Rails執行太多的SQL

Question

我已經開始注意到，我的Rails應用程序生成太多的SQL語句，其中很多都不是必需的。我記得在某個地方讀這可能是一個問題，而且現在隨着我的數據增長，它明顯地減緩了應用程序的速度。

例如，我有發佈有軌道。藝術家可以分配到曲目和發行。當我加載一個只有8首曲目的發行版時，它似乎正在貫穿數據庫中的每一首曲目，以找到這些關係！？！

例如，請參見下文，這是一個非常小的示例，但這些軌道都沒有與發佈實際關聯。它正在通過DB中的每個軌道！

任何常規指針？

Artist Load (0.6ms) SELECT `artists`.* FROM `artists` INNER JOIN `artists_tracks` ON `artists`.`id` = `artists_tracks`.`artist_id` WHERE `artists_tracks`.`track_id` = 12 
    Artist Load (0.6ms) SELECT `artists`.* FROM `artists` INNER JOIN `artists_tracks` ON `artists`.`id` = `artists_tracks`.`artist_id` WHERE `artists_tracks`.`track_id` = 19 
    Artist Load (0.6ms) SELECT `artists`.* FROM `artists` INNER JOIN `artists_tracks` ON `artists`.`id` = `artists_tracks`.`artist_id` WHERE `artists_tracks`.`track_id` = 21 
    Artist Load (0.9ms) SELECT `artists`.* FROM `artists` INNER JOIN `artists_tracks` ON `artists`.`id` = `artists_tracks`.`artist_id` WHERE `artists_tracks`.`track_id` = 22 
    Artist Load (0.7ms) SELECT `artists`.* FROM `artists` INNER JOIN `artists_tracks` ON `artists`.`id` = `artists_tracks`.`artist_id` WHERE `artists_tracks`.`track_id` = 23 
    Artist Load (0.6ms) SELECT `artists`.* FROM `artists` INNER JOIN `artists_tracks` ON `artists`.`id` = `artists_tracks`.`artist_id` WHERE `artists_tracks`.`track_id` = 24 
    Artist Load (0.9ms) SELECT `artists`.* FROM `artists` INNER JOIN `artists_tracks` ON `artists`.`id` = `artists_tracks`.`artist_id` WHERE `artists_tracks`.`track_id` = 25 
    Artist Load (1.0ms) SELECT `artists`.* FROM `artists` INNER JOIN `artists_tracks` ON `artists`.`id` = `artists_tracks`.`artist_id` WHERE `artists_tracks`.`track_id` = 26 
    Artist Load (0.6ms) SELECT `artists`.* FROM `artists` INNER JOIN `artists_tracks` ON `artists`.`id` = `artists_tracks`.`artist_id` WHERE `artists_tracks`.`track_id` = 27 
    Artist Load (0.9ms) SELECT `artists`.* FROM `artists` INNER JOIN `artists_tracks` ON `artists`.`id` = `artists_tracks`.`artist_id` WHERE `artists_tracks`.`track_id` = 28 
    Artist Load (0.6ms) SELECT `artists`.* FROM `artists` INNER JOIN `artists_tracks` ON `artists`.`id` = `artists_tracks`.`artist_id` WHERE `artists_tracks`.`track_id` = 29 
    Artist Load (0.6ms) SELECT `artists`.* FROM `artists` INNER JOIN `artists_tracks` ON `artists`.`id` = `artists_tracks`.`artist_id` WHERE `artists_tracks`.`track_id` = 30 
    Artist Load (0.6ms) SELECT `artists`.* FROM `artists` INNER JOIN `artists_tracks` ON `artists`.`id` = `artists_tracks`.`artist_id` WHERE `artists_tracks`.`track_id` = 31 

這裏涉及到的車型：

class Artist < ActiveRecord::Base 
    has_many :artist_releases 
    has_many :releases, :through => :artist_releases 
    has_many :artists_tracks 
    has_many :tracks, :through => :artists_tracks 
end 

class ArtistRelease < ActiveRecord::Base 
    belongs_to :artist 
    belongs_to :release 
end 

class ArtistsTrack < ActiveRecord::Base 
    belongs_to :artist 
    belongs_to :release 
    belongs_to :track 
end 

class Release < ActiveRecord::Base 
    has_many :artist_releases 
    has_many :artists, :through => :artist_releases 
    accepts_nested_attributes_for :artists, :reject_if => lambda { |a| a[:name].blank? } 
    accepts_nested_attributes_for :artist_releases  
    has_many :releases_tracks, :dependent => :destroy 
    has_many :tracks, :through => :releases_tracks, :order => "releases_tracks.position" 
    accepts_nested_attributes_for :tracks, :reject_if => lambda { |a| a[:name].blank? }, :allow_destroy => :true 
    accepts_nested_attributes_for :releases_tracks  
end 

class ReleasesTrack < ActiveRecord::Base 
    default_scope :order => 'releases_tracks.position ASC' 
    acts_as_list :scope => :release_id 
    belongs_to :release 
    belongs_to :track 
end 

class Track < ActiveRecord::Base 
    has_many :releases_tracks, :dependent => :destroy 
    has_many :releases, :through => :releases_tracks 
    has_many :artists_tracks 
    has_many :artists, :through => :artists_tracks 
    accepts_nested_attributes_for :artists, :reject_if => lambda { |a| a[:name].blank? } 
    accepts_nested_attributes_for :artists_tracks 
end 

Answer 1

從Rails正在生成的查詢，它看起來像你正在做這樣的事情，它單獨加載每個藝術家，當你提到它：

release.tracks.each{ |t| t.artist } 

你應該急切地加載的藝術家，這樣在單個查詢它們預裝了：

release.tracks.includes(:artist).each{ |t| t.artist } 

的includes方法是非常靈活的，並允許您急切地加載多個協會，甚至嵌套協會：

Release.first.includes(:releases_tracks => {:track => :artist}) 

這將加載第一個版本，那麼其所有ReleaseTracks，然後他們所有的曲目，以及所有曲目的藝術家只需4次查詢（每桌1張）。這比單獨加載每條記錄效率更高。

有關Eager Loading Associations in the Active Record Query interface guide部分的詳細信息。