Rails执行太多的SQL

Question

我已经开始注意到，我的Rails应用程序生成太多的SQL语句，其中很多都不是必需的。我记得在某个地方读这可能是一个问题，而且现在随着我的数据增长，它明显地减缓了应用程序的速度。

例如，我有发布有轨道。艺术家可以分配到曲目和发行。当我加载一个只有8首曲目的发行版时，它似乎正在贯穿数据库中的每一首曲目，以找到这些关系！？！

例如，请参见下文，这是一个非常小的示例，但这些轨道都没有与发布实际关联。它正在通过DB中的每个轨道！

任何常规指针？

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 

这里涉及到的车型：

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

从Rails正在生成的查询，它看起来像你正在做这样的事情，它单独加载每个艺术家，当你提到它：

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

你应该急切地加载的艺术家，这样在单个查询它们预装了：

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

的includes方法是非常灵活的，并允许您急切地加载多个协会，甚至嵌套协会：

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

这将加载第一个版本，那么其所有ReleaseTracks，然后他们所有的曲目，以及所有曲目的艺术家只需4次查询（每桌1张）。这比单独加载每条记录效率更高。

有关Eager Loading Associations in the Active Record Query interface guide部分的详细信息。