How to use dynamic keys when parsing YAML with viper?

Question

I have the following yml file:

# config.yml
items:
  name-of-item: # dynamic field
    source: ...
    destination: ...

And I want to use viper to parse it, but name-of-item can be anything, so I'm not sure how to solve that. I know that I could use the following:

// inside config folder
package config

type Items struct {
  NameOfItem NameOfItem
}

type NameOfItem struct {
  Source string
  Destination string
}

// inside main.go
package main

import (
    "github.com/spf13/viper"
    "log"
    "github.com/username/lib/config"
)

func main() {

    viper.SetConfigName("config.yml")
    viper.AddConfigPath(".")

    var configuration config.Item
    if err := viper.ReadInConfig(); err != nil {
        log.Fatalf("Error reading config file, %s", err)
    }

    err := viper.Unmarshal(&configuration)
    if err != nil {
        log.Fatalf("unable to decode into struct, %v", err)
    }
}

In this case, I can unmarshal because I'm declaring NameOfItem, but what should I do if I don't know the name of the field (or in other words, if it's dynamic)?

score 3 · Answer 1 · answered Sep 08 '18 at 16:41

The struct types in Go may not be dynamic (I doubt they may in any other strictly-typed language), so you have to use a two stage process:

Unmarshal the relevant piece of data into a value of type map[string]interface{}.
Post process the result by iterating over the map's keys and use type assertions for the values corresponding to particular keys.

But it's not clear from your question whether your YAML data are really arbitrary or the items key contains a uniform array of items—I mean, each item consists of the source and destination values, just the keys for the items themselves are not known.

In this latter case, the target for the unmarshaling of the items piece should be a map—something like

type Item struct {
  Source string
  Destination string
}

items := make(map[string]Item)

err := viper.Unmarshal(items)

How to use dynamic keys when parsing YAML with viper?

1 Answers1