How to make nested form for model with has_many :through association in Rails 5

Question

So I have two models. One is Meal and the second is called Food. A meal can have multiple food items and a food item can be in multiple meals. Basically a many-to-many association. I did that with a has_many through association. The association model is called MealFood. I want to make a form so that when I'm creating a meal I have the option to select as many food items as I want either through a checkbox or a select option field. How can I achieve that? I'm very new to Rails so I don't even know where to begin. Any help would be amazing! Thanks in advance!

Here's my code so far:

class Meal < ApplicationRecord
    belongs_to :user
    has_many :meal_foods
    has_many :foods, through: :meal_foods
end

class Food < ApplicationRecord
    has_many :meal_foods
    has_many :meals, through: :meal_foods
end

class MealFood < ApplicationRecord
    belongs_to :meal
    belongs_to :food
end

Meals Controller

def index
end

def new
    @meal = Meal.new
    @meal.foods.new
end

def create
    @meal = Meal.new(meal_params)

    if @meal.save
        redirect_to @meal
    else
        render 'new'
    end
end

private

def meal_params
    params.require(:meal).permit(:meal_type, :date, :total_calories, :total_carbohydrates, :total_fat, foods_attributes: [:name, :calories, :proteins, :carbohydrates, :fat])
end

Answer 1

You don't need nested attributes to assign associations. All you really need is checkboxes or a select:

<%= form_with model: @meal do |f| %>
  # ...
  <%= f.collection_select(:foods_ids, Food.all, :id, :name) %>
  <%= f.submit %>
<% end %>

You then need to whitelist the attribute foods_ids attribute:

def food_params
  params.require(:food).permit(:foo, :bar, foods_ids: [])
end

This works as every many to many association has a _ids setter that takes an array of ids and replaces (or creates) the associations as needed.

Nested attributes is only needed if you need pass attributes for the associated records.