How Can I Efficiently Unmarshal Inconsistent String or Array JSON Fields?

Unmarshalling Flexible JSON Fields: Converting Inconsistent String or Array Values

Problem

Unmarshalling JSON can be challenging when a field can be inconsistent, appearing as either a string or an array of strings. This inconsistency can lead to unmarshalling errors, causing your application to fail.

Solution

Use Raw JSON Data Capture

Utilize the json.RawMessage data structure to capture the varying JSON field as raw data. Additionally, leverage the json "-" name tag to conceal the field during the initial decoding phase.

type MyListItem struct {
    Date           string          `json:"date"`
    RawDisplayName json.RawMessage `json:"display_name"`
    DisplayName    []string        `json:"-"`
}

Unmarshal Top-Level JSON

Unmarshall the top-level JSON structure without populating the DisplayName field.

var li MyListItem
if err := json.Unmarshal(data, &li); err != nil {
    // handle error
}

Parse Raw Display Name

Depending on the type of raw data, unmarshall the display name into the appropriate format.

if len(li.RawDisplayName) > 0 {
    switch li.RawDisplayName[0] {
    case '"':
        if err := json.Unmarshal(li.RawDisplayName, &li.DisplayName); err != nil {
            // handle error
        }
    case '[':
        var s []string
        if err := json.Unmarshal(li.RawDisplayName, &s); err != nil {
            // handle error
        }
        // Join arrays with "&&"
        li.DisplayName = strings.Join(s, "&&")
    }
}

Handle Listings Array

Integrate the aforementioned logic into a loop to handle an array of MyListItem objects within a MyListings structure.

var listings MyListings
if err := json.Unmarshal([]byte(data), &listings); err != nil {
    // handle error
}
for i := range listings.CLItems {
    li := &listings.CLItems[i]
    // Parse raw display name
    if len(li.RawDisplayName) > 0 {
        switch li.RawDisplayName[0] {
        case '"':
            if err := json.Unmarshal(li.RawDisplayName, &li.DisplayName); err != nil {
                // handle error
            }
        case '[':
            var s []string
            if err := json.Unmarshal(li.RawDisplayName, &s); err != nil {
                // handle error
            }
            li.DisplayName = strings.Join(s, "&&")
        }
    }
}

Encapsulate Logic

For repeated occurrences of fields with varying types, consider encapsulating the parsing logic in a custom type that implements the json.Unmarshaler interface.

Handle Array Values

To retain the field value as a slice of strings rather than a joined string, modify the multiString type accordingly.

These techniques provide robust methods for handling inconsistent JSON fields, ensuring successful unmarshalling and accurate data retrieval.

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