How to Parse YAML with Dynamic Keys in Go?

Dynamic Key Parsing in YAML with Golang

Introduction

YAML (YAML Ain't Markup Language) is a widely used data serialization format that allows for flexible data representation. However, parsing YAML files with dynamic keys (i.e., keys that may vary) poses a challenge in Go because the language relies heavily on statically typed structures.

Problem Description

You have a YAML file where the keys represent different versions (e.g., V1, V2, V3). The user can specify which versions they support, and the keys may not be in order or skip versions. You want to parse this YAML file into a Go struct, but the struct's fields must match the dynamic keys.

Incorrect Approach

Initially, you attempted to parse the YAML file into an Environment struct, expecting the keys to be present at the root level. However, the actual type of the root element is a map of strings ( representing environment names) to Environment structs.

Solution

To parse YAML with dynamic keys, you need to use a custom UnmarshalYAML method on the struct that will parse the keys and construct the correct struct. In this case, the Environment struct requires an UnmarshalYAML method to unmarshal the root as a map and then parse the individual environments.

Code Sample

Here's an example of a custom UnmarshalYAML method for the Environment struct:

<code class="go">func (e *Environment) UnmarshalYAML(unmarshal func(interface{}) error) error {
    var params struct {
        SkipHeaderValidation bool `yaml:"skip-header-validation"`
    }
    if err := unmarshal(&params); err != nil {
        return err
    }
    var versions map[string]MajorVersion
    if err := unmarshal(&versions); err != nil {
        // Here we expect an error because a boolean cannot be converted to a MajorVersion
        if _, ok := err.(*yaml.TypeError); !ok {
            return err
        }
    }
    e.SkipHeaderValidation = params.SkipHeaderValidation
    e.Versions = versions
    return nil
}</code>

This method first unmarshals the Boolean field SkipHeaderValidation. Then, it attempts to unmarshal the rest of the data into a map of version names to MajorVersion structs. However, because the unmarshaled data is a mix of Booleans and MajorVersion structs, a yaml.TypeError will be thrown. The code ignores this error and assigns the unmarshaled versions map to the Versions field of the Environment struct.

Example Usage

With the custom UnmarshalYAML method in place, you can now unmarshal the YAML data correctly:

<code class="go">var e map[string]Environment
if err := yaml.Unmarshal([]byte(data), &e); err != nil {
    fmt.Println(err.Error())
}
fmt.Printf("%#v\n", e)</code>

This will produce the following output:

map[string]main.Environment{
    "development": {
        SkipHeaderValidation: true,
        Versions:             {
            "V2": {
                Current:                "2.0.0",
                MimeTypes:              {"application/vnd.company.jk.identity+json;", "application/vnd.company.jk.user+json;", "application/vnd.company.jk.role+json;", "application/vnd.company.jk.scope+json;", "application/vnd.company.jk.test+json;"},
                SkipVersionValidation:  false,
                SkipMimeTypeValidation: false,
            },
            "V1": {
                Current:                "1.0.0",
                MimeTypes:              {"application/vnd.company.jk.identity+json;", "application/vnd.company.jk.user+json;", "application/vnd.company.jk.role+json;", "application/vnd.company.jk.scope+json;", "application/vnd.company.jk.test+json;"},
                SkipVersionValidation:  true,
                SkipMimeTypeValidation: true,
            },
        },
    },
}

This demonstrates the successful parsing of the YAML file with dynamic keys into a Go struct.

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