add user scaffold

app/controllers/users_controller.rb

@@ -0,0 +1,53 @@
+class UsersController < ApplicationController
+  before_action :set_user, only: %i[ show update destroy ]
+
+  # GET /users
+  # GET /users.json
+  def index
+    @users = User.all
+  end
+
+  # GET /users/1
+  # GET /users/1.json
+  def show
+  end
+
+  # POST /users
+  # POST /users.json
+  def create
+    @user = User.new(user_params)
+
+    if @user.save
+      render :show, status: :created, location: @user
+    else
+      render json: @user.errors, status: :unprocessable_entity
+    end
+  end
+
+  # PATCH/PUT /users/1
+  # PATCH/PUT /users/1.json
+  def update
+    if @user.update(user_params)
+      render :show, status: :ok, location: @user
+    else
+      render json: @user.errors, status: :unprocessable_entity
+    end
+  end
+
+  # DELETE /users/1
+  # DELETE /users/1.json
+  def destroy
+    @user.destroy
+  end
+
+  private
+    # Use callbacks to share common setup or constraints between actions.
+    def set_user
+      @user = User.find(params[:id])
+    end
+
+    # Only allow a list of trusted parameters through.
+    def user_params
+      params.require(:user).permit(:username)
+    end
+end

app/models/user.rb

@@ -0,0 +1,10 @@
+class User < ApplicationRecord
+  validates :username,
+            presence: true,
+            uniqueness: true,
+            length: { maximum: 14 },
+            allow_nil: false,
+            format: {
+              with: /\A[a-zA-Z0-9]+\z/,
+            }
+end

app/views/users/_user.json.jbuilder

@@ -0,0 +1,2 @@
+json.extract! user, :id, :username, :created_at, :updated_at
+json.url user_url(user, format: :json)

app/views/users/index.json.jbuilder

@@ -0,0 +1 @@
+json.array! @users, partial: "users/user", as: :user

app/views/users/show.json.jbuilder

@@ -0,0 +1 @@
+json.partial! "users/user", user: @user