Rails validate uniqueness of attribute according to another table

I have a model User, which has_many Profile. I also have Report model, which belongs_to Profile.

How can I make sure that one user has only one report? Something like

class Report
  validate_uniqueness_of profile_id, scope: :user 
end

would be great, but of course it doesn't work. (I don't want to attach user field to Report, because it mixes up ownership chain).

Just to give you an idea on how to implement custom validations. Check this

class Report
    validate :unique_user

    def unique_user
        if self.exists?("profile_id = #{self.profile_id}")
          errors.add(:profile_id, "Duplicate user report")
        end
    end
end