How to Generate Dynamic JSON Field Tags in Go for Terraform?

Generating Dynamic JSON Field Tags in Go

In Go, field tags are used for JSON marshalling to specify how struct fields map to JSON keys. However, when dealing with Terraform JSON, there are cases where field names may be dynamic or unknown at compile time. This can pose a challenge when using field tags to generate JSON.

Current Approach with Static Field Tags

The provided code demonstrates an attempt to use field tags to generate JSON for a Terraform file. However, the problem arises with dynamic identifiers like web1, making it infeasible to define static field tags for such names:

type Resource struct {
    AwsResource AwsResource `json:"aws_instance,omitempty"` // Static
}

type AwsResource struct {
    AwsWebInstance AwsWebInstance `json:"web1,omitempty"` // Dynamic
}

Alternative: Using Maps

To handle dynamic field names, alternative strategies must be employed. One viable option is to utilize maps:

type Resource struct {
    AWSInstance map[string]AWSInstance `json:"aws_instance"` // Map of dynamic instance names and their configuration
}

type AWSInstance struct {
    AMI string `json:"ami"`
    Count int `json:"count"`
    SourceDestCheck bool `json:"source_dest_check"`
}

In this approach, the AWSInstance field in the Resource struct is a map that maps dynamic instance names (such as "web1", "web2", etc.) to their respective configuration.

Example Usage

To generate the desired JSON, one can populate the map dynamically:

r := Resource{
    AWSInstance: map[string]AWSInstance{
        "web1": AWSInstance{
            AMI:   "qdx",
            Count: 2,
        },
    },
}

This map can then be marshaled to produce the desired JSON output. A playground example is provided for further illustration.

By leveraging maps, this approach allows for flexible marshalling of JSON with dynamic keys, effectively addressing the challenge of variable field tags in Go.

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