def sql
  billing_requests
  .project(billing_requests[Arel.star])
  .where(
    filter_by_day
      .and(filter_by_merchant)
      .and(filter_by_operator_name)
  )
  .to_sql
end

def filter_by_day
  billing_requests[:created_at].gteq(@start_date).and(
    billing_requests[:created_at].lteq(@end_date)
  )
end

def filter_by_operator_name
  unless @operator_name.blank?
    return billing_requests[:operator_name].eq(@operator_name)
  end
end

def filter_by_merchant
  unless @merchant_id.blank?
    return billing_requests[:merchant_id].eq(@merchant_id)
  end
end

private

def billing_requests
  @table ||= Arel::Table.new(:billing_requests)
end

在方法filter_by_merchant中,当商家ID变为空时,Arel必须返回的值应该忽略AND子句效果？
还有,有更好的方法来处理这种情况吗？