How YouTube Might Track “Most Replayed” Parts of a Video — in Ruby on Rails

[Lomanu4]

Have you ever noticed the spiky graph above the YouTube timeline showing "Most Replayed" sections?

In this post, we’ll build a simplified version of that system using Ruby on Rails — to track, store, and visualize which parts of a video are being re-watched the most.

? Step 1: Database Design


We'll use two tables:

1. videos – to store video metadata
2. video_interactions – to log when a user scrubs, replays, or pauses at a specific second

Migration: videos


# db/migrate/20250515120000_create_videos.rb
class CreateVideos < ActiveRecord::Migration[7.0]
def change
create_table :videos do |t|
t.string :title
t.integer :duration_seconds
t.references :uploaded_by, foreign_key: { to_table: :users }

t.timestamps
end
end
end
Migration: video_interactions


# db/migrate/20250515120500_create_video_interactions.rb
class CreateVideoInteractions < ActiveRecord::Migration[7.0]
def change
create_table :video_interactions do |t|
t.references :video, null: false, foreign_key: true
t.references :user, foreign_key: true
t.string :interaction_type # e.g. 'replay', 'scrub'
t.integer :timestamp_in_video # second in timeline

t.timestamps
end
end
end
? Step 2: Models


# app/models/video.rb
class Video < ApplicationRecord
has_many :video_interactions, dependent: :destroy
end

# app/models/video_interaction.rb
class VideoInteraction < ApplicationRecord
belongs_to :video
belongs_to :user, optional: true

enum interaction_type: { replay: 'replay', scrub: 'scrub', pause: 'pause' }

validates :timestamp_in_video, presence: true
end
? Step 3: API Endpoint to Track Interactions


Let’s make an endpoint to log interactions.


# config/routes.rb
Rails.application.routes.draw do
resources :videos do
resources :video_interactions, only: [:create]
end
end

# app/controllers/video_interactions_controller.rb
class VideoInteractionsController < ApplicationController
def create
interaction = VideoInteraction.new(interaction_params)
if interaction.save
render json: { status: "ok" }, status: :created
else
render json: { errors: interaction.errors.full_messages }, status: :unprocessable_entity
end
end

private

def interaction_params
params.require(:video_interaction).permit(
:video_id,
:user_id,
:interaction_type,
:timestamp_in_video
)
end
end
? Step 4: Aggregating the "Most Replayed" Data


Now let’s create a method to find which parts of the video were replayed the most.


# app/models/video.rb
def replay_heatmap
video_interactions
.where(interaction_type: 'replay')
.group(:timestamp_in_video)
.order(:timestamp_in_video)
.count
end

def most_replayed_second
replay_heatmap.max_by { |_, count| count }&.first
end

This gives you a hash like:


{
5 => 3,
6 => 3,
7 => 14, # ? most replayed second
8 => 2
}
? Step 5: Visualizing on the Frontend


You can send the replay_heatmap hash to your frontend via an API, and render a chart using any JS library like:

• Chart.js
• ApexCharts
• D3.js

GET /videos/123/replay_heatmap

{
"5": 3,
"6": 3,
"7": 14,
"8": 2
}

You can then show a hoverable graph above your video player like YouTube.

? Bonus: Background Summary Table (Optional)


To speed up performance on popular videos, you can pre-aggregate data using a background job:


rails generate model VideoReplaySummary video:references timestamp_in_video:integer replay_count:integer

Use a job (e.g. with Sidekiq or ActiveJob) to refresh this hourly.

? Testing It Locally


Use curl or Postman to simulate a replay:


curl -X POST

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\
-H "Content-Type: application/json" \
-d '{
"video_interaction": {
"user_id": 1,
"interaction_type": "replay",
"timestamp_in_video": 142
}
}'
? Summary

Part	Description
videos table	Stores basic video info
video_interactions	Logs every replay/scrub with timestamp
Aggregation method	Groups interactions by timestamp for heatmap
Visualization	Displays the graph on video timeline

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