Pourquoi ActiveRecord émet-il une requête distincte par jointure?

Question

Je fais une opération de recherche ActiveRecord (via la bibliothèque "will_paginate") comme ceci:

  contacts = Contact.paginate(:all,
    :conditions => conditions_specified,
    :select => "DISTINCT contacts.*",
    :joins => joins,
    :include => [:addresses, :emails, :phone_numbers, {:addresses => :organization}],
    :page => page,
    :per_page => @@PAGE_SIZE,
    :order => 'last_name, first_name, middle_name')

Dans la console, je reçois le résultat indiqué ci-dessous. En définissant le paramètre include , j’espère que cela aboutira à une seule requête. Mais dans la sortie, je vois beaucoup de requêtes. Par exemple, je suis surpris de voir la ligne ...

  Address Load (2.5ms)   SELECT `addresses`.* FROM `addresses` WHERE (`addresses`.contact_id IN (94,106,104,108,121,55,40,75,62,4,67,36,77,64,7,35,24,68,57,117,8,30,85))

dans la sortie ci-dessous. (Je comprends que les requêtes SHOW FIELDS FROM sont mises en cache en production, donc ce n'est pas un problème. Ce sont tous les SELECTS supplémentaires que j'essaie de comprendre.)

Processing ContactController#index (for 127.0.0.1 at 2009-01-03 21:21:18) [GET]
  Session ID: d453897c2f67c29c9a68d3fbc7b94f7b
  ContactSearch Columns (4.4ms)   SHOW FIELDS FROM `contact_searches`
  Contact Load (1.1ms)   SELECT DISTINCT contacts.* FROM `contacts` WHERE (contacts.last_name LIKE '%n%') ORDER BY last_name, first_name, middle_name LIMIT 0, 35
  Contact Columns (2.3ms)   SHOW FIELDS FROM `contacts`
  Address Load (2.5ms)   SELECT `addresses`.* FROM `addresses` WHERE (`addresses`.contact_id IN (94,106,104,108,121,55,40,75,62,4,67,36,77,64,7,35,24,68,57,117,8,30,85))
  Address Columns (2.7ms)   SHOW FIELDS FROM `addresses`
  Email Load (1.7ms)   SELECT `emails`.* FROM `emails` WHERE (`emails`.contact_id IN (94,106,104,108,121,55,40,75,62,4,67,36,77,64,7,35,24,68,57,117,8,30,85))
  Email Columns (1.8ms)   SHOW FIELDS FROM `emails`
  PhoneNumber Load (3.3ms)   SELECT `phone_numbers`.* FROM `phone_numbers` WHERE (`phone_numbers`.contact_id IN (94,106,104,108,121,55,40,75,62,4,67,36,77,64,7,35,24,68,57,117,8,30,85))
  PhoneNumber Columns (3.0ms)   SHOW FIELDS FROM `phone_numbers`
  Organization Columns (2.9ms)   SHOW FIELDS FROM `organizations`
  Organization Load (0.9ms)   SELECT * FROM `organizations` WHERE (`organizations`.`id` IN (99,116,44,6,23,78,107,91,79,119,14,120,71,26,59,94,27,100,62,1,51,29,30,2,65,76,5))
Rendering template within layouts/main
Rendering contact/index
Rendered contact/_field_function_specifier (0.9ms)
Rendered contact/_field_function_specifier (0.6ms)
Rendered contact/_field_function_specifier (0.6ms)
Rendered contact/_field_function_specifier (0.6ms)
Rendered contact/_field_function_specifier (0.6ms)
Rendered contact/_field_function_specifier (0.6ms)
Rendered contact/_field_function_specifier (0.6ms)
Rendered contact/_field_function_specifier (0.6ms)
Rendered contact/_field_function_specifier (0.6ms)
Rendered contact/_field_function_specifier (0.6ms)
Rendered contact/_search_form (14.1ms)
Rendered contact/_quick_view_contact_info (3.2ms)
Rendered contact/_quick_view_contact_info (2.1ms)
Rendered contact/_quick_view_contact_info (1.9ms)
Rendered contact/_quick_view_contact_info (1.9ms)
Rendered contact/_quick_view_contact_info (0.8ms)
Rendered contact/_quick_view_contact_info (2.0ms)
Rendered contact/_quick_view_contact_info (2.4ms)
Rendered contact/_quick_view_contact_info (1.7ms)
Rendered contact/_quick_view_contact_info (1.4ms)
Rendered contact/_quick_view_contact_info (1.7ms)
Rendered contact/_quick_view_contact_info (2.2ms)
Rendered contact/_quick_view_contact_info (1.5ms)
Rendered contact/_quick_view_contact_info (2.0ms)
Rendered contact/_quick_view_contact_info (1.7ms)
Rendered contact/_quick_view_contact_info (1.8ms)
Rendered contact/_quick_view_contact_info (3.6ms)
Rendered contact/_quick_view_contact_info (2.2ms)
Rendered contact/_quick_view_contact_info (2.2ms)
Rendered contact/_quick_view_contact_info (1.9ms)
Rendered contact/_quick_view_contact_info (1.7ms)
Rendered contact/_quick_view_contact_info (2.0ms)
Rendered contact/_quick_view_contact_info (2.1ms)
Rendered contact/_quick_view_contact_info (1.9ms)
Completed in 308ms (View: 104, DB: 27) | 200 OK [http://localhost/contact]

Answer 1

J'ai fait un peu plus de recherches et j'ai trouvé la réponse. J'actualise ceci pour ceux qui recherchent plus tard. La réponse longue est dans cet article de blog .

Il semble que lorsque vous effectuez une recherche comportant à la fois des jointures et une limite, ActiveRecord doit d’abord interroger la table principale pour obtenir les ID des lignes à renvoyer, puis refaire la requête en utilisant uniquement les ID afin de les appliquer. limite.

Puisque j'utilisais will_paginate , c'est ce qui s'est passé lorsque will_paginate a utilisé limite pour exécuter la pagination.