Efficient ActiveRecord has_and_belongs_to_many query

问题

I have Page and Paragraph models with a has_and_belongs_to_many relation. Given a paragraph_id, I'd like to get all matching pages. e.g.:

pages = Paragraph.find(paragraph_id).pages.all

However, this takes two queries. It can be done in one query:

SELECT "pages".* FROM "pages" 
INNER JOIN "pages_paragraphs" ON "pages_paragraphs"."page_id" = "pages"."id" 
WHERE "pages_paragraphs"."paragraph_id" = 123

But can this be done without

using find_by_sql
without modifications to the page_paragraphs table (e.g. adding an id).

Update:

My page model looks like this:

class Page < ActiveRecord::Base
  has_and_belongs_to_many :paragraphs, uniq: true
end

回答1:

With a has_many :through relationship, you could use this:

pages = Page.joins(:pages_paragraphs).where(:pages_paragraphs => {:paragraph_id => 1})

Take a look at Specifying Conditions on the Joined Tables here: http://guides.rubyonrails.org/active_record_querying.html

If you want the pages and paragraphs together:

pages = Page.joins(:pages_paragraphs => :paragraph).includes(:pages_paragraphs => :paragraph).where(:pages_paragraphs => {:paragraph_id => 1})

With a has_and_belongs_to_many:

pages = Page.joins("join pages_paragraphs on pages.id = pages_paragraphs.page_id").where(["pages_paragraphs.paragraph_id = ?", paragraph_id])

回答2:

You can use includes for this :

pages = Paragraph.includes(:pages).find(paragraph_id).pages

回答3:

You can use eager_load for this

pages = Paragraph.eager_load(:pages).find(paragraph_id).pages

I found an article all about this sort of thing: 3 ways to do eager loading (preloading) in Rails 3 & 4 by Robert Pankowecki

来源：https://stackoverflow.com/questions/13625171/efficient-activerecord-has-and-belongs-to-many-query

标签

ruby-on-rails

activerecord

has-and-belongs-to-many