We can use form_for or form_tag to build forms in Rails. But their parameters act differently, which always bites me.

For example, I have to wrap html options into a html key when I change from form_tag to form_for.

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= form_tag '/techloglogies/board', method: :get, class: 'form-inline'

= form_for @search, url: '/techloglogies/board', method: :get, html: { class: 'form-inline' }

And as the 3rd parameter of f.select is options than html_options. So I have to add an empty hash {} there to skip it.

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= select_tag 'step',
  options_for_select(Order::PURE_STEPS.invert, params[:step]),
  class: 'form-control input-sm', style: 'width: 100px'

= f.select :step, Order::PURE_STEPS.invert, {},
  class: 'form-control input-sm', style: 'width: 100px'

Why they acts differently? Isn’t f.select built on select_tag?

The short answer is NO. f.select is not built on select_tag. Let’s dig into the source.

Here’s where form_for defined.

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module ActionView::Helper::FormHelper

  def text_field(object_name, method, options = {})
    Tags::TextField.new(object_name, method, self, options).render
  end

  def form_for(...)
    ...
  end
end

In FormHelper above also defined a class named ActionView::Helpers::FormBuilder. The form_for uses the FormBuilder.

Most methods in FormHelper are delegated by FormBuilder.

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# in FormBuilder
def #{selector}(method, options = {})  # def text_field(method, options = {})
  @template.send(                      #   @template.send(
    #{selector.inspect},               #     "text_field",
    @object_name,                      #     @object_name,
    method,                            #     method,
    objectify_options(options))        #     objectify_options(options))
end                                    # end

Unlike text_field, select helper defined in ActionView::Helpers::FormOptionsHelper. And a select method in FormBuilder delegates the FormOptionsHelper#select.

text_field or select finnally use classes under module ActionView::Helpers::Tags

FormTag and SelectTag are defined in ActionView::Helper::FormTagHelper

ActionView::Helper::FormTagHelper seems totally another implement. Most tags helper are component directly in the methods. eg.

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def text_field_tag(name, value = nil, options = {})
  tag :input, { "type" => "text", "name" => name, "id" => sanitize_to_id(name), "value" => value }.update(options.stringify_keys)
end

select_tag and FormBuilder#select do dot shere the implement. They implement the include_blank and other options separately. Is it a historical reason?

How to avoid converts?

Use form object to always use form_for style.