An attempt at Reform

campaign_message_form.rb

class QuickCampaignMessageForm < Reform::Form
  include Composition

  model :campaign_message

  properties :campaign_id, :name, :subject, validates: {presence: true}, on: :campaign_message
  
  property :file, on: :asset

  property :text, on: :text_asset
end

form_data.rb

{
  "campaign_message" => {
    "campaign_id" => "1",
    "name" => "Text club",
    "subject" => "The message's subject",
    "file" => an_uploaded_file,
    "text" => "Thanks for joining the text club!"
  }
}

models.rb

# Simplified...
class CampaignMessage < ActiveRecord::Base
  has_many :source_assets
end

class SourceAsset < ActiveRecord::Base
  has_many :payload_assets
end

class PayloadAsset < ActiveRecord::Base

end

quick_campaign_messages_controller.rb

class QuickCampaignMessagesController < ApplicationController
  def new
    @form = get_form
  end

  def create
    @form = get_form

    if @form.validate(params[:campaign_message])
      @form.save
      # ... redirect and return ...
    end

    @form.prepopulate! # Maybe?
    render :new
  end

  private

  def get_form
    # This is probably not the Right Way, and where the problems begin. Do 
    # I need an intermediate object here?
    QuickCampaignMessageForm.new(campaign_message: CampaignMessage.new(campaign_id: params[:campaign_id]),
                                 asset: SourceAsset.new,
                                 text_asset: PayloadAsset.new)
  end
end