ruby-on-rails - sql distinct语句和在rails上使用多个表

Question

您将如何以“rails 方式”编写此行？

unique_attendees = CourseSessionsUser.count_by_sql(["SELECT count(DISTINCT csu.user_id) AS count_user_id FROM course_sessions_users csu, course_sessions cs WHERE cs.course_id = ? AND csu.course_session_id = cs.id"], @course.id)

查询本身是：

SELECT count(DISTINCT csu.user_id) AS count_user_id 
FROM  course_sessions_users csu, 
      course_sessions cs 
WHERE cs.course_id = ? 
AND csu.course_session_id = cs.id

Answer 1

count导轨的使用方法

 count = CourseSessionsUser.count('csu.user_id',
  :distinct => true, :conditions => ["cs.course_id = ?", @course.id]
  :joins => "cs LEFT JOIN course_sessions_users csu ON cs.id = csu.course_session_id")

如果条件匹配，这将直接返回非零整数，否则返回零