Rails 中急切加载的问题
我在急于加载相关记录时遇到一些奇怪的行为。我已经在“包含”中指定了所需的关系，并且所有内容都通过四个查询加载。但是，当我迭代选定的记录时，Rails 开始对每个相关实体发出查询，大部分时间都会访问缓存。我无法在具有类似设置的测试应用程序中重现此问题。以下是代码的相关部分：
class Side < ActiveRecord::Base
belongs_to :photo
belongs_to :board
end
class Board < ActiveRecord::Base
belongs_to :address
has_many :sides
end
class Photo < ActiveRecord::Base
has_many :sides
end
class Address < ActiveRecord::Base
belongs_to :city
has_many :boards
end
class City < ActiveRecord::Base
has_many :addresses
end
q = Side.includes({:board => :address}, :photo).joins({:board => :address}, :projects).limit(50)
q.each {|s| s.photo}
连接用于稍后添加谓词。
I'm experiencing some strange behavior while eager loading related records. I've specified needed relations in "includes" and all the stuff is loaded with four queries. But when I iterate over selected records, Rails starts issuing a query for each related entity, hitting the cache most of the time. I wasn't able to reproduce this problem in a test app with similar setup. Here are the relevant parts of the code:
class Side < ActiveRecord::Base
belongs_to :photo
belongs_to :board
end
class Board < ActiveRecord::Base
belongs_to :address
has_many :sides
end
class Photo < ActiveRecord::Base
has_many :sides
end
class Address < ActiveRecord::Base
belongs_to :city
has_many :boards
end
class City < ActiveRecord::Base
has_many :addresses
end
q = Side.includes({:board => :address}, :photo).joins({:board => :address}, :projects).limit(50)
q.each {|s| s.photo}
Joins are for predicates to be added later.
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我相信这是因为 Rails 的“延迟加载”方式 - 因此，在调用
Side.includes(...).limit(50)时，您实际上并没有请求任何记录。可以将其视为在模型上设置范围。我怀疑如果您将
.all添加到末尾，它将在一个查询中加载记录。I believe this is because of the way rails is 'lazy loading' – So where you call
Side.includes(...).limit(50)you're not actually requesting any records. Think of it like setting up ascopeon a model.I suspect if you add
.allto the end it will load the records in one query.