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Previously, applications could only have two types of database configuration objects, `HashConfig` and `UrlConfig`. This meant that if you wanted your config to implement custom methods you had to monkey patch `DatabaseConfigurations` to take a custom class into account. This PR allows applications to register a custom db_config handler so that custom configs can respond to needed methods. This is especially useful for tools like Vitess where we may want to indicate it's sharded, but not give Rails direct access to that knowledge. Using the following database.yml as an example: ```yaml development: primary: database: my_db animals: database: my_animals_db vitess: sharded: 1 ``` We can register a custom handler that will generate `VitessConfig` objects instead of a `HashConfig` object in an initializer: ```ruby ActiveRecord::DatabaseConfigurations.register_db_config_handler do |env_name, name, url, config| next unless config.key?(:vitess) VitessConfig.new(env_name, name, config) end ``` and create the `VitessConfig` class: ```ruby class VitessConfig < ActiveRecord::DatabaseConfigurations::UrlConfig def sharded? vitess_config.fetch("sharded", false) end private def vitess_config configuration_hash.fetch(:vitess_config) end end ``` Now when the application is booted, the config with the `vitess` key will generate a `VitessConfig` object where all others will generate a `HashConfig`. Things to keep in mind: 1) It is recommended but not required that these custom configs inherit from Rails so you don't need to reimplement all the existing methods. 2) Applications must implement the configuration in which their config should be used, otherwise first config wins (so all their configs will be the custom one.) 3) The procs must support 4 arguments to accommodate `UrlConfig`. I am thinking of deprecating this and forcing the URL parsing to happen in the `UrlConfig` directly. 4) There is one tiny behavior change where when we have a nil url key in the config hash we no longer merge it back into the configuration hash. We also end up with a `HashConfig` instead of a `UrlConfig`. I think this is fine because a `nil` URL is...useless. Co-authored-by: John Crepezzi <john.crepezzi@gmail.com> |
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= Active Record -- Object-relational mapping in Rails Active Record connects classes to relational database tables to establish an almost zero-configuration persistence layer for applications. The library provides a base class that, when subclassed, sets up a mapping between the new class and an existing table in the database. In the context of an application, these classes are commonly referred to as *models*. Models can also be connected to other models; this is done by defining *associations*. Active Record relies heavily on naming in that it uses class and association names to establish mappings between respective database tables and foreign key columns. Although these mappings can be defined explicitly, it's recommended to follow naming conventions, especially when getting started with the library. You can read more about Active Record in the {Active Record Basics}[https://guides.rubyonrails.org/active_record_basics.html] guide. A short rundown of some of the major features: * Automated mapping between classes and tables, attributes and columns. class Product < ActiveRecord::Base end The Product class is automatically mapped to the table named "products", which might look like this: CREATE TABLE products ( id bigint NOT NULL auto_increment, name varchar(255), PRIMARY KEY (id) ); This would also define the following accessors: <tt>Product#name</tt> and <tt>Product#name=(new_name)</tt>. {Learn more}[link:classes/ActiveRecord/Base.html] * Associations between objects defined by simple class methods. class Firm < ActiveRecord::Base has_many :clients has_one :account belongs_to :conglomerate end {Learn more}[link:classes/ActiveRecord/Associations/ClassMethods.html] * Aggregations of value objects. class Account < ActiveRecord::Base composed_of :balance, class_name: 'Money', mapping: %w(balance amount) composed_of :address, mapping: [%w(address_street street), %w(address_city city)] end {Learn more}[link:classes/ActiveRecord/Aggregations/ClassMethods.html] * Validation rules that can differ for new or existing objects. class Account < ActiveRecord::Base validates :subdomain, :name, :email_address, :password, presence: true validates :subdomain, uniqueness: true validates :terms_of_service, acceptance: true, on: :create validates :password, :email_address, confirmation: true, on: :create end {Learn more}[link:classes/ActiveRecord/Validations.html] * Callbacks available for the entire life cycle (instantiation, saving, destroying, validating, etc.). class Person < ActiveRecord::Base before_destroy :invalidate_payment_plan # the `invalidate_payment_plan` method gets called just before Person#destroy end {Learn more}[link:classes/ActiveRecord/Callbacks.html] * Inheritance hierarchies. class Company < ActiveRecord::Base; end class Firm < Company; end class Client < Company; end class PriorityClient < Client; end {Learn more}[link:classes/ActiveRecord/Base.html] * Transactions. # Database transaction Account.transaction do david.withdrawal(100) mary.deposit(100) end {Learn more}[link:classes/ActiveRecord/Transactions/ClassMethods.html] * Reflections on columns, associations, and aggregations. reflection = Firm.reflect_on_association(:clients) reflection.klass # => Client (class) Firm.columns # Returns an array of column descriptors for the firms table {Learn more}[link:classes/ActiveRecord/Reflection/ClassMethods.html] * Database abstraction through simple adapters. # connect to SQLite3 ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: 'dbfile.sqlite3') # connect to MySQL with authentication ActiveRecord::Base.establish_connection( adapter: 'mysql2', host: 'localhost', username: 'me', password: 'secret', database: 'activerecord' ) {Learn more}[link:classes/ActiveRecord/Base.html] and read about the built-in support for MySQL[link:classes/ActiveRecord/ConnectionAdapters/Mysql2Adapter.html], PostgreSQL[link:classes/ActiveRecord/ConnectionAdapters/PostgreSQLAdapter.html], and SQLite3[link:classes/ActiveRecord/ConnectionAdapters/SQLite3Adapter.html]. * Logging support for Log4r[https://github.com/colbygk/log4r] and Logger[https://ruby-doc.org/stdlib/libdoc/logger/rdoc/]. ActiveRecord::Base.logger = ActiveSupport::Logger.new(STDOUT) ActiveRecord::Base.logger = Log4r::Logger.new('Application Log') * Database agnostic schema management with Migrations. class AddSystemSettings < ActiveRecord::Migration[7.1] def up create_table :system_settings do |t| t.string :name t.string :label t.text :value t.string :type t.integer :position end SystemSetting.create name: 'notice', label: 'Use notice?', value: 1 end def down drop_table :system_settings end end {Learn more}[link:classes/ActiveRecord/Migration.html] == Philosophy Active Record is an implementation of the object-relational mapping (ORM) pattern[https://www.martinfowler.com/eaaCatalog/activeRecord.html] by the same name described by Martin Fowler: "An object that wraps a row in a database table or view, encapsulates the database access, and adds domain logic on that data." Active Record attempts to provide a coherent wrapper as a solution for the inconvenience that is object-relational mapping. The prime directive for this mapping has been to minimize the amount of code needed to build a real-world domain model. This is made possible by relying on a number of conventions that make it easy for Active Record to infer complex relations and structures from a minimal amount of explicit direction. Convention over Configuration: * No XML files! * Lots of reflection and run-time extension * Magic is not inherently a bad word Admit the Database: * Lets you drop down to SQL for odd cases and performance * Doesn't attempt to duplicate or replace data definitions == Download and installation The latest version of Active Record can be installed with RubyGems: $ gem install activerecord Source code can be downloaded as part of the Rails project on GitHub: * https://github.com/rails/rails/tree/main/activerecord == License Active Record is released under the MIT license: * https://opensource.org/licenses/MIT == Support API documentation is at: * https://api.rubyonrails.org Bug reports for the Ruby on Rails project can be filed here: * https://github.com/rails/rails/issues Feature requests should be discussed on the rails-core mailing list here: * https://discuss.rubyonrails.org/c/rubyonrails-core