rails_admin kills Rails5's belongs_to default presence validation

[ordinaryzelig]

By default, Rails5's belongs_to now requires the association and creates a validation for its presence automatically. For some reason rails_admin kills this default.

To demonstrate: with a brand new rails 5.0.0.1 app, create a basic belongs_to relation:

class Tree < ApplicationRecord
end

class Branch < ApplicationRecord
  belongs_to :tree
end

In the console:

Branch.belongs_to_required_by_default
#=> true
Branch.validators
#=> [#<ActiveRecord::Validations::PresenceValidator:0x007faf72209a58 @attributes=[:tree], @options={:message=>:required}>]
Branch.new.tap(&:validate).errors.full_messages
#=> ["Tree must exist"]

After installing rails_admin gem and running installation generator, the same console commands:

Branch.belongs_to_required_by_default
#=> nil
Branch.validators
#=> []
Branch.new.tap(&:validate).errors.full_messages
#=> []