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Rails联接表呈现错误的sql

  •  2
  • Luke  · 技术社区  · 17 年前

    更新:包含模型关系

    我在波段和事件之间有一个多对多联接表(一个事件有多个波段,一个波段有多个事件…)

    我有一个列出所有事件的主页,还有一个显示过去7天内更新的事件的最近页面。

    有人能解释为什么最近的方法生成包含所有连接的完美sql吗

    @events = Event.find(:all, :include => [:venue, :bands], :conditions => {:updated_at => d1..DateTime.now}, :order => 'events.updated_at desc')
    

    @events = Event.find(:all, :include => [:venue, :bands], :order => 'events.day')
    

    它们都使用相同的视图,没有其他处理/查询。两者之间唯一的区别是:conditions参数及其排序所在的列。这快把我逼疯了!

    对不起,这是我的模型和它们的关系

    event.rb

    has_many :event_bands
    has_many :bands, :through => :event_bands
    belongs_to :venue
    

    has_many :event_bands
    has_many :events, :through => :event_bands
    

    event_.rb

    belongs_to :event
    belongs_to :band
    

    最近生成的sql:

    Processing EventlistController#recent (for 127.0.0.1 at 2009-03-27 14:58:10) [GET]
      [4;35;1mSQL (0.0ms)[0m   [0mSET SQL_AUTO_IS_NULL=0[0m
      [4;36;1mEvent Columns (16.0ms)[0m   [0;1mSHOW FIELDS FROM `events`[0m
      [4;35;1mVenue Columns (15.0ms)[0m   [0mSHOW FIELDS FROM `venues`[0m
      [4;36;1mBand Columns (16.0ms)[0m   [0;1mSHOW FIELDS FROM `bands`[0m
      [4;35;1mEvent Load Including Associations (31.0ms)[0m   [0mSELECT `events`.`id` AS t0_r0, `events`.`is_local` AS t0_r1, `events`.`link` AS t0_r2, `events`.`notes` AS t0_r3, `events`.`band_id` AS t0_r4, `events`.`venue_id` AS t0_r5, `events`.`created_at` AS t0_r6, `events`.`updated_at` AS t0_r7, `events`.`day` AS t0_r8, `events`.`band_list` AS t0_r9, `venues`.`id` AS t1_r0, `venues`.`name` AS t1_r1, `venues`.`link` AS t1_r2, `venues`.`map` AS t1_r3, `venues`.`notes` AS t1_r4, `venues`.`created_at` AS t1_r5, `venues`.`updated_at` AS t1_r6, `bands`.`id` AS t2_r0, `bands`.`name` AS t2_r1, `bands`.`link` AS t2_r2, `bands`.`notes` AS t2_r3, `bands`.`is_local` AS t2_r4, `bands`.`created_at` AS t2_r5, `bands`.`updated_at` AS t2_r6 FROM `events` LEFT OUTER JOIN `venues` ON `venues`.id = `events`.venue_id LEFT OUTER JOIN `event_bands` ON (`events`.`id` = `event_bands`.`event_id`) LEFT OUTER JOIN `bands` ON (`bands`.`id` = `event_bands`.`band_id`) WHERE (`events`.`updated_at` BETWEEN '2009-03-20 14:58:10' AND '2009-03-27 14:58:10')
    

    由main生成的sql

    Processing EventlistController#index (for 127.0.0.1 at 2009-03-27 14:15:47) [GET]
      [4;35;1mSQL (0.0ms)[0m   [0mSET SQL_AUTO_IS_NULL=0[0m
      [4;36;1mEvent Load (0.0ms)[0m   [0;1mSELECT * FROM `events` ORDER BY events.day[0m
      [4;35;1mEvent Columns (15.0ms)[0m   [0mSHOW FIELDS FROM `events`[0m
      [4;36;1mVenue Columns (16.0ms)[0m   [0;1mSHOW FIELDS FROM `venues`[0m
      [4;35;1mVenue Load (0.0ms)[0m   [0mSELECT * FROM `venues` WHERE (`venues`.`id` IN (6,7,8,9,1,2,3,4,5)) [0m
      [4;36;1mEventBand Load (0.0ms)[0m   [0;1mSELECT `event_bands`.* FROM `event_bands` WHERE (`event_bands`.event_id IN (1,24,74,2,75,11,12,3,13,129,76,14,77,78,145,4,146,120,15,130,79,147,5,80,148,144,131,81,132,16,28,82,121,133,83,17,134,84,149,6,18,135,85,122,19,7,136,86,20,9,87,25,137,150,22,138,88,151,124,123,139,89,152,140,90,141,91,8,153,21,142,154,125,143,92,126,10,93,94,155,127,95,128,23,96,97,26,98,99,27,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119)) [0m
      [4;35;1mEventBand Columns (0.0ms)[0m   [0mSHOW FIELDS FROM `event_bands`[0m
      [4;36;1mBand Columns (16.0ms)[0m   [0;1mSHOW FIELDS FROM `bands`[0m
      [4;35;1mBand Load (0.0ms)[0m   [0mSELECT * FROM `bands` WHERE (`bands`.`
    
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  •  3
  •   Brian Guthrie    17 年前

    它们生成两个不同SQL查询的原因是Rails在默认情况下会尝试优化SQL查询以减少连接。当您在查找中包含需要使用其他表的条件列表时,结果SQL当然必须包含联接。但是,如果您所要做的只是预先填充对象与您正在检索的对象的关系,那么Rails认为执行几个快速查询比执行一个大的慢速查询更有效。