Form_with helper in Ruby on Rails

Introduction

The form_with helper makes it easy to create forms in Rails. It helps you build forms that send data to your app without writing HTML manually.

When you want to create a form to add or update a record in your database.

When you need a form that works with AJAX for a smoother user experience.

When you want Rails to handle form URLs and HTTP methods automatically.

When you want to keep your form code clean and DRY (Don't Repeat Yourself).

Syntax

Ruby on Rails

form_with(model: @record, local: true) do |form|
  # form fields here
end

The model: option links the form to a specific database record.

Use local: true to submit the form with a normal HTTP request instead of AJAX.

Examples

Creates a form for a User model with a text field for the name and a submit button.

Ruby on Rails

form_with(model: @user) do |form|
  form.text_field :name
  form.submit "Save"
end

Creates a search form that sends a GET request to the /search URL.

Ruby on Rails

form_with(url: '/search', method: :get, local: true) do |form|
  form.text_field :query
  form.submit "Search"
end

Creates a form for a Post model with a text area for content and a default submit button.

Ruby on Rails

form_with(model: @post, local: true) do |form|
  form.text_area :content
  form.submit
end

Sample Program

This example shows a simple Rails controller and view using form_with. The form lets users enter a post's title and body. When submitted, it creates a new post in the database.

Ruby on Rails

class PostsController < ApplicationController
  def new
    @post = Post.new
  end

  def create
    @post = Post.new(post_params)
    if @post.save
      redirect_to @post
    else
      render :new
    end
  end

  private

  def post_params
    params.require(:post).permit(:title, :body)
  end
end

# In app/views/posts/new.html.erb
<%= form_with(model: @post, local: true) do |form| %>
  <div>
    <%= form.label :title %><br>
    <%= form.text_field :title %>
  </div>
  <div>
    <%= form.label :body %><br>
    <%= form.text_area :body %>
  </div>
  <div>
    <%= form.submit "Create Post" %>
  </div>
<% end %>

Important Notes

By default, form_with submits forms using AJAX unless you set local: true.

The helper automatically sets the correct HTTP method (POST, PATCH) based on whether the model is new or existing.

Use form_with instead of older helpers like form_for for modern Rails apps.

Summary

form_with helps you build forms easily in Rails.

It links forms to models and handles URLs and methods automatically.

Use local: true to disable AJAX if you want normal form submission.

Practice

1. What is the main purpose of the form_with helper in Rails?

easy

A. To create HTML forms linked to models with automatic URL and method handling

B. To style forms using CSS classes automatically

C. To validate form inputs on the client side

D. To generate JavaScript code for form animations

Solution

1. Step 1: Understand the role of form_with

   The form_with helper builds forms in Rails and connects them to models.

2. Step 2: Identify what it automates

   It automatically sets the form's URL and HTTP method based on the model's state.

3. Final Answer:

   To create HTML forms linked to models with automatic URL and method handling -> Option A

4. Quick Check:

   form_with purpose = create linked forms [OK]

Hint: Remember: form_with links forms to models automatically [OK]

Common Mistakes:

• Thinking form_with styles forms
• Confusing form_with with client-side validation
• Assuming form_with generates JavaScript animations

2. Which of the following is the correct syntax to create a form using form_with for a model instance @post with normal (non-AJAX) submission?

easy

A. form_with @post, remote: true do |form|

B. form_with model: @post, local: true do |form|

C. form_with model: @post, remote: true do |form|

D. form_with url: posts_path, method: :get do |form|

Solution

1. Step 1: Identify how to disable AJAX

   Using local: true disables AJAX and makes the form submit normally.

2. Step 2: Check the correct syntax for model binding

   The correct syntax is form_with model: @post, local: true do |form|.

3. Final Answer:

   form_with model: @post, local: true do |form| -> Option B

4. Quick Check:

   local: true disables AJAX [OK]

Hint: Use local: true for normal form submission [OK]

Common Mistakes:

• Omitting local: true for non-AJAX forms
• Using remote: true which enables AJAX
• Using method :get for form submission instead of post

3. Given the following code snippet, what will be the HTTP method used when submitting the form?

form_with model: @article do |form|
  # form fields here
end

medium

A. DELETE if @article is persisted, POST if new

B. GET always

C. POST if @article is a new record, PATCH if it is persisted

D. PUT always

Solution

1. Step 1: Understand form_with method behavior

   When linked to a model, form_with uses POST for new records and PATCH for existing records.

2. Step 2: Apply to @article

   If @article is new, method is POST; if persisted, method is PATCH.

3. Final Answer:

   POST if @article is a new record, PATCH if it is persisted -> Option C

4. Quick Check:

   form_with method depends on record state [OK]

Hint: New record = POST, existing record = PATCH [OK]

Common Mistakes:

• Assuming GET is used for form submission
• Confusing PUT with PATCH
• Thinking DELETE is used automatically

4. Identify the error in this form_with usage:

form_with model: @user, local: true do
  text_field :name
end

medium

A. Missing block variable for form builder

B. local: true is invalid option

C. text_field should be text_field_tag

D. model: @user should be url: user_path(@user)

Solution

1. Step 1: Check block syntax

   The block must accept a form builder variable, e.g., do |form|.

2. Step 2: Check usage of form builder methods

   Inside the block, form builder methods like form.text_field :name must be called.

3. Final Answer:

   Missing block variable for form builder -> Option A

4. Quick Check:

   Block needs form variable for fields [OK]

Hint: Always use |form| block variable with form_with [OK]

Common Mistakes:

• Omitting the block variable
• Calling form builder methods without prefix
• Confusing text_field with text_field_tag

5. You want to create a form for a new @comment that submits normally (no AJAX), includes a text area for content, and a submit button labeled 'Post Comment'. Which of the following is the correct complete form_with usage?

hard

A. form_with @comment do |form|
form.text_field :content
form.submit
end

B. form_with model: @comment, remote: true do |form|
form.text_area :content
form.submit 'Post Comment'
end

C. form_with url: comments_path, method: :post do |form|
form.text_area :content
form.submit 'Post Comment'
end

D. form_with model: @comment, local: true do |form|
form.text_area :content
form.submit 'Post Comment'
end

Solution

1. Step 1: Ensure normal submission

   Use local: true to disable AJAX and submit normally.

2. Step 2: Use correct form builder methods

   Use form.text_area :content for the text area and form.submit 'Post Comment' for the button label.

3. Final Answer:

   form_with model: @comment, local: true do |form| form.text_area :content form.submit 'Post Comment' end -> Option D

4. Quick Check:

   local: true + text_area + submit label [OK]

Hint: Use local: true and form builder methods for fields [OK]

Common Mistakes:

• Using remote: true which enables AJAX
• Using text_field instead of text_area for content
• Omitting the submit button label