Generating Go structs from OpenAPI schemas

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It's possible that you're using OpenAPI specifications to describe the format of your API, and as noted in Use a (JSON) Schema for the Interface Portion of your RESTful API is something I really recommend for codifying part of your API documentation.

If you're on a Go project, it may be that you're currently manually translating from OpenAPI data types to structs, which is time consuming and has the risk of drifting between your definitions and implementations.

One of the benefits of using a very structured format like the format in OpenAPI specifications is that you can programmatically generate the structs.

Example OpenAPI specification

We'll base this on the OpenAPI specification demo from the Petstore.

Note that if you have an OpenAPI schema that uses $refs, we will need to bundle the OpenAPI document into a single file.

Generating the structs

We can take advantage of the great oapi-codegen project, to give us a generator that we can use to produce our types.

We can install it as a command-line utility by running:

# optionally, pin to a version
go install github.com/deepmap/oapi-codegen/cmd/oapi-codegen

This then allows us to use the really handy Go directive go:generate which we embed in (any) source file in the project:

//go:generate oapi-codegen --package=main -generate=types -o ./petstore.gen.go https://petstore3.swagger.io/api/v3/openapi.json

This allows us to execute the oapi-codegen when we execute go generate on the command-line.

This then allows us to write the following code, which can utilise the generated code:

package main

//go:generate oapi-codegen --package=main -generate=types -o ./petstore.gen.go https://petstore3.swagger.io/api/v3/openapi.json

import (
	"encoding/json"
	"fmt"
)

func main() {
	var u User
	err := json.Unmarshal([]byte(`
{
	"email": "example@example.com",
	"id": 123
}
	`), &u)
	if err != nil {
		panic(err)
	}
	fmt.Println(u)
	fmt.Println(*u.Email)
	fmt.Println(*u.Id)
}

This then produces the following output:

{0xc0000103e0 <nil> 0xc0000183d0 <nil> <nil> <nil> <nil> <nil>}
example@example.com
123

Written by Jamie Tanna's profile image Jamie Tanna on , and last updated on .

Content for this article is shared under the terms of the Creative Commons Attribution Non Commercial Share Alike 4.0 International, and code is shared under the Apache License 2.0.

#blogumentation #go #openapi.

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