Passing data to partials in Ruby on Rails - Performance & Optimization

Performance: Passing data to partials

MEDIUM IMPACT

This affects page load speed and rendering performance by controlling how much data is sent and how many partial templates are rendered.

Rendering a list of items using partials

Ruby on Rails

<%= render partial: 'item', collection: @items.includes(:associated_data) %>

Preloading associated data and passing only needed data reduces database calls and speeds up rendering.

📈 Performance Gainreduces database queries to 1 and partial renders remain N but faster

Rendering a list of items using partials

Ruby on Rails

<%= render partial: 'item', collection: @items %>

Passing the entire collection without limiting or preloading causes multiple partial renders and possible N+1 queries.

📉 Performance Costtriggers N partial renders and multiple database queries if not optimized

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Passing entire objects with all data	N partial renders	N reflows	High paint cost due to large HTML	[X] Bad
Passing limited, preloaded data	N partial renders	N reflows	Lower paint cost with smaller HTML	[OK] Good

Rendering Pipeline

When Rails renders partials with passed data, it processes the template, executes Ruby code, and generates HTML. Large or complex data increases template processing time and memory usage.

→Template Rendering

→Ruby Execution

→HTML Generation

⚠️ BottleneckTemplate Rendering with large or complex data

Core Web Vital Affected

LCP

This affects page load speed and rendering performance by controlling how much data is sent and how many partial templates are rendered.

Optimization Tips

1Pass only the data needed by the partial to reduce processing time.

2Preload associations to avoid multiple database queries (N+1 problem).

3Limit the size of collections or objects passed to partials to improve rendering speed.

Performance Quiz - 3 Questions

What is a performance risk when passing entire large objects to Rails partials?

A Faster rendering due to more data availability

B Increased memory usage and slower rendering

C No impact on performance

D Improves browser caching

DevTools: Performance

How to check: Record a performance profile while loading the page with partials. Look for long scripting or rendering times related to template rendering.

What to look for: High scripting time or long tasks indicate heavy partial rendering; optimized partials show shorter scripting and rendering times.

Practice

1. What is the main purpose of using locals when rendering partials in Rails?

easy

A. To create a new controller action

B. To include JavaScript files in the partial

C. To define CSS styles for the partial

D. To pass data as local variables to the partial

Solution

1. Step 1: Understand partials in Rails

   Partials are reusable view templates that help keep code DRY (Don't Repeat Yourself).

2. Step 2: Role of locals

   The locals option lets you pass data as local variables to the partial, making it flexible and dynamic.

3. Final Answer:

   To pass data as local variables to the partial -> Option D

4. Quick Check:

   Passing data to partials = locals [OK]

Hint: Remember: locals pass data inside partials [OK]

Common Mistakes:

• Confusing locals with JavaScript or CSS inclusion
• Thinking partials create controller actions
• Assuming locals define styles

2. Which of the following is the correct syntax to pass a local variable named user with value @user to a partial called _profile.html.erb?

easy

A. <%= render partial: profile, user: @user %>

B. <%= render 'profile', locals: { user: @user } %>

C. <%= render profile, user: @user %>

D. <%= render partial: 'profile', locals: user: @user %>

Solution

1. Step 1: Identify correct render syntax with locals

   The correct way to pass locals is using locals: { key: value } inside the render call.

2. Step 2: Check each option

   <%= render 'profile', locals: { user: @user } %> uses locals: { user: @user } which is correct syntax. Others miss the locals hash or have wrong syntax.

3. Final Answer:

   <%= render 'profile', locals: { user: @user } %> -> Option B

4. Quick Check:

   Use locals hash to pass variables [OK]

Hint: Use locals: { var: value } to pass data [OK]

Common Mistakes:

• Omitting the locals keyword
• Passing variables directly without locals hash
• Incorrect hash syntax for locals

3. Given the following code in a Rails view:

user = User.new(name: 'Alice')
<%= render 'user_info', locals: { user: user } %>

And the partial _user_info.html.erb contains:

<p>Name: <%= user.name %></p>

What will be the rendered output?

medium

A. Name: Alice

B. Name:

C. Error: undefined local variable 'user'

D. Name: user

Solution

1. Step 1: Understand locals passing

   The variable user is passed as a local variable to the partial with value User.new(name: 'Alice').

2. Step 2: Partial uses user.name

   The partial accesses user.name, which is 'Alice', so it renders the name correctly.

3. Final Answer:

   <p>Name: Alice</p> -> Option A

4. Quick Check:

   Locals pass data correctly = Alice shown [OK]

Hint: Locals become variables inside partial [OK]

Common Mistakes:

• Forgetting to pass locals causes undefined variable error
• Assuming instance variables are automatically available
• Confusing variable names inside partial

4. Identify the error in this code snippet:

<%= render partial: 'comment', locals: { comment: @comment } %>

# In _comment.html.erb
<p><%= comment_text %></p>

What will happen when this view is rendered?

medium

A. Error: undefined local variable or method 'comment_text'

B. Renders the comment text correctly

C. Syntax error in render call

D. Renders nothing inside the paragraph

Solution

1. Step 1: Check locals passed to partial

   The locals hash passes comment: @comment, so inside the partial the variable comment is available.

2. Step 2: Check variable used in partial

   The partial uses comment_text, which is not defined or passed, causing an undefined variable error.

3. Final Answer:

   Error: undefined local variable or method 'comment_text' -> Option A

4. Quick Check:

   Variable name mismatch causes error [OK]

Hint: Variable names must match locals keys exactly [OK]

Common Mistakes:

• Using wrong variable name inside partial
• Assuming instance variables are available without passing
• Not passing required locals

5. You want to render a list of products using a partial _product.html.erb and pass each product as a local variable named item. Which code correctly renders the list with locals?

hard

A. <%= render product, collection: @products, as: :item %>

B. <%= render partial: 'product', collection: @products, locals: { item: product } %>

C. <%= render partial: 'product', collection: @products, as: :item %>

D. <% @products.each do |p| %>
<%= render 'product', locals: { item: product } %>
<% end %>

Solution

1. Step 1: Understand rendering collections with locals

   Rails allows rendering a collection with render partial: 'name', collection: array, as: :var to pass each element as a local variable.

2. Step 2: Check each option

   <%= render partial: 'product', collection: @products, as: :item %> correctly uses partial, collection, and as: :item to pass each product as item local variable. The <% @products.each do |p| %> <%= render 'product', locals: { item: product } %> <% end %> uses manual loop but product is undefined (loop var is p). The <%= render partial: 'product', collection: @products, locals: { item: product } %> has wrong locals usage (product undefined). The <%= render product, collection: @products, as: :item %> misses partial keyword and unquoted product undefined.

3. Final Answer:

   <%= render partial: 'product', collection: @products, as: :item %> -> Option C

4. Quick Check:

   Use collection + as for locals in partial [OK]

Hint: Use collection and as: :var for lists [OK]

Common Mistakes:

• Passing locals inside collection render incorrectly
• Forgetting to use as: to rename local variable
• Using manual loops instead of collection render