go-map-schema

Overview
Use Case
Do I Really Need This?
Examples
- Usage
- Full Code
- Output
Universal Type Names

Overview

go-map-schema is a tiny library that's useful for comparing a map (usually from JSON) to a struct, and finding any fields that are missing or that have incompatible types.

Use Case

The most common usage would be for an API that accepts JSON from clients.

Before we can fulfill the request we need to know if the JSON matches what we expect. By verifying the JSON before we even try to json.Unmarshal it into a struct, we can be sure the JSON will be safely converted with no loss of data or swallowed errors.

As a result, you end up with an API that has

strict type checking, and
can give the client helpful error messages when the request is invalid

Do I Really Need This?

For something like an API, it's worth asking who is going to be the client that is using the API. Is this an API for your single page app, or are you providing a service and it will be used by other developers?

If the API is "internal" and only used within the context of your site, I don't think it's necessary. But if someone is trying to use your API, I think it's worth having. API documentation is rarely perfect, and giving the client a readable error can help them debug when a request throws a 400 Bad Request.

Examples

Read below for a quick look at how you can use go-map-schema.

For more examples, check out the examples/ directory.

Usage

Suppose we have a Person model

type Person struct {
    FirstName string `json:"first_name"`
    LastName  string `json:"last_name"`
    Age       int    `json:"age"`
}

and a client comes along and makes a POST request with this JSON.

{
    "first_name": "Jessie",
    "age": "26"
}

We can unmarshal the JSON into a map to make it easier to work with, and then compare it with the Person model.

src := make(map[string]interface{})
json.Unmarshal(payload, &src)

dst := Person{}
results, err := schema.CompareMapToStruct(&dst, src)

After comparing we now have a CompareResults instance stored in results.

type CompareResults struct {
	MismatchedFields []FieldMismatch
	MissingFields    []string
}

With this, we can quickly see which fields have mismatched types, as well as any fields that are in the Person struct but not the JSON.

Full Code

package main

import (
    "encoding/json"
    "fmt"

    schema "github.com/Kangaroux/go-map-schema"
)

type Person struct {
    FirstName string `json:"first_name"`
    LastName  string `json:"last_name"`
    Age       int    `json:"age"`
}

func main() {
    // The JSON payload (src).
    payload := `{
        "first_name": "Jessie",
        "age": "26"
    }`
    m := make(map[string]interface{})
    json.Unmarshal([]byte(payload), &m)

    // The struct we want to load the payload into (dst).
    p := Person{}
    results, _ := schema.CompareMapToStruct(&p, m, nil)

    fmt.Println("missing fields:   ", results.MissingFields)
    fmt.Println("mismatched fields:", results.MismatchedFields)
}

Output

missing fields:    [last_name]
mismatched fields: [expected "age" to be a int but it's a string]

FieldMismatch.String() returns a simple user friendly message describing what the issue is. You can of course use your own custom message instead.

Universal Type Names

By default, CompareMapToStruct will use the DetailedTypeName func when reporting a type mismatch. The detailed type name includes some extra information that you may not want a client to see.

For example, trying to convert a float64 into an int32 will report a mismatch between float64 and int32.

If you don't want to include bit size, you can use SimpleTypeName which converts floatX -> float, intX -> int, uintX -> uint.

opts := &schema.CompareOpts{
    TypeNameFunc: schema.SimpleTypeName,
}

schema.CompareMapToStruct(dst, src, opts)

lingling2012/go-map-schema