accepts_nested_attributes_for in Rails 5.2

Getting accepts_nested_attributes_for to work can be a bit tricky, and most of the walkthroughs I found are a bit dated, so here's a quick update.

Our models will be User and Address:

class User < ApplicationRecord
  has_many :addresses
  accepts_nested_attributes_for :addresses
end

class Address < ApplicationRecord
end

Now, assuming we have a UsersController, with all the standard resource actions omitted, we try to add addresses to our permitted params:

class UsersController < ApplicationController
  def user_params
    params.require(:user).permit(:name, :email, :addresses)
  end
end

Logically this seems like it would work, but if we look in the rails server logs, we will see:

Unpermitted parameter: :addresses_attributes

So we add it to our permitted params:

  def user_params
    params.require(:user).permit(:name, :email, :addresses_attributes)
  end

and refresh. Lo and behold, we get the same error!

Unpermitted parameter: :addresses_attributes

The secret is that address_attributes is a hash, and we need to communicate that to Rails:

  def user_params
    params.require(:user).permit(:name, :email, addresses_attributes: {})
  end

Et voilà!