在 Rails 中进行计数的正确方法是什么？

Question

I have a rails app with many following pieces of code:

Our active community of <%= Account.find_all_by_admin(false).count %>

My question is is this the right way to do counts on views? It seems so "dirty" is there a more railish, way to do counts? I'm thinking named scopes perhaps, but I just want to be sure that these type of things won't have a greater impact in performance.

Thank You,

Answer 1

You don't need a name scope to perform a count.

Account.where(:admin => false).count

But named scopes are an excellent way to make your code more reusable.

Named scopes don't have any noticeable performance impact on your application.

Answer 2

I would recommend you to avoid direct access to database in my templates because then you're losing a bit of flexibility when it comes to caching.

Try to prepare all the data you need to render in your action instead and then use meaningful instance variables like @number_of_accounts or @accounts.count .

This will make your views cleaner and easier to debug and also a bit more DRY if you render action in different formats (html, json, etc)

As to how do you get your numbers - it doesn't really matter that much, just move away from find_* methods towards scoping and write readable code

Answer 3

In rails 3 a simple call to count issues a simple count request:

Contact.count

is resolved as:

SELECT COUNT(*) AS count_id FROM "contacts"

a find all by field name will resolve as:

Contact.find_all_by_country("Canada")

SELECT "contacts".* FROM "contacts" WHERE ("contacts"."country" = 'Canada')

I would recommend indexing your admin column for faster lookups and this can be translated into a named scope, but that by itself will only predefine the query, not optimize it.

It is important to note that if you issue

Contact.find_all_by_country("Canada").count

count is a method on the array class and doesn't actually issue a count on the database:

Contact.find_all_by_country("Canada").count

SELECT "contacts".* FROM "contacts" WHERE ("contacts"."country" = 'Canada')

Answer 4

A named scope shouldn't have an impact on performance

scope :not_admin, where(:admin => false)

Then you can have Account.not_admin.count

Edited per DGM's comment: To check the generated SQL in a console, compare Account.not_admin.to_sql with Account.find_all_by_admin(false).to_sql

Answer 5

You can use following query instead of Account.where(:admin => false).count

Account.select(:id).where(:admin => false).count

Just select one column, instead of selecting all. It generates the following query and it is faster than the previous one:

SELECT COUNT("accounts"."id") FROM "accounts" where admin = false

Answer 6

You should create an instance variable in the controller and use it in the view. This will make you view clear.

@accounts_count = Account.where(admin: false).count

And use it in the view like `<%= @accounts_count %>

If you want to loop though the results along with the count, you can use the following way:

@accounts = Account.where(admin: false)

in view, write like below:

<%= @accounts.count %>
// For looping account
<% @accounts.each do |account| %>
  # do some stuff
<% end %>

But the above way will fire 2 queries, 1 for count and other for the loop.

Correct Approach

Use size instead of count . It will fire 1 query but you need to change the order of count in your view like:

// For looping account
<% @accounts.each do |account| %>
  # do some stuff
<% end %>
<%= @accounts.size %>

If you want the same order, then load the user object first like below

<%= @accounts.load.size %>
// For looping account
<% @accounts.each do |account| %>
  # do some stuff
<% end %>