How to prevent saving invalid ids in associative table in Rails?

I have two tables in my Rails app: Category and Service. I have also created an associative table CategoriesService between them and everything works right except validation of ids on the CategoriesService table - I just noticed I am able to create an association with a non-existent record. I wonder how could I fix it properly - I suspect that Rails should let us create some database-level validation of that, which could be faster and cleaner. I defined my models that way:

class Category < ApplicationRecord
  has_and_belongs_to_many :services
end

class Service < ApplicationRecord
  has_and_belongs_to_many :categories
end

class CategoriesService < ApplicationRecord
end

I was thinking that creating has_and_belongs_to_many relationship would ensure this kind of validation itself, but I can see I was wrong. How should I solve this?

Solution

From the Rails style guide:

Prefer has_many :through to has_and_belongs_to_many. Using has_many :through allows additional attributes and validations on the join model.

In your case:

class Category < ApplicationRecord
  has_many :categories_services
  has_many :services, through: :categories_services
end

class Service < ApplicationRecord
  has_many :categories_services
  has_many :categories, through: :categories_services
end

class CategoriesService < ApplicationRecord
  belongs_to :category
  belongs_to :service

  # if not using Rails 5:
  validates :category, presence: true
  validates :service, presence: true
  # if using Rails 5, a `belongs_to` will auto-validate the presence
end

Using the join model (and not the has_and_belongs_to_many), you have a better control of the many-to-many relation:

you (can) have the created_at and updated_at fields on the join table, automatically managed by Rails as usual
you could improve your join model to have, for example, a column position and then you could offer a feature of sorting the services of a specific category, a favorite boolean column, etc.

Additionally, you can (and I recommend you to do so) enforce this validation by adding some constraints in your Database:

# PostgreSQL
CREATE TABLE categories_services (
  id SERIAL PRIMARY KEY,
  category_id integer REFERENCES categories NOT NULL,
  service_id integer REFERENCES services NOT NULL,
  created_at timestamp NOT NULL DEFAULT now(),
  updated_at timestamp NOT NULL DEFAULT now()
);