Fixing Go-jsonschema With Enums In Array Type AnyOf

Nov 12, 2025 by Alex Johnson 52 views

This article delves into a specific issue encountered while using the go-jsonschema generator, focusing on scenarios where it fails to correctly handle anyOf schemas that include an array type containing enum items alongside other types. We'll dissect the problem, explore the root cause, and understand the impact on users.

Understanding the Issue

The core problem arises when the go-jsonschema generator encounters a JSON schema employing the anyOf keyword. This keyword signifies that a particular field can adhere to one of several specified schema definitions. The complication occurs when one of these definitions involves an array whose items are restricted to a specific set of enumerated values (enums), and another definition specifies a different data type, such as a simple string with its own set of enums. This scenario, while perfectly valid under the JSON Schema specification, exposes a limitation in the go-jsonschema generator's current implementation.

The Error Message

When the generator stumbles upon such a schema, it throws the following error message:

could not generate type for field "dayOfWeek": invalid type "array": unexpected type "array" here

This message indicates that the generator is unable to reconcile the array type with the context in which it appears within the anyOf construct. It suggests that the generator's logic is not designed to handle an array type directly within an anyOf schema in certain cases, particularly when that array contains an enum.

A Concrete Test Case

To illustrate this issue, consider the following JSON schema:

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "type": "object",
    "properties": {
        "dayOfWeek": {
            "anyOf": [
                {
                    "type": "array",
                    "items": {
                        "enum": [
                            "Monday",
                            "Tuesday",
                            "Wednesday",
                            "Thursday",
                            "Friday",
                            "Saturday",
                            "Sunday"
                        ]
                    }
                },
                {
                    "type": "string",
                    "enum": [
                        "All",
                        "Weekdays"
                    ]
                }
            ]
        }
    }
}

In this schema, the dayOfWeek field is defined using anyOf. It can either be an array of weekday strings (e.g., ["Monday", "Wednesday", "Friday"]) or a single string with specific values like "All" or "Weekdays". This pattern is commonly used to provide flexibility in how data is represented.

Expected Behavior

The expected behavior of the generator is to create a Go type that can seamlessly handle both possibilities. Ideally, the generated code should allow the dayOfWeek field to accept either an array of weekday strings or a single string with the allowed shorthand values. This would enable developers to work with the data in a type-safe manner, regardless of which format is used in the JSON data.

Unveiling the Root Cause

The root cause of this issue lies within the pkg/codegen/utils.go file of the go-jsonschema generator, specifically around line 170. The getTypeForSchema function, responsible for determining the appropriate Go type for a given JSON schema, contains a conditional block that explicitly rejects array and object types in certain contexts:

case schemas.TypeNameObject, schemas.TypeNameArray:
    return nil, fmt.Errorf("%w %q here", errUnexpectedType, jsType)

This code snippet reveals a deliberate limitation in the generator's design. It seems that the generator is not equipped to handle array and object types within the specific context of an anyOf schema. This restriction prevents the generation of code that can accommodate the flexible input patterns that anyOf is intended to support.

Reproducing the Issue: A Step-by-Step Guide

To reproduce the issue and observe the error firsthand, follow these steps:

Save the JSON schema provided earlier as tests/data/core/anyOf/anyOf.8.json. This creates a file containing the problematic schema within the generator's test directory.
Run the following command in your terminal:
```
go test ./tests -v -run "TestCore/core/anyOf/anyOf.8"
```
This command executes the generator's test suite, specifically targeting the test case associated with the anyOf.8.json schema file. The -v flag enables verbose output, providing more detailed information about the test execution.
Observe the failure. The output will display the error message described earlier, confirming that the generator is unable to process the schema correctly.

Impact and Implications

This issue has a significant impact on users of the go-jsonschema generator. It prevents them from defining schemas with flexible input patterns that are perfectly valid according to the JSON Schema specification. The inability to use anyOf with array types containing enums restricts the expressiveness of schemas and forces developers to resort to workarounds or alternative approaches. This limitation can lead to more complex code, reduced type safety, and increased development effort.

Addressing the Challenge

Overcoming this limitation requires modifications to the go-jsonschema generator's code. The getTypeForSchema function needs to be updated to handle array and object types within anyOf schemas gracefully. This might involve creating a union type in Go that can represent the different possible types specified within the anyOf construct. Alternatively, the generator could generate code that uses type assertions or reflection to handle the different types at runtime.

The solution should ensure that the generated code is type-safe and efficient, while also adhering to the principles of the JSON Schema specification. Thorough testing is essential to validate the fix and ensure that it does not introduce any new issues.

Potential Solutions and Workarounds

While a comprehensive fix requires modifying the go-jsonschema generator, some workarounds may be possible in the interim. These workarounds might involve restructuring the schema or using alternative validation techniques.

Restructuring the Schema

One approach is to restructure the schema to avoid using anyOf with an array type containing enums. This might involve creating separate properties for the different possible types and using conditional logic in the application code to handle them accordingly. However, this approach can make the schema more complex and less readable.

Alternative Validation Techniques

Another option is to use alternative validation techniques, such as custom validation functions or external validation libraries. These techniques can be used to validate the data against the schema at runtime, providing the necessary type safety and error checking. However, this approach can add complexity to the application code and may not be as efficient as using the generated Go types.

Conclusion

The go-jsonschema generator's failure to handle anyOf schemas with array types containing enums is a significant limitation that impacts users who need to define flexible input patterns. Addressing this issue requires modifications to the generator's code to properly handle these types within the anyOf construct. While workarounds may be possible, a comprehensive fix is necessary to ensure that the generator can fully support the JSON Schema specification. By resolving this issue, the go-jsonschema generator can become a more powerful and versatile tool for Go developers working with JSON data.

For more information about JSON Schema and its capabilities, visit the official JSON Schema website. You will find detailed specifications, tutorials, and examples to help you understand and utilize JSON Schema effectively.