review_up_down_votes.md

Review Votes (Up/Down)

The Task

"Users should be able to upvote or downvote reviews"

What Makes This Harder Than Simple Upvotes

• Three states: upvoted, downvoted, not voted
• Can change vote (upvote → downvote)
• Net score calculation (upvotes - downvotes)
• More complex UI (two buttons, current state indication)

---

What They Need to Build

• New review_votes table with value column
• ReviewVote model
• Vote/change vote/remove vote functionality
• Display score and current vote state

---

Good Solution

1. Migration

# db/migrate/xxx_create_review_votes.rb
class CreateReviewVotes < ActiveRecord::Migration[7.1]
  def change
    create_table :review_votes do |t|
      t.references :user, null: false, foreign_key: true
      t.references :review, null: false, foreign_key: true
      t.integer :value, null: false
      t.timestamps
    end

    add_index :review_votes, [:user_id, :review_id], unique: true
  end
end

Look for: value column with null: false, unique composite index.

---

2. Model

# app/models/review_vote.rb
class ReviewVote < ApplicationRecord
  belongs_to :user
  belongs_to :review

  validates :review_id, uniqueness: { scope: :user_id }
  validates :value, inclusion: { in: [1, -1] }
end

# app/models/review.rb (add)
has_many :review_votes, dependent: :destroy

def score
  review_votes.sum(:value)
end

def vote_from(user)
  return nil unless user
  review_votes.find_by(user: user)
end

# app/models/user.rb (add)
has_many :review_votes, dependent: :destroy

Look for: Value validation with inclusion, score method using sum, helper method for finding user's vote.

---

3. Routes

# config/routes.rb (add)
resources :reviews, only: [] do
  resource :vote, only: [:create, :destroy], controller: 'review_votes'
end

Generates:

• POST /reviews/:review_id/vote
• DELETE /reviews/:review_id/vote

---

4. Controller

# app/controllers/review_votes_controller.rb
class ReviewVotesController < ApplicationController
  before_action :require_authenticated_user
  before_action :set_review

  def create
    value = params[:value].to_i

    unless [1, -1].include?(value)
      return redirect_back fallback_location: @review.book, alert: "Invalid vote"
    end

    vote = current_user.review_votes.find_or_initialize_by(review: @review)
    vote.value = value

    if vote.save
      redirect_back fallback_location: @review.book
    else
      redirect_back fallback_location: @review.book, alert: "Could not save vote"
    end
  end

  def destroy
    vote = current_user.review_votes.find_by(review: @review)
    vote&.destroy
    redirect_back fallback_location: @review.book
  end

  private

  def set_review
    @review = Review.find(params[:review_id])
  end
end

Look for:

• Value validation in controller (allowlist)
• find_or_initialize_by pattern for upsert
• Scoped to current_user

---

5. View

<%# app/views/reviews/_vote_controls.html.erb %>
<%# locals: (review:) %>

<div class="vote-controls">
  <span class="score">Score: <%= review.score %></span>

  <% if current_user %>
    <% current_vote = review.vote_from(current_user) %>

    <%= button_to review_vote_path(review),
        params: { value: 1 },
        method: :post,
        class: current_vote&.value == 1 ? "btn-voted" : "btn-vote" do %>
      Upvote
    <% end %>

    <%= button_to review_vote_path(review),
        params: { value: -1 },
        method: :post,
        class: current_vote&.value == -1 ? "btn-voted" : "btn-vote" do %>
      Downvote
    <% end %>

    <% if current_vote %>
      <%= button_to "Clear", review_vote_path(review), method: :delete, class: "btn-clear" %>
    <% end %>
  <% end %>
</div>

Look for:

• Passing value as param
• Visual indication of current vote state
• Clear vote option when voted
• Login check

---

Alternative Approaches (All Acceptable)

Controller: Separate actions vs single action

# Separate actions (more RESTful, more code)
def upvote
  vote_with_value(1)
end

def downvote
  vote_with_value(-1)
end

# vs single action with param (shown above)
def create
  value = params[:value].to_i
  # ...
end

Model: Integer vs enum

# Integer (shown above)
validates :value, inclusion: { in: [1, -1] }

# Enum (also acceptable)
enum vote_type: { upvote: 1, downvote: -1 }

---

Discussion Points

Question	What You're Looking For
"50 reviews on page, each calling .score - problem?"	N+1 awareness, could preload or add counter cache
"What if we want to sort reviews by score?"	Add scope: scope :by_score, -> { ... } with join/subquery
"Should users vote on their own reviews?"	Business logic awareness, could add validation
"Integer value vs enum - tradeoffs?"	Enum more readable, integer more flexible for future values

---

Common Mistakes

• Not validating value in controller (security issue - user could pass any integer)
• Creating new record instead of updating existing vote
• N+1 when displaying vote state for many reviews
• Forgetting to handle the "change vote" case
• Not scoping queries to current_user

---

Bonus Points If They...

• Add scope :by_score to Review model
• Mention counter cache for performance
• Add validation preventing self-votes
• Extract vote checking into a helper method
• Consider what happens if review is deleted (dependent destroy)

---

Time Expectation

• Target: 30-40 minutes
• Fast candidate: 25 minutes
• Struggling: 45+ minutes (offer hints on find_or_initialize_by)

---

Hints to Give If Stuck

If stuck on...	Hint
Changing existing vote	"What ActiveRecord method finds OR creates a record?"
Passing value to controller	"Can button_to accept additional params?"
Showing current vote state	"Could the Review model have a helper method?"
Vote validation	"What values should be allowed? How do you validate that?"