Let me start by saying that I love the idea of simple_form. An easy way to generate forms with a standard - and automated - way of displaying errors when they occur and just generally give your whole site a consistent look and feel with very little overhead.

On my latest project, I started adding simple_form, and for the vanilla forms/field this certainly proved to be easy, however once I moved onto my more bespoke fields things soon became far more complicated.

To do this I needed to use ‘custom render’, and typically of an open source application the documentation only covered vanilla use cases, so creating a custom render was an undocumented process. Normally this would not have been a problem as I am happy to dig into the code and search for help on google. Unfortunately simple_form didn’t live up to its name, as the internals of the gem are anything but simple. I’m sure the architecture fits together easily once you understand it, but for me, it all felt like black magic.

As such, I won’t be using simple_form on my current project. Not because I think it is bad software or unfit for purpose, I have chosen not to use it, as even if I got it all working - and I feel confident that given the time I could - I would be fearful of future changes, as each time I needed to change it I would need to spend time and effort remembering how it worked. I feel this extra effort outweighs any benefit provided by the convenience provided.

This may be due to the complex nature of my application, or it may be that simple_form really is only suitable for vanilla forms.

Final solution

For reference I have provided the code I used in my final solution below:

Helper method ‘form_helper.rb’

module FormHelper
  def field(f, field_name:, hint: nil, &block)
    errors = Array(field_name).flat_map { |fn| f.object.errors[fn]   }.uniq.sort
    render 'form/field', hint: hint, errors: errors, block: block
  end

  def field_tag(value:, error_proc: ->(_) { nil }, hint: nil, &block)
    errors = [error_proc.call(value)].compact
    render 'form/field', hint: hint, errors: errors, block: block
  end
end

View partial ‘form/_field.html.slim’

.form-group class=('has-danger' if errors.any?)
  = capture(&block)
  - if errors.any?
    .errors
      - errors.each do |error|
        .error= error
  - if hint
    small.text-muted=hint

Example usage

Simple field

= field(f, field_name: :date) do
  = f.label :date
  = f.date_field :date

Multiple fields with shared error messaging

= field(f, field_name: [:price, :currency])
  = f.label :price
    = f.text_field :price, type: :number, step: '0.01', min: '0'
    = f.text_field :currency

Using ’*_tag’ helpers

= field_tag(value: params[:ticker], error_proc: ->(value) { 'please enter ticker' if value.blank? }) do
  = label_tag :ticker
  = text_field_tag :ticker, params[:ticker]