example.rb

# /app/views/families/show.html.haml

= link_to 'Send Message', new_family_message_path(:family_id => @family.id, :category => 'Family')

# /app/controllers/messages_controller.rb

def new
  @category = params[:category]
  render :action => "messages/#{@category}_form", :layout => true
end

# /app/views/messages/_family_form.html.haml

- form_for :message,:url => new_family_messages_path(:family_id => X) do |f|
  = f.text_field :subject
  = f.text_field :body
  = f.submit
  = hidden_field_tag :category, @category

# /app/controllers/messages_controller.rb

# params => { :xxxxx_id => x, :category => 'xxxx', :message => { ... } }
def create
  @category = params[:category] # => 'Family'
  @parent_id = params["#{@category.downcase}_id"] # => params[:family_id]
  @messageable=@category.constantize.find(@parent_id) # => Family.find(...)
  @messageable.messages.build(params[:message])
  #.... save and shit
end

# Resources
# * http://api.rubyonrails.org/classes/ActiveSupport/CoreExtensions/String/Inflections.html