Partial templates in Ruby on Rails - Step-by-Step Execution

Concept Flow - Partial templates

Start Rendering View

↓

Encounter Partial Call

↓

Locate Partial File (_partial.html.erb)

↓

Render Partial with Passed Locals

↓

Insert Partial Output into Main View

↓

Continue Rendering Main View

↓

Complete Rendering

When rendering a view, Rails finds the partial file, renders it with any given data, then inserts its output into the main view before continuing.

Execution Sample

Ruby on Rails

<%= render partial: "user", locals: { user: @user } %>

This code renders the '_user.html.erb' partial, passing the @user object as a local variable named user.

Execution Table

Step	Action	Details	Result
1	Start rendering main view	Main view begins processing	No output yet
2	Encounter partial call	Render partial 'user' with locals {user: @user}	Prepare to render partial
3	Locate partial file	Find '_user.html.erb' in views folder	Partial file found
4	Render partial	Use local variable user = @user	Partial HTML generated with user data
5	Insert partial output	Embed partial HTML into main view output	Main view output updated
6	Continue main view rendering	Process remaining main view content	Final combined HTML output
7	Complete rendering	All view code processed	Full HTML sent to browser

💡 Rendering completes after partial is inserted and main view finishes

Variable Tracker

Variable	Start	After Partial Render	Final
@user	User object from controller	Passed as local 'user' to partial	Unchanged
user	N/A	Local variable in partial with @user value	Scope ends after partial render

Key Moments - 3 Insights

Why do we prefix partial filenames with an underscore?

Rails uses the underscore to identify partial files. The execution_table step 3 shows Rails locating '_user.html.erb' as the partial file.

How does the partial get access to the data it needs?

Data is passed explicitly via locals, as shown in step 2 and 4 where the user variable is passed and used inside the partial.

What happens if the partial is missing?

Rails will raise an error at step 3 when it tries to locate the partial file and cannot find it.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the partial file located?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Check the 'Locate partial file' action in the execution_table

According to variable_tracker, what is the value of 'user' after the partial render?

A Nil

B Undefined

C Same as @user from controller

D A new unrelated object

💡 Hint

Look at the 'After Partial Render' column for 'user' in variable_tracker

If you remove the locals hash from the render call, what will happen?

A Partial renders with no data, likely causing errors

B Partial renders normally with @user accessible

C Rails automatically passes all controller variables

D Partial file is not found

💡 Hint

Refer to key_moments about how data is passed to partials

Concept Snapshot

Partial templates in Rails are reusable view snippets.
They are stored with filenames starting with an underscore (_).
Use <%= render partial: "name", locals: {var: value} %> to include them.
Locals pass data explicitly to the partial.
Rails renders the partial and inserts its output into the main view.
This helps keep views clean and DRY.

Full Transcript

Partial templates in Rails let you reuse small pieces of view code. When the main view is rendering, it sees a call to render a partial. Rails then finds the partial file, which always starts with an underscore, like '_user.html.erb'. It renders this partial using any local variables you pass in, such as a user object. The partial's HTML output is then inserted back into the main view's output. This process continues until the entire main view is rendered and sent to the browser. If the partial file is missing, Rails will raise an error during rendering. Using partials keeps your views organized and avoids repeating code.

Practice

1. What is the main purpose of using partial templates in Rails views?

easy

A. To define routes for the application

B. To speed up database queries

C. To handle user authentication

D. To reuse small pieces of view code across different pages

Solution

1. Step 1: Understand what partial templates are

   Partial templates are small view files that can be included in other views to avoid repeating code.

2. Step 2: Identify the main use case

   The main use is to reuse small pieces of view code across different pages, improving maintainability.

3. Final Answer:

   To reuse small pieces of view code across different pages -> Option D

4. Quick Check:

   Partial templates = reuse view code [OK]

Hint: Partial templates reuse view parts, not logic or routes [OK]

Common Mistakes:

• Thinking partials speed up database queries
• Confusing partials with authentication logic
• Assuming partials define routes

2. Which of the following is the correct syntax to render a partial named _menu.html.erb in a Rails view?

easy

A. <%= partial 'menu' %>

B. <%= render 'menu' %>

C. <%= include 'menu' %>

D. <%= render_partial 'menu' %>

Solution

1. Step 1: Recall the Rails partial rendering syntax

   Rails uses render 'partial_name' to include partials, omitting the underscore and file extension.

2. Step 2: Match the correct syntax

   The correct syntax is <%= render 'menu' %> to render _menu.html.erb.

3. Final Answer:

   <%= render 'menu' %> -> Option B

4. Quick Check:

   Render partial = render 'name' [OK]

Hint: Use render 'partial_name' without underscore or extension [OK]

Common Mistakes:

• Using 'partial' instead of 'render'
• Using 'include' which is not a Rails helper
• Adding underscore in render argument

3. Given the following code in a Rails view:

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

What will be the local variable name inside the _product.html.erb partial for each item?

medium

A. item

B. product

C. @products

D. collection

Solution

1. Step 1: Understand collection rendering with 'as'

   When rendering a collection with render partial: 'name', collection: ..., as: :var, each element is passed as var inside the partial.

2. Step 2: Identify the local variable name

   Here, as: :item means inside _product.html.erb each element is accessible as item.

3. Final Answer:

   item -> Option A

4. Quick Check:

   Collection with 'as' sets local variable name [OK]

Hint: Use 'as' option to rename local variable in collection partial [OK]

Common Mistakes:

• Assuming default variable name 'product' instead of 'item'
• Using instance variable '@products' inside partial
• Confusing 'collection' as variable name

4. What is wrong with this code snippet in a Rails view?

<%= render 'user', user: @user %>

Assuming the partial is named _user.html.erb and expects a local variable user.

medium

A. The syntax should be render partial: 'user', locals: { user: @user }

B. The partial name should include the underscore: render '_user'

C. The code is correct and will work as expected

D. You cannot pass local variables to partials

Solution

1. Step 1: Check the shorthand syntax for passing locals

   Rails allows passing locals directly by naming the variable in the render call, like render 'user', user: @user.

2. Step 2: Confirm partial naming rules

   The underscore is omitted in the render call; Rails finds _user.html.erb automatically.

3. Final Answer:

   The code is correct and will work as expected -> Option C

4. Quick Check:

   Passing locals shorthand is valid [OK]

Hint: Passing locals as named args in render is valid shorthand [OK]

Common Mistakes:

• Thinking underscore is required in render argument
• Believing locals must be inside a hash named 'locals'
• Assuming locals cannot be passed at all

5. You want to render a list of comments using a partial _comment.html.erb. Each comment should be passed as comment local variable. Which is the best way to do this in Rails?

hard

A. <%= render partial: 'comment', collection: @comments %>

B. <%= render 'comment', comments: @comments %>

C. <%= render partial: 'comment', collection: @comments, as: :comments %>

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

Solution

1. Step 1: Understand collection rendering default variable

   When rendering a collection with render partial: 'comment', collection: @comments, Rails passes each element as comment by default (partial name without underscore).

2. Step 2: Evaluate other options

   <%= render 'comment', comments: @comments %> passes the whole array as a single local, not each comment. <%= render partial: 'comment', collection: @comments, as: :comments %> passes each as comments (plural). <%= render partial: 'comment', locals: { comment: @comments } %> passes the entire array as one variable, not individual comments.

3. Final Answer:

   <%= render partial: 'comment', collection: @comments %> -> Option A

4. Quick Check:

   Collection rendering uses partial name as local by default [OK]

Hint: Render collection with partial name to get default local variable [OK]

Common Mistakes:

• Passing whole array as one local variable
• Using 'as: :comments' which sets plural local name
• Confusing locals hash with collection rendering