How to Handle Inconsistent JSON Fields: Strings vs. String Arrays?

Unmarshalling Inconsistent JSON Fields with Arrays and Strings

Unmarshalling JSON can be challenging when the data is inconsistent, such as when a field can be either a string or an array of strings. To handle this scenario, we can employ various strategies.

Capturing Varying Data Using RawMessage

For fields with varying types, we can use json.RawMessage to capture the unparsed data. This allows us to manipulate the field after unmarshalling the top-level JSON.

Hiding Fields with "-" Tag

To hide the "DisplayName" field from automatic decoding, we can use the "-" tag in the struct definition. This prevents the application from assigning values to that field during top-level unmarshalling.

Determining Field Type and Populating

Once the top-level JSON has been decoded, we can inspect the type of the RawDisplayName field to determine whether it is a string or an array. If it is a string, we unmarshal it into a string slice. If it is an array, we join the values with "&&" per the original question.

Custom Unmarshal Interface

For scenarios with multiple fields of varying types, it can be beneficial to encapsulate the parsing logic in a custom type that implements the json.Unmarshaler interface. This allows for centralized control over the data conversion process.

Example Implementation

Here is an example implementation that handles the specified JSON:

package main

import (
    "encoding/json"
    "fmt"
    "strings"
)

type multiString string

func (ms *multiString) UnmarshalJSON(data []byte) error {
    *ms = ""
    if len(data) > 0 {
        switch data[0] {
        case '"':
            var s string
            if err := json.Unmarshal(data, &s); err != nil {
                return err
            }
            *ms = multiString(s)
        case '[':
            var s []string
            if err := json.Unmarshal(data, &s); err != nil {
                return err
            }
            *ms = multiString(strings.Join(s, "&&"))
        }
    }
    return nil
}

type MyListItem struct {
    Date           string      `json:"date"`
    DisplayName    multiString `json:"display_name"`
}

type MyListings struct {
    CLItems []MyListItem `json:"myitems"`
}

func main() {
    var listings MyListings
    err := json.Unmarshal([]byte(`{
        "date": "30 Apr",
        "display_name": "Mr Smith"
    },{
        "date": "30 Apr",
        "display_name": ["Mr Smith", "Mr Jones"]
    }`), &listings)
    if err != nil {
        fmt.Println(err)
        return
    }

    fmt.Println(listings)
}

By using these techniques, we can effectively unmarshal JSON with fields of varying types, ensuring that the application can handle the data correctly.

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