How to Create Form in Rails: Syntax and Example

In Rails, you create forms using the form_with helper method, which generates HTML form tags linked to your model or URL. Inside form_with, you use form fields like text_field and submit to build the form inputs and buttons.

Syntax

The basic syntax of a form in Rails uses the form_with helper. It can be linked to a model or a URL. Inside the block, you add form fields and buttons.

• form_with(model: @object): Creates a form tied to a model instance.
• form_with(url: some_path): Creates a form that submits to a specific URL.
• form.text_field :attribute: Adds a text input for the model's attribute.
• form.submit: Adds a submit button.

ruby

form_with(model: @post) do |form|
  form.text_field :title
  form.text_area :content
  form.submit "Save"
end

Example

This example shows a simple form for a Post model with title and content fields. It uses form_with to generate the form and submit button.

erb

<!-- app/views/posts/new.html.erb -->
<%= form_with(model: @post) do |form| %>
  <div>
    <%= form.label :title %><br>
    <%= form.text_field :title %>
  </div>
  <div>
    <%= form.label :content %><br>
    <%= form.text_area :content %>
  </div>
  <div>
    <%= form.submit "Create Post" %>
  </div>
<% end %>

Output

<form action="/posts" method="post">
  <div>
    <label for="post_title">Title</label><br>
    <input type="text" name="post[title]" id="post_title">
  </div>
  <div>
    <label for="post_content">Content</label><br>
    <textarea name="post[content]" id="post_content"></textarea>
  </div>
  <div>
    <input type="submit" value="Create Post">
  </div>
</form>

Common Pitfalls

Common mistakes when creating forms in Rails include:

• Not passing a model or URL to form_with, which causes the form not to submit correctly.
• Forgetting to permit parameters in the controller, leading to errors when saving data.
• Using form_tag instead of form_with in modern Rails, which is legacy.
• Not including labels for accessibility.

erb

Wrong:
<%= form_with do |form| %>
  <%= form.text_field :name %>
  <%= form.submit %>
<% end %>

Right:
<%= form_with(model: @user) do |form| %>
  <%= form.label :name %>
  <%= form.text_field :name %>
  <%= form.submit %>
<% end %>

Quick Reference

Here is a quick summary of key form helpers in Rails:

Helper	Purpose
form_with(model: @object)	Creates a form linked to a model instance
form_with(url: path)	Creates a form that submits to a specific URL
form.text_field :attribute	Adds a text input field
form.text_area :attribute	Adds a multi-line text area
form.label :attribute	Adds a label for an input
form.submit	Adds a submit button

Key Takeaways

Use form_with with a model or URL to create forms in Rails.

Always include labels for accessibility and clarity.

Permit parameters in your controller to allow form data saving.

Avoid legacy helpers like form_tag in new Rails apps.

Test your form to ensure it submits data correctly and renders inputs.