Ruby on Rails update triggered ArgumentError in Mailer functions

We have recently migrated our codebase from Ruby 2.7.x and Rails 5.2.x to Ruby 3.2.2 and Rails 6.1.7.3. Previously, our mailers were defined like this:

class ApplicationMailer < ActionMailer::Base
...
end

class MyMailer < ApplicationMailer  
  def custom(user: , timestamp: Time.now)
    @user = user
    @timestamp = timestamp
  end
end

And we would deliver emails like this: MyMailer.custom(user: @myuser).deliver_now. With the update, this now returns an ArgumentError: wrong number of arguments (given 1, expected 0; required keyword: user) (ArgumentError).

We have found that we are supposed to pass the arguments in a params hash and read from there, but that seems pretty cumbersome and unelegant compared to our previous solution. So my questions are,

• Which Ruby (or Rails) code change made our previous solution fail?
• And is there a way we can still define the Mailer methods with custom keyword arguments?

This is the way that we are apparently supposed to pass parameters:

class MyMailer < ApplicationMailer  
  def custom
    @user = params[:user]
    @timestamp = params[:timestamp] || Time.now
  end
end

ActionMailer still accepts passing arguments on the mailer actions. Howevber, it appears that the keyword argument semantics in Ruby 3.2 are not fully expected by Rails 6.1 which causes the issue you are seeing.

Rails 6.1 was released before Ruby 3.2 was a thing. As recent Ruby releases have changed some semantics with keywords arguments, it's appears likely that Rails 6.1 is not (fully) compatible with Ruby 3.2.

While I have not found any authoritative documentation about this, for the latest commit on the Rails 6.1.7.3 tag, the actionmailer tests fail a test regarding keyword arguments with Ruby 3.2.

As such, you have two possible avenues to fix this issue:

• You revert to Ruby 3.1 with your Ruby 6.1 app
• Or you could further upgrade your Rails version to > 7.0.1

As a quick fix, you could probably now first downgrade your Ruby version, then start upgading your Rails application further until you can bump your Ruby version again.