rails活动记录关联sql行为

Question

I have 2 models:

# models/car.rb
class Car < ActiveRecord::Base
  belongs_to :model
end

and

# models/manufacturer.rb
class Manufacturer < ActiveRecord::Base
  has_many :cars
end

When I'm executing command in rails console Car.find(1).manufacturer it shows me that one more sql query was executed SELECT manufacturers.* FROM manufacturers WHERE manufacturers.id = 54 LIMIT 1 ,

so I am interested is it usual (for production, first of all) behavior, when a lot of sql queries being executed just to get some object property? what about performance?

UPDATE, ANSWER: I got an answer from another source: I was told it's "necessary evil" as a payment for abstraction

Answer 1

This is not a "necessary evil" and your intuition that the second query is needless is correct. What you need to do is use :include / includes to tell Rails to do a JOIN to get the associated objects in the same SELECT . So you could do this:

Car.find 1, :include => :manufacturer

# or, in Rails 3 parlance:

Car.includes(:manufacturer).find 1

Rails calls this "eager loading" and you can read more about it in the documentation (scroll down to or Ctrl+F for "Eager loading of associations").

If you always want to eager-load the associated objects you can declare default_scope in your model:

class Car
  belongs_to :manufacturer

  default_scope :include => :manufacturer

  # or Rails 3:

  default_scope includes(:manufacturer)
end

However you shouldn't do this unless you really need the associated Manufacturer every time you show a Car record.