Rails 8과 MySQL

ruby -v
which rails

ruby 2.6.10p210 (2022-04-12 revision 67958) [universal.arm64e-darwin25]

brew install ruby
export PATH="/opt/homebrew/opt/ruby/bin:/opt/homebrew/lib/ruby/gems/4.0.0/bin:$PATH"
ruby -v

gem install rails -v '~> 8.0'
rails -v

-d, [--database=DATABASE]  # Preconfigure for selected database
                            # Default: sqlite3
                            # Possible values: mysql, trilogy, postgresql, sqlite3, mariadb-mysql, mariadb-trilogy

rails new . --database=mysql --skip-git

brew install mysql-client
bundle config set build.mysql2 --with-mysql-dir=/opt/homebrew/opt/mysql-client
bundle install

mysql -S /tmp/mysql.sock -u root -e "
CREATE DATABASE IF NOT EXISTS rails_guide;
CREATE USER IF NOT EXISTS 'rails_app'@'127.0.0.1' IDENTIFIED BY 'railspass123';
GRANT ALL PRIVILEGES ON rails_guide.* TO 'rails_app'@'127.0.0.1';
"

default: &default
  adapter: mysql2
  encoding: utf8mb4
  max_connections: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  username: rails_app
  password: railspass123
  host: 127.0.0.1
  port: 3306

development:
  <<: *default
  database: rails_guide

test:
  <<: *default
  database: rails_guide_test

Mysql2::Error::ConnectionError: Access denied for user 'rails_app'@'127.0.0.1' to database 'rails_guide_test'
Couldn't create 'rails_guide_test' database. Please check your configuration.
Created database 'rails_guide'

mysql -S /tmp/mysql.sock -u root -e "
GRANT ALL PRIVILEGES ON rails_guide_test.* TO 'rails_app'@'127.0.0.1';
"
rails db:create

Database 'rails_guide' already exists
Created database 'rails_guide_test'

rails generate model Post title:string body:text
rails db:migrate

class CreatePosts < ActiveRecord::Migration[8.1]
  def change
    create_table :posts do |t|
      t.string :title
      t.text :body

      t.timestamps
    end
  end
end

mysql -S /tmp/mysql.sock -u root -D rails_guide -e "SHOW TABLES; DESCRIBE posts;"

Tables_in_rails_guide
ar_internal_metadata
posts
schema_migrations

Field       Type          Null  Key  Default  Extra
id          bigint        NO    PRI  NULL     auto_increment
title       varchar(255)  YES        NULL
body        text          YES        NULL
created_at  datetime(6)   NO         NULL
updated_at  datetime(6)   NO         NULL

# config/routes.rb
Rails.application.routes.draw do
  resources :posts, only: [:index, :create]
end

# app/controllers/posts_controller.rb
class PostsController < ApplicationController
  skip_forgery_protection only: :create

  def index
    posts = Post.order(created_at: :desc)
    render json: posts.as_json(only: [:id, :title, :body, :created_at])
  end

  def create
    post = Post.create!(post_params)
    render json: post.as_json(only: [:id, :title, :body, :created_at]), status: :created
  end

  private

  def post_params
    params.require(:post).permit(:title, :body)
  end
end

rails server -p 3210

curl -s http://127.0.0.1:3210/posts
# []

curl -s -X POST http://127.0.0.1:3210/posts \
  -H "Content-Type: application/json" \
  -d '{"post":{"title":"Hello VillageSQL","body":"First post from Rails"}}'
# {"id":1,"title":"Hello VillageSQL","body":"First post from Rails","created_at":"2026-08-28T19:40:37.844Z"}

curl -s http://127.0.0.1:3210/posts
# [{"id":1,"title":"Hello VillageSQL","body":"First post from Rails","created_at":"2026-08-28T19:40:37.844Z"}]

mysql -S /tmp/mysql.sock -u root -D rails_guide -e "SELECT id, title, body, created_at FROM posts;"

id  title              body                     created_at
1   Hello VillageSQL   First post from Rails    2026-08-28 19:40:37.844329

max_connections: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>

RuntimeError: Ambiguous configuration: 'pool' (10) and 'max_connections' (20) are set to different values. Prefer just 'max_connections'.

# threads. This includes Active Record's `pool` parameter in `database.yml`.
threads_count = ENV.fetch("RAILS_MAX_THREADS", 3)
threads threads_count, threads_count