ソースのみを貼っていきます。。。

carrierwaveを使ってS3に画像をアップロードできたので、備忘録で書いておきます。 *** ところどころソースに出てくる:imageには注目

/config/routes.rb


Rails.application.routes.draw do
  root "users#index"
  resources :users, only: [:new, :create]
end

/controllers/users_controller.rb


class UsersController < ApplicationController
  def index
    @users = User.all
  end

  def new
    @user = User.new
  end

  def create
    @user = User.new(user_params)

    if @user.save
      redirect_to root_url, notice: 'Add User'
    else
      render :new
    end
  end

  private

  def user_params
    params.fetch(:user, {}).permit(:image)
  end
end

/uploaders/image_uploader.rb

class ImageUploader < CarrierWave::Uploader::Base

    # ストレージにS3を指定
    storage :fog

    # 画像ごとに保存するディレクトリを変えたいのでオーバーライド
    def store_dir
        # 例えばidごとにディレクトリを分けてみる
        "avater/#{model.id}"
    end

    # ファイル名を書き換える
    def filename
        # 例えば avater_1.jpg みたいなファイル名にしてみる
        "avater_#{model.id}.#{file.extension}" if original_filename
    end

    # キャッシュ先のディレクトリを指定
    def cache_dir
        "cache"
    end

end

/config/initializers/carrierwave.rb


# /config/initializers/carrierwave.rb
CarrierWave.configure do |config|
    config.fog_credentials = {
        provider: 'AWS',
        aws_access_key_id: '',
        aws_secret_access_key: '',
        region: 'ap-northeast-1',  # Tokyoの場合
    }
    config.cache_storage = :fog # キャッシュにS3を指定

    # 環境毎にバケットを分けます
    case Rails.env
    when 'production'
        config.fog_directory  = 'バケットname'
    when 'staging'
        config.fog_directory  = 'バケットname'
    when 'development'
        config.fog_directory  = 'バケットname'
    when 'test'
        config.fog_directory  = 'バケットname'
    end
end

/db/migrate/20181121140557_create_users.rb

class CreateUsers < ActiveRecord::Migration[5.1]
  def change
    create_table :users do |t|
      t.string :image
      t.timestamps
    end
  end
end

/app/views/users/index.html.erb

<h1>Users Index</h1>
<%= link_to 'Add user', new_user_path %>

<ul>
  <% @users.each do |user| %>
    <li>
    </li>
  <% end %>
</ul>

/app/views/users/new.html.erb

<h1>Add User</h1>

<%= form_with model: @user, local: true do |form| %>
  <div>
    <%= form.label :name %>
    <%= form.text_field :name %>
  </div>

  <div>
    <%= form.label :image %>
    <%= form.file_field :image %>
  </div>

  <div>
    <%= form.submit %>
  </div>
<% end %>

/app/models/user.rb

class User < ActiveRecord::Base
    mount_uploader :image, ImageUploader
end

/Gemfile


source 'https://rubygems.org'

git_source(:github) do |repo_name|
  repo_name = "#{repo_name}/#{repo_name}" unless repo_name.include?("/")
  "https://github.com/#{repo_name}.git"
end


# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 5.1.6'
# Use sqlite3 as the database for Active Record
gem 'sqlite3'
# Use Puma as the app server
gem 'puma', '~> 3.7'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'
# See https://github.com/rails/execjs#readme for more supported runtimes
# gem 'therubyracer', platforms: :ruby





gem 'carrierwave',github: 'carrierwaveuploader/carrierwave'
gem "fog"
gem 'rmagick'





# Use CoffeeScript for .coffee assets and views
gem 'coffee-rails', '~> 4.2'
# Turbolinks makes navigating your web application faster. Read more: https://github.com/turbolinks/turbolinks
gem 'turbolinks', '~> 5'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.5'
# Use Redis adapter to run Action Cable in production
# gem 'redis', '~> 4.0'
# Use ActiveModel has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use Capistrano for deployment
# gem 'capistrano-rails', group: :development

group :development, :test do
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug', platforms: [:mri, :mingw, :x64_mingw]
  # Adds support for Capybara system testing and selenium driver
  gem 'capybara', '~> 2.13'
  gem 'selenium-webdriver'
end

group :development do
  # Access an IRB console on exception pages or by using <%= console %> anywhere in the code.
  gem 'web-console', '>= 3.3.0'
  gem 'listen', '>= 3.0.5', '< 3.2'
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
  gem 'spring-watcher-listen', '~> 2.0.0'
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]

参考

【Rails】ActiveStorageを使ってお手軽にファイルアップロードを試す
 carrierwaveを使ってS3に画像をアップロードする