_form.html.erb

<!-- posts/_form.html.erb -->
<%= simple_form_for @form do |f| %>
  <%= f.input :user_id %>
  <%= f.input :subject %>
  <%= f.input :content %>
  <%= f.submit %>
<% end %>

application_form.rb

class ApplicationForm
  include ActiveModel::Model
  include ActiveModel::Attributes

  attr_reader :context
  attribute :id, :integer

  def self.from_params(params)
    new.tap do |form|
      params.each do |k,v|
        if form.attributes.include?(k)
          form.public_send("#{k}=", v)
        end
      end
    end
  end

  def self.from_model(model)
    new.tap { |form| form.map_model(model) }
  end

  # self.user_id = model.user_id
  # self.title = model.title
  def map_model(model)
    raise ArgumentError, "not defined."
  end

  def self.model_name
    ActiveModel::Name.new(self, nil, form_name.to_s.camelize)
  end

  def self.acts_like(val)
    @acts_like = val
  end

  def self.form_name
    @acts_like || inferred_name
  end

  def self.inferred_name
    self.name.demodulize.underscore.to_sym
  end

  def persisted?
    id.present? && id.to_i > 0
  end

  def to_key
    [id]
  end

  def to_model
    self
  end

  def to_param
    id.to_s
  end

  def attributes
    super.except(:id)
  end
end

new_form.rb

# forms/posts/new_form.rb
module Posts
  class NewForm < ApplicationForm
    acts_like :post

    attribute :user_id, :integer
    attribute :subject, :string
    attribute :content, :string

    validates :user_id, presence: true
  end
end

posts_controller.rb

class PostsController < ApplicationController
  def new
    @form = Posts::NewForm.new
  end

  def create
    @form = Posts::NewForm.from_params(post_params)
    unless @form.valid?
      render :new
    end
  end

  private

  def post_params
    params.require(:post).permit(:subject, :user_id, :content)
  end
end