ruby 28 lines · 8 steps

Recursively merging nested config hashes in Ruby

A module that deep-merges two hashes, combining nested hashes and arrays instead of clobbering them.

Explained by highlit
Intermediate recursion hash-merge immutability block-arguments configuration
1module ConfigMerge
2 module_function
3 
4 def deep_merge(base, override)
5 base.merge(override) do |_key, base_val, override_val|
6 if base_val.is_a?(Hash) && override_val.is_a?(Hash)
7 deep_merge(base_val, override_val)
8 elsif base_val.is_a?(Array) && override_val.is_a?(Array)
9 (base_val + override_val).uniq
10 else
11 override_val
12 end
13 end
14 end
15 
16 def deep_merge!(base, override)
17 override.each do |key, override_val|
18 base_val = base[key]
19 base[key] =
20 if base_val.is_a?(Hash) && override_val.is_a?(Hash)
21 deep_merge!(base_val, override_val)
22 else
23 override_val
24 end
25 end
26 base
27 end
28end
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1Passing a block to `merge` lets you decide per-key how conflicting values combine instead of blindly overwriting.
  2. 2Recursing into nested hashes lets a merge preserve deep structure rather than replacing whole subtrees.
  3. 3Offering both a pure and a bang variant gives callers the choice between safety and in-place efficiency.

Related explainers

ruby 
module DeepFreeze
  module_function
 
  def call(obj)

Recursively freezing nested Ruby data

recursion immutability pattern matching
Intermediate 9 steps
typescript 
import { Module, Injectable } from '@nestjs/common';
import { ConfigModule, ConfigService } from '@nestjs/config';
import * as Joi from 'joi';
 

Validated, typed config in NestJS

configuration validation dependency-injection
Intermediate 6 steps
ruby 
class Webhooks::StripeController < ApplicationController
  skip_before_action :verify_authenticity_token
  skip_before_action :authenticate_user!
 

How a Rails Stripe webhook controller works

webhooks signature verification event dispatch
Intermediate 7 steps
java 
public record ServerConfig(String host, int port, boolean tls, List<String> allowedOrigins) {
 
    public ServerConfig {
        Objects.requireNonNull(host, "host is required");

Validating config with a Java record

records validation immutability
Intermediate 9 steps
python 
from functools import lru_cache
import math
 
 

Memoizing number theory with lru_cache

memoization number-theory caching
Intermediate 8 steps
ruby 
class ReportGenerator
  def initialize(account, period)
    @account = account
    @period = period

Memoized report metrics in Ruby in Rails

memoization query-composition service-object
Intermediate 7 steps

Share this explainer

Here's the card — post it anywhere.

Recursively merging nested config hashes in Ruby — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code