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Dynamic Polymorphic Forms in Rails 7

15 November 2023 about a 5 minute read

It was time to move my Meal Planning app from AirTable to a custom application. AirTable’s pricing for the size of the database wasn’t awful, but I’d moved past the free account limits. And there were some features I wanted to add that were hard with the basic AirTable features.

It’s been a while, so let’s write a Rails app!

I exported CSVs from AirTable, ran bundle && rails new and got to test-driving. Rails 7 and Resource scaffolding just worked as I expected. Recipes and Restaurants came together very quickly.

As soon as I tried to model a calendar of days, where days could have one or more restaurants and one or more recipes (a key feature of the current solution), things got complex.

I ran into three problems:

  1. Rails and forms with has_many associations
  2. But the association is polymorphic
  3. But the form really needs to be dynamic in order to add/remove things from the association

Complex Forms

I had my Recipe and Restaurant models and full controllers. Now I wanted to add a Day model that has a date as well has has_many :recipes as a starting point.

Rails has gotten pretty mature when it comes to the fields_for helper that basically gives you a nested form that includes your association. The Complex Forms section of the Action View Form Helpers Rails Guide talks about People with more than one address. This is great for explaining how form_with and fields_for work together.

To keep things simple, I only tried to add and edit one Recipe on a Day. It was working great. Now it was time to make the association polymorphic.

Polymorphic has-and-belongs-to-many

I needed to be able to add one recipe or one restaurant. I tried several different modelings of the associations to get a polymorphic has_many to work and couldn’t get all the tests passing. Then I found this StackOverflow post that was. I needed a separate join table with some has_many through’s’.

Of course! I needed something that looked more like:

Day <-> Meal (Course <-> [Recipe | Restaurant])

Which got me to these models:

class Day < ApplicationRecord  
  validates :date, uniqueness: true  
  validates :date, presence: true  
  has_many :meals, dependent: :destroy  

class Meal < ApplicationRecord  
  belongs_to :day  
  belongs_to :course, polymorphic: true  

class Recipe < ApplicationRecord  
  validates :name, presence: true  
  has_many :meals, as: :course  
  has_many :days, through: :meals  

class Restaurant < ApplicationRecord  
  validates :name, presence: true  
  has_many :meals, as: :course  
  has_many :days, through: :meals  

(Note: I’m not thrilled with the name of course, but it’s close enough.)

Now with the form, I could put one Recipe select and one Restaurant select and be able to have one or the other. There were some bugs, sure. But I was creating, updating, and saving Days. Progress!

Next question - how do I get the form properly dynamic, adding as many recipes and restaurants as I wanted?

Dynamic Complex Polymorphic Forms

There’s a nice caveat at the Rails Guide for forms:

Rather than rendering multiple sets of fields ahead of time you may wish to add them only when a user clicks on an “Add new address” button. Rails does not provide any built-in support for this.

Whoops. I’m on my own.

To recap, I knew that what I wanted was for a Day form to start with no Meals, but then be able to add or delete many Recipes and Restaurants and submit that form at the end to create or update the Day.

Time to learn Turbo and Hotwire.

I found two great pages that helped immensely. First, was Alexis Chávez’s guide to Turbo & Hotwire. The concepts were clear - it’s a bit like old RJS, but more formal thanks to how Controllers have evolved. But how do I structure the form?

Back to StackOverflow, and a nicely detailed question Rails 7 Dynamic Nested Forms with hotwire/turbo frames. It models cocktails with ingredients and looked a lot like what I needed.

I had some patterns! Time for some code.

DaysController gets new methods

First, a couple of new routes:

resources :days do  
  get :new_meal, on: :collection, path: "new_meal/:course_type"  
  delete :destroy_meal, on: :collection, path: "destroy_meal/(:id)"  

Then in the Day form I added two Turbo links that call the #new_meal method: one to add a Recipe and one to add a Restaurant.

  = link_to new_meal_days_path("Recipe"), data: { turbo_method: :get } do  
    = "Add Recipe"  
  = link_to new_meal_days_path("Restaurant"), data: { turbo_method: :get } do  
    = "Add Restaurant"

Then in DaysController:

def new_meal  
  helpers.fields model: do |f| 
    f.fields_for :meals,  
                   Process.clock_gettime(Process::CLOCK_REALTIME, :millisecond) do |ff|  
      render turbo_stream: 
                                   partial: "meal_fields", 
                                   locals: {f: ff, klass: params[:course_type] })  

Copying from the “Cocktails” post, this method:

  • Creates a form helper that will build the tags for a Day and then one Meal
  • Passes the fields_for form helper (ff) to the view, along with the course_type that came in from the GET

And then the meals_fields view…

- div_id = "meal_#{f.index}"  
%div{id: div_id}  
  = f.hidden_field :id  
  = f.hidden_field :course_type, value: course_type  
    = f.collection_select :course_id, 
                          prompt: "Select a #{course_type}..."  
    = link_to destroy_meal_days_path(id:, div_id: div_id), 
              data: { turbo_method: :delete } do  

This builds a div with a unique ID (from the timestamp in the controller method) that renders:

  • Hidden fields for the Meal id and course_type
  • A select with that type’s list (yay, String#constantize!)
  • A link to destroy this section of the form, if the user so chooses

And that link routes back to DaysController#destory_meal

def destroy_meal  
  Meal.find(params[:id]).destroy! if params[:id]  
  render turbo_stream: turbo_stream.remove(params[:div_id])  

Which will…

  • Find and destroy the Meal if this is a Day#edit and so an id is provided; but ignores it if not
  • Removes this Meal’s div inside the Day form

It’s Alive!

completed form

Phew! Thanks, Internet!


This article is tagged with web development, ruby, rails, and forms.