Passing data to partials in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will this partial render output?

Given the following partial _user.html.erb and its usage, what will be rendered?

<!-- _user.html.erb -->
<p>User: <%= user.name %></p>

<!-- In a view -->
<%= render partial: 'user', locals: { user: OpenStruct.new(name: 'Alice') } %>

A <p>User: user.name</p>

B <p>User: Alice</p>

C Error: undefined local variable or method `user`

D <p>User: </p>

💡 Hint

Remember that locals passes variables to partials as local variables.

The partial receives user as a local variable with the value OpenStruct.new(name: 'Alice'). So user.name outputs 'Alice'.

📝 Syntax

intermediate

Which option correctly passes a collection to a partial?

You want to render a list of @products using the _product.html.erb partial. Which code correctly passes the collection?

A <%= render partial: 'product', locals: { products: @products } %>

B <%= render 'product', products: @products %>

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

D <%= render partial: 'product', collection: @products, as: :products %>

💡 Hint

Use the collection: option to render a partial for each item.

Option C uses collection: which renders the partial once per item in @products, assigning each item to product local variable.

🔧 Debug

advanced

Why does this partial raise an error?

Consider this partial and its render call:

<!-- _item.html.erb -->
<p>Item: <%= item.title %></p>

<!-- In a view -->
<%= render partial: 'item' %>

Why does this raise an error?

A Because the partial expects a local variable 'item' which was not passed

B Because partials cannot be rendered without a collection

C Because the partial name must be 'items' to match the variable

D Because the render call is missing the 'locals:' keyword

💡 Hint

Check what variables the partial uses and what is passed in the render call.

The partial uses item but the render call does not pass any local variable named item. This causes a NameError.

❓ state_output

advanced

What is the output of this nested partial rendering?

Given these partials and render calls:

<!-- _comment.html.erb -->
<p>Comment: <%= comment.text %></p>

<!-- _post.html.erb -->
<h2><%= post.title %></h2>
<%= render partial: 'comment', collection: post.comments %>

<!-- In a view -->
<%= render partial: 'post', locals: { post: OpenStruct.new(title: 'Hello', comments: [OpenStruct.new(text: 'Nice!'), OpenStruct.new(text: 'Great!')]) } %>

What will be rendered?

A <h2>Hello</h2><p>Comment: post.comments</p>

B Error: undefined local variable 'comment' in _comment.html.erb

C <h2>Hello</h2><p>Comment: </p><p>Comment: </p>

D <h2>Hello</h2><p>Comment: Nice!</p><p>Comment: Great!</p>

💡 Hint

Remember that collection: passes each item as a local variable named after the partial.

The post partial renders the comment partial twice, once for each comment, passing each comment as comment local variable.

🧠 Conceptual

expert

Which option best describes how locals work in Rails partials?

Choose the most accurate description of how locals work when rendering partials in Rails.

A Locals are variables passed explicitly to partials and are only available inside that partial's scope

B Locals are global variables accessible in all views and partials once set

C Locals automatically include all instance variables from the parent view

D Locals are used to pass data between controllers and views only

💡 Hint

Think about variable scope and how data is shared with partials.

Locals are explicitly passed variables to partials and exist only within that partial's scope. They do not affect global or instance variables.