Passing data to partials in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a partial in Rails?

A partial is a reusable view template in Rails that helps you break down complex views into smaller, manageable pieces.

beginner

How do you pass local variables to a partial in Rails?

You pass local variables using the locals option in the render method, like render partial: 'name', locals: { variable: value }.

intermediate

What happens if you don't pass a needed local variable to a partial?

The partial will raise an error because it expects the variable to be defined. Always pass required locals to avoid this.

intermediate

How can you pass a collection of objects to a partial?

Use render partial: 'item', collection: @items. Rails will render the partial once for each item, assigning each to a local variable named after the partial.

beginner

Why use partials with passed data instead of repeating code?

Partials with passed data keep your code DRY (Don't Repeat Yourself), making views easier to maintain and update.

How do you pass a local variable named user with value @user to a partial named profile?

A render partial: 'profile', locals: { user: @user }

B render 'profile', user: @user

C render partial: 'profile', user: @user

D render 'profile', locals: @user

Use the locals hash to pass local variables to partials.

What local variable name does Rails assign when rendering a collection with render partial: 'item', collection: @items?

A collection

B items

C item

D partial

Rails uses the partial name as the local variable for each item in the collection.

What will happen if a partial expects a local variable but it is not passed?

A The variable will be automatically created

B The partial will render with a nil value

C Rails will ignore the missing variable

D Rails will raise an error

Rails raises an error if a required local variable is missing in a partial.

Which of these is the correct way to render a partial without passing any locals?

A render 'partial_name'

B All of the above

C render partial: 'partial_name'

D render partial: 'partial_name', locals: {}

All these ways correctly render a partial without locals.

Why is it good to use partials with passed data in Rails views?

A To keep views organized and avoid repetition

B To avoid using controllers

C To slow down rendering

D To repeat code easily

Partials help keep views clean and avoid repeating code.

Explain how to pass data to a partial in Rails and why it is useful.

Think about how you share information with a friend when showing them a part of your work.

Describe how rendering a collection with a partial works in Rails.

Imagine handing out the same form to many people, one for each.

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