Ruby on Rails转换为SQL查询

Question

I have did the first one, and want to check if I got it right or not? also I have no idea how to do number 2 Ruby ORM Consider the following two active record definitions over the tables “customers” and “orders”. The orders table has a foreign key “cust_key” that references the primary key of “customers”, which is also named “cust_key”.

Table:
customers-
          cust_key
          address

orders-
          order key
          cust_key
          order_priority
          total_price

1 class Customer < ActiveRecord::Base
2 set_table_name "customers"
3 set_primary_key "cust_key"
4 has_many :orders, :foreign_key => "cust_key”
5 End

1 class Order < ActiveRecord::Base
2 set_table_name "orders"
3 belongs_to :customer, :foreign_key => "cust_key"
4 set_primary_key "order_key”
5 end

Consider the following piece of Ruby-on-Rails code.
1 <table>
2 <% Customers.all.each.do |c| %>
3 <tr>
4 <td><%= c.address %></td>
5 <td><%= c.orders.count() %></td>
6 </tr>
7 <% end %>
8 </table>

Questions:

1. Provide the SQL queries that will result from executing this piece of Ruby-on-Rails. How many SQL queries in total will be executed?

2 queries
SELECT address FROM customers

     SELECT COUNT(*) FROM orders where orders.cust_key= customers.cust_key;

1. Write a jsp fragment that issues only one SQL query and creates identical html with the Ruby-on-Rails fragment above.

Answer 1

I know that this is an old question, and you've probably forgotten it but ... If you change:

<% Customers.all.each.do |c| %>
  <tr>
    <td><%= c.address %></td>
    <td><%= c.orders.count() %></td>
  </tr>
<% end %>

to

<% Customers.includes(:orders).each.do |c| %>
  <tr>
    <td><%= c.address %></td>
    <td><%= c.orders.length %></td>
  </tr>
<% end %>

You'll display the customer information in one SQL call.

The includes method, changes the request so that it uses a single query with a join, to gather all customers with their corresponding order records

Note that I've used length rather than count, as count will call a SQL count for each customer (c) object, whereas length will look at the length of orders data already gathered in the first SQL call.