Understanding Tricky Concepts for Junior Developers

# config/routes.rb

# ❌ Common Mistake: Adding routes without understanding them
resources :posts
resources :comments
resources :users

# ✅ Better: Understanding what each route does
resources :posts do
  # This creates routes like:
  # GET    /posts          -> posts#index    (shows all posts)
  # GET    /posts/new      -> posts#new      (shows form for new post)
  # POST   /posts          -> posts#create   (creates a new post)
  # GET    /posts/:id      -> posts#show     (shows one post)
  # GET    /posts/:id/edit -> posts#edit     (shows edit form)
  # PATCH  /posts/:id      -> posts#update   (updates a post)
  # DELETE /posts/:id      -> posts#destroy  (deletes a post)
end

# Want to see all your routes? Run:
# rails routes

# Three main sources of params:

# 1. From the URL
# URL: /posts/123
get '/posts/:id' # -> params[:id] will be "123"

# 2. From form submissions
# In your view (posts/new.html.erb):
<%= form_with model: @post do |f| %>
  <%= f.text_field :title %>
  # When submitted, params will have:
  # params[:post][:title]
<% end %>

# 3. From query strings
# URL: /posts?category=ruby
# -> params[:category] will be "ruby"

# In your controller:
class PostsController < ApplicationController
  def show
    # params is a hash-like object
    puts params # Try this to see what's in params!

    # Common pattern:
    @post = Post.find(params[:id])
  end

  def create
    # ❌ Common Mistake: Using params directly
    @post = Post.create(params[:post]) # UNSAFE!

    # ✅ Better: Use strong parameters
    @post = Post.create(post_params)
  end

  private

  def post_params
    # This is called "strong parameters"
    params.require(:post).permit(:title, :content)
  end
end

# app/controllers/posts_controller.rb
class PostsController < ApplicationController
  def index
    # ❌ Common Mistake: Using a regular variable
    posts = Post.all # The view can't see this!

    # ✅ Better: Using an instance variable
    @posts = Post.all # The view can see this!
  end
end

# app/views/posts/index.html.erb
<% @posts.each do |post| %>  # This works!
  <%= post.title %>
<% end %>

<%# In your view (.html.erb file) %>

<% name = "Alice" %>   
<%# Just runs Ruby code, doesn't show anything %>

<%= name %>           
<%# Runs Ruby code AND shows the result %>

<%# Common examples: %>

<%# ❌ Common Mistake: Using <% when you want to show something %>
<% post.title %>      <%# Nothing appears! %>

<%# ✅ Better: Use <%= when you want to show something %>
<%= post.title %>     <%# Shows the title! %>

<%# When to use each: %>
<% @posts.each do |post| %>  <%# Just Ruby code, no output %>
  <%= post.title %>          <%# Want to show this! %>
<% end %>

# belongs_to: "I belong to ONE thing"
# has_many: "I have MANY things"

# Example: A Post has many Comments
class Post < ApplicationRecord
  has_many :comments
  # This means:
  # - A post can have many comments
  # - You can do: @post.comments
  # - You can do: @post.comments.create(...)
end

class Comment < ApplicationRecord
  belongs_to :post
  # This means:
  # - Each comment belongs to ONE post
  # - You can do: @comment.post
  # - Comments MUST have a post_id
end

# Using associations:
# ❌ Common Mistake: Not understanding the relationship
@post.comments = Comment.create(...)  # Wrong!

# ✅ Better: Using the relationship properly
# Creating a comment for a post:
@comment = @post.comments.create(content: "Great post!")

# Finding a post's comments:
@comments = @post.comments  # Gets all comments for this post

class PostsController < ApplicationController
  def show
    @post = Post.find(params[:id])

    # Different ways to render:

    # 1. Implicit render (no render statement)
    # Rails automatically renders show.html.erb

    # 2. Explicit render of a view
    render 'show'  # Same as above

    # 3. Rendering a different view
    render 'preview'  # Renders preview.html.erb

    # 4. Rendering JSON (for APIs)
    render json: @post

    # 5. Rendering with status
    render 'show', status: :not_found  # 404 status
  end

  def create
    @post = Post.new(post_params)

    # ❌ Common Mistake: Forgetting about render vs redirect
    render 'index'  # This might cause errors!

    # ✅ Better: Redirect after changing data
    if @post.save
      redirect_to @post  # Goes to show page
    else
      render 'new'      # Shows the form again
    end
  end
end

class Post < ApplicationRecord
  # Callbacks run in this order:
  before_validation :cleanup_title
  after_create :notify_followers
  before_destroy :can_be_deleted?

  private

  # ❌ Common Mistake: Doing too much in callbacks
  after_save :do_everything  # Don't do this!

  # ✅ Better: Simple, focused callbacks
  before_validation :cleanup_title
  def cleanup_title
    self.title = title.strip if title.present?
  end

  # This runs after creating a new post
  after_create :notify_followers
  def notify_followers
    # Send notifications about new post
  end

  # This runs before deleting a post
  before_destroy :can_be_deleted?
  def can_be_deleted?
    throw(:abort) if comments.exists?
  end
end

# Start the console
rails c

# Find things
Post.first          # Get the first post
Post.last           # Get the last post
Post.find(123)      # Find post with id 123
Post.where(published: true)  # Find all published posts

# Create things
post = Post.new(title: "Hello")  # Just creates in memory
post.save                        # Saves to database

# Or do it in one step:
Post.create(title: "Hello")      # Creates and saves

# Update things
post = Post.find(123)
post.update(title: "New title")  # Updates the post

# Delete things
post = Post.find(123)
post.destroy                     # Deletes the post

# Reload things (get fresh from database)
post.reload

# Check if valid
post.valid?        # Returns true or false
post.errors        # See any errors