undefined method `find_by' for Memo:Module

Question

前提・実現したいこと

工程管理をメモできるアプリを作ってます。
views/memos/show.html.rbにuser情報を表示させようと思い、memosとusersテーブルの紐づけの実装中に下記のエラーが発生しました。
memosの情報とusersの情報を紐づけて、views/memos/show.html.rbに表示させる為にはどうすればいいでしょうか。
教えて頂けると幸いです。宜しくお願いします。

発生している問題・エラーメッセージ

Processing by MemosController#show as HTML
  Parameters: {"id"=>"3"}
Completed 500 Internal Server Error in 21ms (ActiveRecord: 0.0ms | Allocations: 1383)


  
NoMethodError (undefined method `find_by' for Memo:Module):
  
app/controllers/memos_controller.rb:7:in `show'

該当のソースコード

######db/migrate/20210106215236_add_user_id_to_user.rb

class AddUserIdToUser < ActiveRecord::Migration[6.0]
  def change
    add_column :users, :memo_id, :integer
  end
end

######db/schema.rb

ActiveRecord::Schema.define(version: 2021_01_06_215236) do

  # These are extensions that must be enabled in order to support this database
  enable_extension "plpgsql"

  create_table "memos", force: :cascade do |t|
    t.string "product_name", limit: 30, null: false
    t.string "order_number", limit: 6, null: false
    t.datetime "delivery_date"
    t.integer "quantity"
    t.string "process"
    t.text "remarks"
    t.datetime "created_at", precision: 6, null: false
    t.datetime "updated_at", precision: 6, null: false
    t.integer "user_id"
  end

  create_table "users", force: :cascade do |t|
    t.string "name", limit: 30, null: false
    t.string "email", null: false
    t.string "password_digest", null: false
    t.datetime "created_at", precision: 6, null: false
    t.datetime "updated_at", precision: 6, null: false
    t.string "remember_digest"
    t.integer "memo_id"
    t.index ["email"], name: "index_users_on_email", unique: true
  end
end

######app/controllers/memos_controller

class MemosController < ApplicationController
  def index
    @memos = Memo.all
  end

  def show
    @memo = Memo.find_by(id: params[:id])
    @user = User.find_by(id: @memo.user_id)
  end

  def new
    @memo = Memo.new
  end

  def create
    @memo = Memo.new(
      content: params[:content],
      user_id: @current_user.id
    )
    if @memo.save
      flash[:success] = "「#{@memo.product_name}」を登録しました。"
      redirect_to @memo
    else
      render 'memo/new'
    end
  end

  def edit
    @memo = Memo.find(params[:id])
  end

  def update
    memo = Memo.find(params[:id])
    memo.update!(memo_params)
    flash[:success] = "「#{@memo.product_name}」を編集しました。"
    redirect_to tasks_url
  end

  def destroy
    memo = Memo.find(params[:id])
    memo.destroy
    flash[:success] = "「#{@memo}」を削除しました。"
    redirect_to tasks_url
  end

  private

  def memo_params
    params.require(:task).permit(:product_name, :order_number,
                                 :delivery_date, :quantity,
                                 :process, :remarks)
  end
end

######app/controllers/users_controller

class UsersController < ApplicationController
  before_action :logged_in_user, only: [:index, :edit, :update, :destroy]
  before_action :correct_user, only: [:edit, :update]
  before_action :admin_user, only: :destroy

  def show
    @user = User.find(params[:id])
  end

  def new
    @user = User.new
  end

  def create
    @user = User.new(user_params)
    if @user.save
      log_in @user
      flash[:success] = "登録が完了！"
      redirect_to @user
    else
      render 'new'
    end
  end

  def edit
    @user = User.find(params[:id])
  end

  def update
    @user = User.find(params[:id])
    if @user.update_attributes(user_params)
      flash[:success] = "Profile updated"
      redirect_to @user
    else
      render 'edit'
    end
  end

  def index
    @users = User.paginate(page: params[:page])
  end

  def destroy
    User.find(params[:id]).destroy
    flash[:success] = "User deleted"
    redirect_to users_url
  end

  private

    def user_params
      params.require(:user).permit(:name, :email, :password,
                                   :password_confirmation)
    end

    def logged_in_user
      unless logged_in?
        store_location
        flash[:danger] = "Please log in."
        redirect_to login_url
      end
    end

    def correct_user
      @user = User.find(params[:id])
      redirect_to(root_url) unless current_user?(@user)
    end

    def admin_user
      redirect_to(root_url) unless current_user.admin?
    end
end

######app/views/memos/show.html.erb

<% provide(:title, @memo.product_name) %>
<nav class="row justify-content-end">
<%= link_to '製品一覧', memos_path, class: 'nav-link' %>
  <div class="col-md-4">
    <table class= "table table table-hover">
      <tbody>
        <tr><th>ID</th><td><%= @memo.id %></td></tr>
        <tr><th>品名</th><td><%= @memo.product_name %></td></tr>
        <tr><th>注番</th><td><%= @memo.order_number %></td></tr>
        <tr><th>製造日</th><td><%= @memo.created_at %></td></tr>
        <tr><th>納期</th><td><%= @memo.delivery_date %></td></tr>
        <tr><th>手配数量</th><td><%= @memo.quantity %></td></tr>
        <tr><th>作業工程</th><td><%= @memo.process %></td></tr>
        <tr><th>備考</th><td><%= @memo.remarks %></td></tr>
        <tr><th>更新日時</th><td><%= @memo.updated_at %></tr>
        <td><%= link_to "編集", edit_memo_path, class: "btn btn-primary" %></td>
        <td><%= link_to "削除", @memo, method: :delete,
                            data: { confirm: "「#{@memo.product_name}」を削除します。よろしいですか？" },
                            class: 'btn btn-danger' %></td>
      </tbody>
    </table>
  </div>
  <%= @user.name %>
</nav>

######app/models/user.rb

class User < ApplicationRecord
  attr_accessor :remember_token
  before_save { self.email = email.downcase }
  validates :first_name, presence: true, length: { maximum: 50 },
                           uniqueness: { scope: :user_id}

  VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+.[a-z]+\z/i
  validates :email, presence: true, length: { maximum: 255 },
                      format: { with: VALID_EMAIL_REGEX },
                      uniqueness: { case_sensitive: false }

  has_secure_password
  validates :password, presence: true, length: { minimum: 6 }, allow_nil: true

  has_many :memos

  class << self
    def digest(string)
      cost = ActiveModel::SecurePassword.min_cost ? BCrypt::Engine::MIN_COST :
                                                    BCrypt::Engine.cost
      BCrypt::Password.create(string, cost: cost)
    end

    def new_token
      SecureRandom.urlsafe_base64
    end
  end

  def remember
    self.remember_token = User.new_token
    update_attribute(:remember_digest, User.digest(remember_token))
  end

  def authenticated?(remember_token)
    return false if remember_digest.nil?
    BCrypt::Password.new(remember_digest).is_password?(remember_token)
  end

  def forget
    update_attribute(:remember_digest, nil)
  end
end

######app/models/memo.rb

class Memo < ApplicationRecord
  belongs_to :user
end

試したこと

memosテーブルとuser_idの紐づけはできていると思ってます。
memos_controller.rbにuserテーブルを紐づける為の記述。
user.rb、mermo.rbにデータベースを関連させる為の記述。

補足情報（FW/ツールのバージョンなど）

ruby 2.7.1p83
Rails 6.0.3.4
Ubuntu 20.04 LTS

Answer 1

Userがhas_many memos で　Memo が belongs_o userなら、databaseの構造が間違っています。
users にある memo_id をなくし、memos に user_id を追加してください。
そうすれば　
user のmemoは user.memos で得られます
memoの持ち主は memo.user です