[Ruby on Rails] Follow function undefined method ʻid'for nil: NilClass error resolution

Premise / What you want to achieve

I want to implement a follow function. ruby 2.5.1p57 Rails 5.2.4.2

The following error message occurred while implementing the follow function.

Problems / error messages that are occurring

error

undefined method `id' for nil:NilClass

Corresponding source code

Table (migration file)

relationships table

class CreateRelationships < ActiveRecord::Migration[5.2]
  def change
    create_table :relationships do |t|
      t.integer :follower_id, foreign_key: true
      t.integer :following_id, foreign_key: true

      t.timestamps null: false
    end

    add_index :relationships, :follower_id
    add_index :relationships, :following_id
    add_index :relationships, [:follower_id, :following_id], unique: true
  end
end

model

relationship.rb

class Relationship < ApplicationRecord
  belongs_to :follower, class_name: "User"
  belongs_to :following, class_name: "User"
  validates :follower_id, presence: true
  validates :following_id, presence: true
end

user.rb

class User < ApplicationRecord
  devise  :database_authenticatable, :registerable,
          :recoverable, :rememberable, :validatable
  has_many :posts, dependent: :destroy
  has_many :likes, dependent: :destroy
  has_many :liked_posts, through: :likes, source: :post
  has_many :comments
  
  def already_liked?(post)
    self.likes.exists?(post_id: post.id)
  end

  has_many :following_relationships, class_name: "Relationship", foreign_key: "follower_id", dependent: :destroy
  has_many :followings, through: :following_relationships, source: :following
  has_many :follower_relationships, class_name: "Relationship", foreign_key: "following_id", dependent: :destroy
  has_many :followers, through: :follower_relationships, source: :follower

  def following?(user)
    following_relationships.find_by(following_id: user.id)
  end

  def follow!(other_user)
    following_relationships.create!(following_id: user.id)
  end

  def unfollow!(other_user)
    following_relationships.find_by(following_id: user.id).destroy
  end
  
end

routing

routes.rb

Rails.application.routes.draw do
  devise_for :users
  root to: "posts#index"

  resources :posts do
    resources :comments, only: :create
    resources :likes, only: [:create, :destroy]
  end

  resources :users do
    member do
      get :following, :followers
    end
  end

  resources :relationships, only: [:create, :destroy]
end

View

ruby:_follow_form.html.haml

- unless current_user?(@user)
  #follow_form
  - if current_user.following?(@user)
    = render 'unfollow'
  - else
    = render 'follow'

controller

users_controller.rb

class UsersController < ApplicationController

  def show
    
  end
  
  def edit
    
  end

  def update
    if current_user.update(user_params)
      redirect_to root_path
    else
      render :edit
    end
  end

  def following
    @user = User.find(id: params[:id])
    binding.pry
    @users = @user.followings
    render 'show_following'
  end

  def followers
    @user = User.find(id: params[:id])
    binding.pry
    @users = @user.followers
    render 'show_follower'
  end

  private

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

end

What I tried

-It was confirmed that the association of user.rb (user model) was set up correctly. -Checked all over for spelling mistakes in user.rb (user model) and _follow_form.html.haml (partial template).

Meaning of error message

error

following_relationships.find_by(following_id: user.id)

undefined method `id' for nil:NilClass

-I tried to call the id method for nil, but I couldn't find it. ・ User is nil

→ What causes user to become nil? ?? ??

Found error causes and solutions

-@User was not defined in the show action of users_controller.rb. The error screen says NoMethodError in Users # show, so I should be suspicious of the show action in users_controller.rb.

I solved it by defining @user as follows in the show action of users_controller.rb.

users_controller.rb

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

With params [: id], the value sent with the path is received from the screen before the transition.

Thanks to this article, I was able to resolve the error

I found it on a site called teratail, an engineer's yahoo wisdom bag. [Ruby on Rails] Follow function undefined method ʻid'for nil: NilClass error message appears