How to Use form_for in Rails: Syntax and Examples

In Rails, form_for is a helper method used to create a form tied to a model object, automatically setting the form's action and method. You pass the model instance to form_for, and inside the block, use form builder methods like f.text_field to generate input fields linked to model attributes.

Syntax

The form_for helper takes a model object and a block. Inside the block, a form builder variable (commonly f) is used to create form fields linked to the model's attributes.

• model: The instance of the model you want to build the form for.
• form builder (f): Used to generate input fields like text fields, checkboxes, etc.
• submit button: Adds a button to submit the form.

erb

<%= form_for(@article) do |f| %>
  <%= f.label :title %>
  <%= f.text_field :title %>

  <%= f.label :content %>
  <%= f.text_area :content %>

  <%= f.submit "Save" %>
<% end %>

Example

This example shows a form for an Article model with title and content fields. When submitted, it sends data to the appropriate controller action to create or update the article.

erb

<!-- app/views/articles/_form.html.erb -->
<%= form_for(@article) do |f| %>
  <div>
    <%= f.label :title %><br>
    <%= f.text_field :title %>
  </div>

  <div>
    <%= f.label :content %><br>
    <%= f.text_area :content %>
  </div>

  <div>
    <%= f.submit "Save Article" %>
  </div>
<% end %>

Output

<form action="/articles" method="post">
  <div>
    <label for="article_title">Title</label><br>
    <input type="text" name="article[title]" id="article_title">
  </div>
  <div>
    <label for="article_content">Content</label><br>
    <textarea name="article[content]" id="article_content"></textarea>
  </div>
  <div>
    <input type="submit" name="commit" value="Save Article">
  </div>
</form>

Common Pitfalls

Common mistakes when using form_for include:

• Not passing a model instance, which causes errors or unexpected behavior.
• Using form_for with a symbol instead of a model instance (use form_with for symbols).
• Forgetting to include the authenticity token, which Rails adds automatically with form_for.
• Not using the form builder methods (f.text_field, f.label) inside the block, which breaks the connection to the model.

Example of wrong usage and correct usage:

erb

<!-- Wrong: passing symbol instead of model instance -->
<%= form_for(:article) do |f| %>
  <%= f.text_field :title %>
<% end %>

<!-- Right: pass model instance -->
<%= form_for(@article) do |f| %>
  <%= f.text_field :title %>
<% end %>

Quick Reference

Tips for using form_for:

• Always pass a model instance to form_for.
• Use the form builder variable to create fields linked to model attributes.
• The form's action and method are set automatically based on the model's state (new or existing record).
• Use f.submit to add a submit button.
• For non-model forms or symbols, prefer form_with in Rails 5.1+.

Key Takeaways

Use form_for with a model instance to create forms tied to that model.

Inside the form_for block, use the form builder methods to generate input fields.

form_for automatically sets the form action and HTTP method based on the model state.

Avoid passing symbols to form_for; use form_with for non-model forms.

Always include a submit button using f.submit to allow form submission.