当使用has_many：通过rails时，查询可以从所有三个表中产生字段吗？

Question

In my app, the main objects are Accounts and Phones, with a typical has_many :through Contacts, eg:

Account:
has_many :contacts
has_many :phones, :though => contacts

Phone:
has_many :contacts
has_many :accounts, :though => :contacts

Contact:
belongs_to :account
belongs_to :phone

Contacts has fields signup_status, name There is one Contact per unique Account/Phone pair

For an account with id = 123, which has 5 contacts, each contact having one phone, is there a query that would yield all 5 rows and include all the account fields AND contact fields AND phone fields?

Answer 1

You can use eager loading of associations to get all the data you need in one active record query

@account = Account.includes(:contacts, :phones).find(123)

, which will actually translate into three SQL queries:

SELECT "accounts".* FROM "accounts" WHERE "accounts"."id" = $1 LIMIT 1  [["id", 123]]
SELECT "contacts".* FROM "contacts" WHERE "contacts"."account_id" IN (123)
SELECT "phones".* FROM "phones" WHERE "phones"."id" IN (<phone ids found in prev query>)

All of the records will be loaded into memory and become available through @account. To get the array of contacts and phones, just call @account.contacts and @account.phones , respectively. Note that these calls will not result in re-issued SQL queries, which is the beauty of eager loading.

Answer 2

ActiveRecord isn't quite smart enough to do all that with one SQL query. You can get pretty close, however, by using includes, which will avoid n+1 queries.

Account.includes(:contacts => :phones).where(:id => 123)

ActiveRecord will execute one query to load all Account records, one query to load all Contacts, and one query to load all Phones. See the link below to the documentation for the reason behind this.

if you really wanted to get everything in one SQL query (which can have drawbacks) you should look at ActiveRecord::Associations::Preloader (documentation)