simple_form lessons

07 Mar 2014,

So I’ve messed with simple_form, another great project from José Valim.

I’ve gathered some of my lessons learned in this post. First of all I’m skipping the basics, since that’s covered just fine in the README.

Notes On Configuration

Remember to flip your rails server, when you do configuration changes. They will not be loaded on the fly in development.


Wrappers was one of the things I had to dig in to, before figuring out how they work. The documentation explains how to set them up, but not really how they work.

So a wrapper is a configuration set that tells simple_form, how to render the components your form use. But what does that mean?

Let’s look at a simple wrapper, looking something like this:

config.wrappers :simple, tag: :div, error_class: :field_error, class: :form_field do |b|
  b.use :html5
  b.use :label
  b.use :input
  b.use :hint,  wrap_with: { tag: :span, class: :hint }
  b.use :error, wrap_with: { tag: :span, class: :error }

When you use this wrapper in a form, like this

# Here the wrapper is applied to all of the elements of the form
<%= simple_form_for @profile, wrapper: :simple do |f| %>
  <%= f.input :first_name %>
<% end %>

# Here it is only applied to the single field
<%= simple_form_for @profile do |f| %>
  <%= f.input :first_name, wrapper: :simple %>
<% end %>

Yes yes, but what does it do? Well … It wraps. Our first_name input field friend from above, will be wrapped, like this

<div class="form_field string required profile_first_name">
  <label class="string required control-label" for="profile_first_name">
    First name<abbr title="required">*</abbr>
  <input class="string required" id="profile_first_name" name="profile[first_name]" size="50" type="text">

So there’s a lot going on, and simple_form is indeed a powerful ally in maintaning forms across an application.

You can even nest the wrappers.

Custom input

Custom input types goes into the app/inputs folder. They will be loaded by simple_form.

Let’s take a look at a simple example. I want to add a weight type, thus appending the text kg to the input field.

It is as simple as this

class WeightInput < SimpleForm::Inputs::NumericInput
  def input
    out = ''
    out << super # Render what the super class has to offer
    out << template.content_tag(:span, I18n.t(''))

You can build your output, prepending, appending and pretty much do what you like with the fields.

The above turns into this

<input class="numeric weight optional" type="number" name="..."><span>Kg</span>

More to come…