Form helpers (form_with) in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output HTML of this form_with helper?

Consider this Rails view code:

form_with(model: @post, local: true) do |form|
  concat form.label :title
  concat form.text_field :title
  concat form.submit 'Save'
end

What HTML does this generate if @post is a new record?

Ruby on Rails

form_with(model: @post, local: true) do |form|
  concat form.label :title
  concat form.text_field :title
  concat form.submit 'Save'
end

A

<form action="/posts" method="post">
  <label for="post_title">Title</label>
  <input type="text" name="post[title]" id="post_title" />
  <input type="submit" value="Save" />
</form>

B

<form action="/posts/1" method="post">
  <label for="post_title">Title</label>
  <input type="text" name="post[title]" id="post_title" />
  <input type="submit" value="Save" />
</form>

C

<form action="/posts" method="get">
  <label for="post_title">Title</label>
  <input type="text" name="post[title]" id="post_title" />
  <input type="submit" value="Save" />
</form>

D

<form action="/posts/1" method="patch">
  <label for="post_title">Title</label>
  <input type="text" name="post[title]" id="post_title" />
  <input type="submit" value="Save" />
</form>

💡 Hint

Think about what URL and HTTP method Rails uses for new records with form_with.

When @post is a new record, Rails generates a form that submits to the collection path (e.g., /posts) using POST method to create a new resource.

❓ state_output

intermediate

What is the value of the hidden input generated by form_with for an existing record?

Given this code snippet:

form_with(model: @post, local: true) do |form|
  concat form.hidden_field :id
end

If @post.id is 42, what is the value attribute of the hidden input?

Ruby on Rails

form_with(model: @post, local: true) do |form|
  concat form.hidden_field :id
end

A <input type="hidden" name="post[id]" id="post_id" value="" />

B <input type="hidden" name="post[id]" id="post_id" value="42" />

C <input type="hidden" name="post[id]" id="post_id" value="0" />

D <input type="hidden" name="post[id]" id="post_id" />

💡 Hint

Hidden fields reflect the model's attribute values.

The hidden field for :id will have the value of the model's id attribute, which is 42 here.

📝 Syntax

advanced

Which form_with syntax correctly disables remote AJAX submission?

You want to create a form that submits synchronously (not via AJAX). Which option correctly disables remote submission?

A form_with(model: @user, remote: false) do |form| ... end

B form_with(model: @user, submit_via_ajax: false) do |form| ... end

C form_with(model: @user, ajax: false) do |form| ... end

D form_with(model: @user, local: true) do |form| ... end

💡 Hint

Check the official Rails option for disabling remote forms.

The correct option to disable remote AJAX submission is local: true. The remote option is deprecated.

🔧 Debug

advanced

Why does this form_with raise an error?

Given this code:

form_with(url: posts_path, method: :post) do |form|
  concat form.text_field :title
  concat form.submit
end

Why does this raise an error?

Ruby on Rails

form_with(url: posts_path, method: :post) do |form|
  concat form.text_field :title
  concat form.submit
end

A Because <code>form.text_field</code> requires a model object to infer the field name.

B Because <code>method: :post</code> is invalid; it should be a string.

C Because <code>form.submit</code> must have a label argument.

D Because <code>url</code> option cannot be used with <code>form_with</code>.

💡 Hint

Think about what form.text_field needs to generate the input name.

When using form_with with a URL instead of a model, form helpers like text_field need an object or name to generate proper input names. Without a model or scope, it raises an error.

🧠 Conceptual

expert

How does form_with decide the HTTP method for an existing record?

When you use form_with(model: @article) and @article is persisted, which HTTP method does Rails use and why?

A Rails uses PUT because it replaces the entire resource.

B Rails uses POST because it always submits forms with POST.

C Rails uses PATCH because it updates the existing record partially.

D Rails uses GET because it fetches the resource for editing.

💡 Hint

Think about RESTful conventions for updating resources.

Rails uses PATCH method for existing records to update them partially, following RESTful conventions.