arel "most frequent"

frequency_extension.rb

module FrequencyExtension
  def most_frequent
    frequency_order.take
  end

  def frequency_order
    group(arel_attribute(primary_key).order(arel_attribute(primary_key).count.desc)
  end
end

user.rb

class User
  has_many :clubs
  has_many :sports, through: :clubs, extend: FrequencyExtension
end

@user.sports.most_frequent