I'm writing an application in Go that will recieve two forms of JSON:
Example 1:
{"book_data":{"title":"book-title","page_number":457}}
Example 2:
{"book_data":{"collection":214},"books":{"data":[{"title":"book-title","page_number":457},{"title":"book-title","page_number":354}]}}
I thought that I could create a struct like the following and unmarshal JSON into it:
type Book struct {
Title string `json:"title"`
PageNumber int `json:"page_number"`
}
but that only works for the first example.
How can I handle JSON from both examples?
You can first unmarshal partly in json.RawMessage to next decide depending of unmarshalled payload. And you also can just unmarshal in more generic structure. Something like
type Book struct {
Title string `json:"title"`
PageNumber int `json:"page_number"`
}
type BookShelf struct {
BookData struct {
Book
Collection int `json:"collection"`
} `json:"book_data"`
Books struct {
Data []Book `json:"data"`
} `json:"books"`
}
which for me looks readable, meaningful and handy enough for further processing.
Why not unmarshal to a map[string]interface{} and then use the result to see which form you need to handle? You can then deserialize with a specific struct type for each form.
Another way would be to use the following package to check for differing attributes, so you can decide which struct to use for the real unmarshaling.
https://github.com/go-xmlpath/xmlpath/tree/v2
You can unmarshal to map because your key is string and value may be anything like - map[string]interface{}. If you are not sure any data type or value then use interface{} beacause it can store any value. Then use result to see which form it is, And deserialize to specific struct type.
Another way to convert JSON to go struct is use this tool.
https://mholt.github.io/json-to-go/
Related
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.
I have a struct -
type User struct {
Uid string `firestore:"uid"`
FcmToken string `firestore:"fcmtoken"
}
How do I convert It to map using json.Marshal(user) ,
I know it can be done when struct fields are annotated with json:"fieldname" but I don't know how to do the same when it is annotated with firestore, or is it even possible?
I have used the word annotation, which may not be what it is called, please correct me!
A field tag can contain multiple key/value pairs. See the struct tag documentation for more details.
Edit the field tags to include whatever JSON configuration you want:
type User struct {
Uid string `firestore:"uid" json:"uid"`
FcmToken string `firestore:"fcmtoken" json:"tid"`
}
It is not possible to make the JSON package use the firestore tags or vice versa.
I have a dynamic json object which I want to unmarshal in my Go app. The problem is that some parts of the json are dynamically named, so I don't know what to put in the struc type json tags. To illustrate my problem, please see this playground: https://play.golang.org/p/q8J0VVO7uj
As you can see the s1 can perfectly be unmarshalled, because the struct type indeed has tag description. But s2 cannot be unmarshalled.
So my question is: how can I solve this? Can I make use of interfaces here?
Use a map for dynamic keys:
type ElvisEvent struct {
Timestamp int64 `json:"timestamp"`
Type string `json:"type"`
AssetID string `json:"assetId"`
Metadata struct {
} `json:"metadata"`
ChangedMetadata map[string]struct {
OldValue interface{} `json:"oldValue"`
NewValue interface{} `json:"newValue"`
} `json:"changedMetadata"`
}
playground example
I'm trying to pull reddit content from the json API into a custom structure for the client. the structure i've come up with in go for this is
type Subreddit struct {
offset int
num_of_posts int
subscribers: int
thumbnail string
children []post
}
type post struct {
type string
url string
thumbnail string
submitted_by string
upvotes int
downvotes int
}
unfortunately the reddit json isn't formatted even close to this and in addition i'll want to filter out url's i can't support etc.
The only way i know to do it this is to create an interface for each of the "children" in the source data, and iterate through each child manually, creating an individual "post" for each interface. and pushing them into the subreddit object's post array.
For reference the data is formatted like http://www.reddit.com/r/web_design/.json
Is this the right way to do this? Or is there a faster way. It seems like a lot of overhead for such a small task, but i'm a PHP Javascript dev, so It's just unusual for me I suppose.
Before I even start to answer the question:
Remember that your struct fields must be exported in order to be used with the encoding/json package.
Secondly I must admit I am not entirely sure what you meant with the entire create an interface for each of the "children" part. But it sounded complicated ;)
Anyway, to your answer:
If you wish to use the standard encoding/json package to unmarshal the json, you must use an intermediate structure unless you will use a similar structure as the one used by Reddit.
Below you can find an example of how parts of the Reddit structure might be mapped to Go structs. By Unmarshalling the json into an instance of RedditRoot, you can then easily iterate over the Children , remove any unwanted child, and populate your Subreddit struct:
type RedditRoot struct {
Kind string `json:"kind"`
Data RedditData `json:"data"`
}
type RedditData struct {
Children []RedditDataChild `json:"children"`
}
type RedditDataChild struct {
Kind string `json:"kind"`
Data *Post `json:"data"`
}
type Post struct {
Type string `json:"-"` // Is this equal to data.children[].data.kind?
Url string `json:"url"`
Thumbnail string `json:"thumbnail"`
Submitted_by string `json:"author"`
Upvotes int `json:"ups"`
Downvotes int `json:"downs"`
}
I am trying to unmarshal a json response from the server into various types, but I
am not finding how to do it.
The types which work are :-
type ServerResponse struct {
Total int
Data []User
}
type User struct {
Name string
Age int
}
and I can successfully unmarshal the json and receive the expected User type.
What I want to do is handle various server responses and convert after the
fact. eg.
type ServerResponse struct {
Total int
Data []ServerItem
}
type User struct {
ServerItem
Name string
Age int
}
type Book struct {
ServerItem
Name string
Author string
}
Then use either User(response.Data) or response.Data.(User) to make it a
concrete type so that later functions type check correctly.
Please could anyone let me know where to start looking to solve this issue.
I don't think that this can be done easily. Just decode to map[string]interface{} and create your stuff from this.
Here I wrote a simple program that parses json http://play.golang.org/p/51eiswgznR.
You may also want to read the encoding/json docs http://golang.org/pkg/encoding/json/