rich_text_field.rb

module RichTextHelper
  cattr_accessor(:id, instance_accessor: false) { 0 }

  def rich_text_field_tag(name, value = nil, options = {})
    options.symbolize_keys!

    attributes = {
      class: "formatted_content #{options[:class]}".squish
    }

    attributes[:autofocus] = true if options[:autofocus]
    attributes[:placeholder] = options[:placeholder] if options[:placeholder]
    attributes[:input] = options[:input] || "trix_input_#{RichTextHelper.id += 1}"
    attributes[:toolbar] = options[:toolbar] if options[:toolbar]

    editor_tag = content_tag("trix-editor", "", attributes)
    input_tag = hidden_field_tag(name, value, id: attributes[:input])

    editor_tag + input_tag
  end
end

module ActionView
  module Helpers
    include RichTextHelper

    module Tags
      class RichText < Base
        include RichTextHelper
        delegate :dom_id, to: ActionView::RecordIdentifier

        def render
          options = @options.stringify_keys
          add_default_name_and_id(options)
          options["input"] ||= dom_id(object, [options["id"], :trix_input].compact.join("_"))
          rich_text_field_tag(options.delete("name"), value_before_type_cast(object), options)
        end
      end
    end

    module FormHelper
      def rich_text_field(object_name, method, options = {})
        Tags::RichText.new(object_name, method, self, options).render
      end
    end

    class FormBuilder
      def rich_text_field(method, options = {})
        @template.rich_text_field(@object_name, method, objectify_options(options))
      end
    end
  end
end

@javan Thanks for this!

For rails master branch this did not work for me.
https://gist.github.com/javan/d316ff74ee189f5d2c2e#file-rich_text_field-rb-L36

I fixed it with:

rich_text_field_tag(options.delete("name"), options.delete("value") { value_before_type_cast }, options)