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In a multiple database application, associations can't join across databases. When set, this option tells Rails to make 2 or more queries rather than using joins for associations. Set the option on a has many through association: ```ruby class Dog has_many :treats, through: :humans, disable_joins: true has_many :humans end ``` Then instead of generating join SQL, two queries are used for `@dog.treats`: ``` SELECT "humans"."id" FROM "humans" WHERE "humans"."dog_id" = ? [["dog_id", 1]] SELECT "treats".* FROM "treats" WHERE "treats"."human_id" IN (?, ?, ?) [["human_id", 1], ["human_id", 2], ["human_id", 3]] ``` This code is extracted from a gem we use internally at GitHub which means the implementation here is used in production daily and isn't experimental. I often get the question "why can't Rails do this automatically" so I figured I'd include the answer in the commit. Rails can't do this automatically because associations are lazily loaded. `dog.treats` needs to load `Dog`, then `Human` and then `Treats`. When `dog.treats` is called Rails pre-generates the SQL that will be run and puts that information into a reflection object. Because the SQL parts are pre-generated, as soon as `dog.treats` is loaded it's too late to skip a join. The join is already available on the object and that join is what's run to load `treats` from `dog` through `humans`. I think the only way to avoid setting an option on the association is to rewrite how and when the SQL is generated for associations which is a large undertaking. Basically the way that Active Record associations are designed, it is currently impossible to have Rails figure out to not join (loading the association will cause the join to occur, and that join will raise an error if the models don't live in the same db). The original implementation was written by me and Aaron. Lee helped port over tests, and I refactored the extraction to better match Rails style. Co-authored-by: Lee Quarella <leequarella@gmail.com> Co-authored-by: Aaron Patterson <aaron@rubyonrails.org> |
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Welcome to Rails
What's Rails?
Rails is a web-application framework that includes everything needed to create database-backed web applications according to the Model-View-Controller (MVC) pattern.
Understanding the MVC pattern is key to understanding Rails. MVC divides your application into three layers: Model, View, and Controller, each with a specific responsibility.
Model layer
The Model layer represents the domain model (such as Account, Product,
Person, Post, etc.) and encapsulates the business logic specific to
your application. In Rails, database-backed model classes are derived from
ActiveRecord::Base
. Active Record allows you to present the data from
database rows as objects and embellish these data objects with business logic
methods.
Although most Rails models are backed by a database, models can also be ordinary
Ruby classes, or Ruby classes that implement a set of interfaces as provided by
the Active Model module.
Controller layer
The Controller layer is responsible for handling incoming HTTP requests and
providing a suitable response. Usually, this means returning HTML, but Rails controllers
can also generate XML, JSON, PDFs, mobile-specific views, and more. Controllers load and
manipulate models, and render view templates in order to generate the appropriate HTTP response.
In Rails, incoming requests are routed by Action Dispatch to an appropriate controller, and
controller classes are derived from ActionController::Base
. Action Dispatch and Action Controller
are bundled together in Action Pack.
View layer
The View layer is composed of "templates" that are responsible for providing appropriate representations of your application's resources. Templates can come in a variety of formats, but most view templates are HTML with embedded Ruby code (ERB files). Views are typically rendered to generate a controller response or to generate the body of an email. In Rails, View generation is handled by Action View.
Frameworks and libraries
Active Record, Active Model, Action Pack, and Action View can each be used independently outside Rails. In addition to that, Rails also comes with Action Mailer, a library to generate and send emails; Action Mailbox, a library to receive emails within a Rails application; Active Job, a framework for declaring jobs and making them run on a variety of queuing backends; Action Cable, a framework to integrate WebSockets with a Rails application; Active Storage, a library to attach cloud and local files to Rails applications; Action Text, a library to handle rich text content; and Active Support, a collection of utility classes and standard library extensions that are useful for Rails, and may also be used independently outside Rails.
Getting Started
-
Install Rails at the command prompt if you haven't yet:
$ gem install rails
-
At the command prompt, create a new Rails application:
$ rails new myapp
where "myapp" is the application name.
-
Change directory to
myapp
and start the web server:$ cd myapp $ bin/rails server
Run with
--help
or-h
for options. -
Go to
http://localhost:3000
and you'll see: "Yay! You’re on Rails!" -
Follow the guidelines to start developing your application. You may find the following resources handy:
Contributing
We encourage you to contribute to Ruby on Rails! Please check out the Contributing to Ruby on Rails guide for guidelines about how to proceed. Join us!
Trying to report a possible security vulnerability in Rails? Please check out our security policy for guidelines about how to proceed.
Everyone interacting in Rails and its sub-projects' codebases, issue trackers, chat rooms, and mailing lists is expected to follow the Rails code of conduct.
License
Ruby on Rails is released under the MIT License.