How Can I Efficiently Unmarshal Inconsistent JSON Fields That Are Sometimes Strings and Sometimes Arrays of Strings in Go?

Inconsistent JSON Field Handling: Unmarshaling as a String or Array

When working with JSON data that exhibits inconsistencies, it can be challenging to unmarshal fields that can vary in type, such as being either a string or an array of strings. This article addresses this issue and provides practical solutions.

The Problem

Consider the following JSON structure where the "display_name" field is either a string or an array of strings:

{
    "date": "30 Apr",
    "display_name": "Mr Smith"
},
{
    "date": "30 Apr",
    "display_name": ["Mr Smith", "Mr Jones"],
}

Attempting to unmarshal this data with the following Go structures will result in an error:

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

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

Solution

To overcome this challenge, one can utilize the versatility of json.RawMessage, which allows for the capture of varying field data. Additionally, the "-" field name can be used to hide the "DisplayName" field from the decoder, allowing the application to populate it after the top-level JSON has been decoded.

Modified Go Structure

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

Unmarshaling Process

Unmarshalling the top-level JSON:

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

Reconstructing the "display_name" field based on the type of raw data:

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 "&&" per OP's comment on the question.
        li.DisplayName = strings.Join(s, "&&")
    }
}

For multiple occurrences of this inconsistent field within a data model, a custom type and implementation of the json.Unmarshaler interface can be employed to encapsulate the logic.

Custom Unmarshaler

type multiString string

func (ms *multiString) UnmarshalJSON(data []byte) error {
    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
}

Usage with Custom Unmarshaler

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

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

var listings MyListings
if err := json.Unmarshal([]byte(data), &listings); err != nil {
    log.Fatal(err)
}

These solutions provide a comprehensive approach to efficiently handle inconsistent JSON fields, allowing for flexible mapping between data structures and JSON representations.

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