Rails 3:延迟加载与预先加载 [英] Rails 3: Lazy loading versus eager loading

问题描述

在 Rails 3 中，这些是相同的还是不同的?它们有何不同?

In Rails 3, are these the same, or different? How do they differ?

o = Appointment.find(297)
o.service


o = Appointment.includes(:service).find(297)
o.service

推荐答案

我不确定，但看起来您在 Appointment 类中有 belongs_to :serivce和 has_many :appointments Service 类.正确吗?

I'm not sure, but it looks like you have belongs_to :serivce in the Appointment class and has_many :appointments the Service class. Correct?

在那种情况下，您的 2 个示例之间不会有任何区别.在这两种情况下，Rails 都会执行 2 个查询:

In that case there won't be any difference between your 2 examples. Rails will execute 2 queries in both cases:

Appointment Load (0.0ms)  SELECT "appointments".* FROM "appointments" WHERE ("appointments"."id" = 1) LIMIT 1
Service Load (0.0ms)  SELECT "services".* FROM "services" WHERE ("services"."id" = 1) LIMIT 1

另一方面，如果你打电话给:

If, on the other hand, you were calling:

s = Service.find(123)

然后执行以下操作:

s.appointments.find(1)
s.appointments.find(2)

等等.在代码的许多地方，对数据库的查询将与这些调用的数量一样多(Rails 3 在这里非常聪明，所以如果你执行 s.appointments.each 它实际上会在 1 个查询中获取所有约会).

etc. in many places in the code, then there would be as many queries to the database as the number of these calls (Rails 3 is pretty smart here, so if you executed s.appointments.each it would actually fetch all the appointments in 1 query).

在这种情况下，最好调用:

In that case it'd be better to call:

s = Service.include(:appointments).find(123)

因为 Rails 将只执行 2 个查询:一个获取 Service，另一个获取所有约会:

because then Rails will execute only 2 queries: one to fetch the Service and one to fetch all the appointments:

Service Load ( 0.0ms )  SELECT "services".* FROM "services" WHERE ("services"."i
d" = 123) LIMIT 1
Appointment Load ( 0.0ms )  SELECT "appointments".* FROM "appointments" WHERE ("
appointments".service_id = 123) 

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