What is the current behavior?

Currently, we can set the relationship id_method_name only as a symbol or string. This does not work well when you want to use custom id value, because you have to create method with same value on object class, aka put serialization logic in to non-serializer class. Example:

class User
  has_many :candies

  def api_candy_ids # I do not want to put this method here - it's used for serialization only!
    candies.map { |candy| "#{candy.id}-nom-nom" }
  end
end

class UserSerializer
  has_many :candies, id_method_name: :api_candy_ids
end

What is the new behavior?

This PR allows to set id_method_name as a lambda/Proc so custom id logic can be defined directly in serializer:

class User
  has_many :candies
  # no need to write #api_candy_ids method here, yay!
end

class UserSerializer
  has_many :candies, id_method_name: -> (user) { user.candies.map { |candy| "#{candy.id}-nom-nom" } }
end

With lambda value, id_method_name key is a bit misleading, but I want to make this change as small as possible. Key renaming, deprecation & co might be done with separate PR.

Checklist

Please make sure the following requirements are complete:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes /
  features)
• All automated checks pass (CI/CD)