Question

嘿我试图在rails中创建一个sql查询如何将这个sql查询转换为rails actvie记录。查询是：

   SELECT  f.id, COUNT( f.id  ) AS total               
   FROM    feeds AS f LEFT JOIN feed_likes AS l
    ON f.id = l.feed_id
    GROUP   BY f.id

有两个表Feed表，另一个是feed_likes。

Answer 1

YourClassName.select("feeds.id, feeds.content, count(feed_likes.id) as likes").joins("left join feed_likes on feeds.id = feed_likes.feed_id").group("feeds.id, feeds.content")

你在这里使用ActiveRecord并没有太多收获，你可以直接使用Arel或者只使用find_by_sql（）

Answer 2

帮助

 Feed.joins("LEFT JOIN feed_likes ON feeds.id = feed_likes.feed_id").group("feeds.id").select("feeds.id, feeds.content, COUNT(feed_likes.id) as likes")

匹配查询执行此操作

 Feed.joins("LEFT JOIN feed_likes ON feeds.id = feed_likes.feed_id").group("feeds.id").select("feeds.id, feeds.content, COUNT(feed_likes.id) as likes").to_sql

Answer 3

要在ActiveRecord中创建此查询，您可以使用每个模型的Arel表为您生成SQL。假设您有两个ActiveRecord类Feed和FeedLike：

f = Feed.arel_table
fl = FeedLike.arel_table

query = f.join(fl, Arel::Nodes::OuterJoin).on(f[:id].eq(fl[:feed_id]))
         .project(f[:id], f[:id].count.as("Total")) 
         .group(f[:id])

然后，您可以在where子句中使用查询

在查询上调用to_sql将返回以下内容

＆＃34; SELECT \＆＃34; feeds \＆＃34;。\＆＃34; id \＆＃34;，COUNT（\＆＃34; feeds \＆＃34;。\＆＃34 ; id \＆＃34;）AS Total

FROM \＆＃34; feeds \＆＃34; LEFT OUTER JOIN \＆＃34; feed_likes \＆＃34;   ON \＆＃34; feeds \＆＃34;。\＆＃34; id \＆＃34; = \＆＃34; feed_likes \＆＃34;。\＆＃34; feed_id \

GROUP BY \＆＃34; feeds \＆＃34;。\＆＃34; id \＆＃34;＆＃34;

Rails左连接查询，如何创建复杂查询

3 个答案: