Rails：ActiveRecord渴望负载和N + 1查询

Question

I'm using Rails 3.2.8, and have models

class Subject < ActiveRecord::Base
  has_and_belongs_to_many :users
end
class User < ActiveRecord::Base
  has_and_belongs_to_many :subjects
end

in the SubjectsController, index

@subjects = Subject.includes(:users).all

I use includes to do eager load, and In the view file, I want to display all users of a subject by this:

<%= subject.users.count if not subject.users.nil? %>

the problem is I got db hit for every subject when display user count, which I think is the N+1 Query problem

anything I missed or did wrong?

btw: I'm using MySQL

Answer 1

The count method always generates SQL COUNT query. Since you have already loaded all the entries, try to use length to avoid extra query:

<%= subject.users.length if not subject.users.nil? %>

Answer 2

<%= subject.users.count if not subject.users.nil? %>

Count always do sql query and don't do any caching

length ans size are alias of each other, the difference between count and length is that you can pass parameter to count but to length or size and count don't do any caching.

as follow

[1,2,3].count{|x| x > 2}
result => 1

so use any of the following statements

<%= subject.users.length if not subject.users.nil? %>

<%= subject.users.size if not subject.users.nil? %>

Answer 3

First, subject.users returns an Array, which will never be nil. Second, try size instead of count . So the code could be:

<%= subject.users.size if not subject.users.empty? %>