eager loading and lazy loading in rails

Question

I am confused about eager loading and lazy loading, is there any difference in the performance of rails queries?

Is there any way to implement both ways?

Answer 1

Eager Loading

One way to improve performance is to cut down on the number of SQL queries. You can do this through eager loading.

User.find(:all, :include => :friends)

Here you are firing only two queries :

1) One for all users.

2) One for all friends of users .

Lazy Loading :

When you have an object associated with many objects like a User has many Friends and you want to display a list as in Orkut you fire as many queries as there are friends, plus one for the object itself.

users = User.find(:all)

Then query for each user friend , like :

users.each do |user|
  friend = Friend.find_by_user_id(user.id)
end

Here

1) One query for all users.

2) N query for N no. of users friends .

Take a look on : Rails 3: Lazy loading versus eager loading

Hope that will help you to understand this .