tried
My next attempt was to try:
// struct I unmarshal my original data into
type SignedResult struct {
Raw json.RawMessage `json:"random"`
Signature string `json:"signature"`
...
}
// struct I marshal my data back into
type VerifiedSignatureReq {
Raw map[string]interface{} `json:"random"`
Signature string `json:"signature"`
}
// ... getData is placeholder for function that gets my data
response := SignedResult{}
x, _ := json.Unmarshal(getData(), &response)
// do some post-processing with SignedResult that does not alter `Raw` or `Signature`
var object map[string]interface{}
json.Unmarshal(x.Raw, &object)
// now correctly generates the JSON structure.
y := json.Marshal(VerifiedSignatureReq{Raw: object, Signature: x.Signature}
// but now this is not the same JSON string as received!
The issue with this approach is that there are minor byte-wise differences in the spacing between the data. It no longer looks exactly the same when catted to a file.
I cannot use string(x.Raw) either because it escapes certain characters when marshaled with \.
You will need a custom type with its own marshaler, in place of json.RawMessage for your VerifiedSignatureReq struct to use. Example:
type VerifiedSignatureReq {
Raw RawMessage `json:"random"`
Signature string `json:"signature"`
}
type RawMessage []byte
func (m RawMessage) MarshalJSON() ([]byte, error) {
return []byte(m), nil
}
I'm testing out how to unmarshal a json response from an API I'm using.
The json looks something like the following;
body := []byte(`[
{"name":"Name1", "value":100.00},
{"name":"Name2", "value":200.00}
]`)
I've searched around for various ways to do this, but fail to get this to work. The unmarshal returns zero values. I also get an error "unexpected end of JSON input" (I've removed the error handling in the example).
Full code example - https://play.golang.org/p/VMdWuAm6HS
Reference:
https://godoc.org/encoding/json#RawMessage
Golang json Unmarshal "unexpected end of JSON input"
How to unmarshal json into interface{} in golang?
Your input JSON can be modeled with a simple []Obj where Obj is your type:
type Obj struct {
Name string `json:"name"`
Value float32 `json:"value"`
}
Nothing else is required, really:
body := []byte(`[
{"name":"Name1", "value":100.00},
{"name":"Name2", "value":200.00}]`)
var res []Obj
err := json.Unmarshal(body, &res)
fmt.Printf("%#v\n%v\n", res, err)
Output contains the data from the input JSON (try it on the Go Playground):
[]main.Obj{main.Obj{Name:"Name1", Value:100}, main.Obj{Name:"Name2", Value:200}}
<nil>
Back to your code:
Where you're going wrong is that you use this model:
type Obj struct {
Name string `json:"name"`
Value float32 `json:"value"`
}
type Result struct {
Data json.RawMessage
}
var res []Result
But this res variable would model the following JSON:
[
{"Data":{"name":"Name1", "value":100.00}},
{"Data":{"name":"Name2", "value":200.00}}
]
I think you can see the difference: the elements of the array here are JSON Objects with a "Data" field, which then are modeled with your Obj. This input JSON is then properly parsed with your original parsing code, you can try it on the Go Playground:
[{{"name":"Name1", "value":100.00}} {{"name":"Name2", "value":200.00}}]
&main.Obj{Name:"Name1", Value:100}
&main.Obj{Name:"Name2", Value:200}
I have the following json:
{"results":
[{"columns":["room_id","player_name","player_ip"],
"types":["integer","text","text"],
"values":[[1,"alice","127.0.0.1"]
[1,"bob","127.0.0.1"]],
"time":0.00018839100000000002}]}
where values can have any number of elements [] inside them. When i try to unmarshal the json into a struct, the "values" tag does not get parsed properly
struct:
type queryResults struct {
Results []struct {
Columns []string `json:"columns"`
Types []string `json:"types"`
Values []struct {
Room_id int
Player_name string
Player_ip string
} `json:"values"`
Time float64 `json:"time"`
} `json:"results"`
}
Code:
//jsonString is the string input to Unmarshal
resultjson := queryResults{}
json.Unmarshal([]byte(jsonString), &resultjson)
fmt.Printf("%+v",resultjson)
Current Output:
{Results:
[{Columns:[room_id player_name player_ip]
Types:[integer text text]
Values:[{room_id:0 player_name: player_ip:}
{room_id:0 player_name: player_ip:}]
Time:0.00018839100000000002}]}
Expected Output:
{Results:
[{Columns:[room_id player_name player_ip]
Types:[integer text text]
Values:[{room_id:1 player_name:alice player_ip:127.0.0.1}
{room_id:1 player_name:bob player_ip:127.0.0.1}]
Time:0.00018839100000000002}]}
Json arrays should be unmarshalled into Go slices or arrays. Looks like you are trying to unmarshal the arrays inside values to a struct
"values": [[1,"alice","127.0.0.1"], [1,"bob","127.0.0.1"]]
Above array of arrays should be unmarshalled into a Go slice of slices.
try,
type queryResults struct {
Results []struct {
Columns []string `json:"columns"`
Types []string `json:"types"`
Values [][]interface{} `json:"values"`
Time float64 `json:"time"`
} `json:"results"`
}
in Go Playground
And don't ignore errors. If you tried,
err := json.Unmarshal([]byte(jsonString), &resultjson)
if(err != nil){
fmt.Println(err)
}
you could have seen the error.
The problem is that your struct definition expects "values" to contain an array of objects, but your actual json contains an array of arrays. If you check the result of json.Unmarshal() you'll see it reports an error about this. try on golang playground
error json: cannot unmarshal array into Go value of type struct { Room_id int; Player_name string; Player_ip string }
As far as i know you can't map this directly into the struct, you'd need to read it into an array then post process it into your final type. Here's an example that successfully parses the json [the post parsing conversion is left as an exercise for the reader]
{Results:[{Columns:[room_id player_name player_ip]
Types:[integer text text]
Values:[[1 alice 127.0.0.1] [1 bob 127.0.0.1]]
Time:0.00018839100000000002}]}
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.