Por que ActiveRecord emitir uma consulta separada por participar?

Question

Estou fazendo uma operação de busca ActiveRecord (via a biblioteca "will_paginate") como este:

  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')

No console, eu recebo a saída mostrada abaixo. Ao definir o parâmetro include, eu espero que isso resultaria em apenas uma consulta. Mas na saída eu estou vendo muitas consultas. Por exemplo, eu estou surpreso de ver a linha ...

  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))

na saída abaixo. (Eu entendo que as consultas SHOW FIELDS FROM são armazenados em cache na produção de modo que não é um problema. É tudo o SELECIONA extras estou tentando descobrir.)

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

Eu fiz um pouco mais de pesquisa e encontrei a resposta. Estou atualizando este para quem busca mais tarde. A resposta longa é na este post .

Parece que quando você faz uma descoberta que tem tanto junta e um limite, ActiveRecord tem que primeiro consultar a tabela principal para obter os IDs das linhas para retornar, em seguida, refazer a consulta utilizando apenas os IDs, a fim de aplicar o limite.

Uma vez que eu estava usando will_paginate, isso é o que aconteceu como will_paginate usado consultas limited para fazer a paginação.