Using custom form builder in rails.

application_form_builder.rb

# app/builders/application_form_builder.rb
class ApplicationFormBuilder < ActionView::Helpers::FormBuilder
  def form_group(method, options = {}, &block)
    content = @template.capture(&block)
    content += help_block_with_errors(method) if object.errors[method].any?

    options[:class] ||= ""
    options[:class] += " form-group"

    @template.content_tag :div, content, options
  end

  def label(method, text = nil, options = {}, &block)
    text ||= @template.t(".#{method}.label")
    text += I18n.t("helpers.field.required") if options[:required]

    options[:class] ||= ""

    super(method, text, options, &block)
  end

  %w[text_field file_field text_area datetime_field email_field password_field url_field].each do |field_helper|
    define_method field_helper do |method, options = {}|
      options[:class] ||= ""
      options[:class] += " form-control"
      options[:class] += " is-invalid" if object.errors[method].any?

      options[:placeholder] ||= @template.t(".#{method}.placeholder", default: "")

      super(method, options)
    end
  end

  def submit(value = nil, options = {})
    unless value.is_a? String
      options = value
      value = @template.t(".submit")
    end

    super(value, options)
  end

  private

  def help_block_with_errors(method)
    @template.content_tag(
      :div, object.errors[method].join(", "), class: "invalid-feedback"
    )
  end
end

form_builder.rb

# config/initializers/form_builder.rb
# Disable .field_with_errors wrapper for validation messages
ActionView::Base.field_error_proc = proc do |html_tag, instance|
  html_tag.html_safe
end
# Tell rails which custom form builder to use by default
ActionView::Base.default_form_builder = "ApplicationFormBuilder"

new.html.erb

<%# app/views/registrations/new.html.erb %>
<%= form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f| %>
  <%= f.form_group :email do %>
    <%= f.label :email %>
    <%= f.text_field :email %>
  <% end %>

  <%= f.form_group :nick do %>
    <%= f.label :nick %>
    <%= f.text_field :nick %>
  <% end %>

  <%= f.form_group :name do %>
    <%= f.label :name %>
    <%= f.text_field :name %>
  <% end %>

  <%= f.form_group :password do %>
    <%= f.label :password %>
    <% if @minimum_password_length %>
      <small class="form-text text-muted">
        <%= t('.min_password_length', length: @minimum_password_length) %>
      </small>
    <% end %>
    <%= f.password_field :password, autocomplete: "off" %>
  <% end %>

  <%= f.form_group :password_confirmation do %>
    <%= f.label :password_confirmation %>
    <%= f.password_field :password_confirmation, autocomplete: "off" %>
  <% end %>
<% end %>

sk.yml

sk:
  user_registrations:
    form:
        name:
          label: Meno a priezvisko
          placeholder: Ján Galaxián
        email:
          label: E-mail
          placeholder: [email protected]
        password:
          label: Heslo
        password_confirmation:
          label: Heslo znovu
        submit: Odoslať