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 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: Product#name and Product#name=(new_name).

Learn more

Associations between objects defined by simple class methods.

class Firm < ActiveRecord::Base
  has_many   :clients
  has_one    :account
  belongs_to :conglomerate
end

Learn more

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

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

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

Inheritance hierarchies.

class Company < ActiveRecord::Base; end
class Firm < Company; end
class Client < Company; end
class PriorityClient < Client; end

Learn more

Transactions.

# Database transaction
Account.transaction do
  david.withdrawal(100)
  mary.deposit(100)
end

Learn more

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

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 and read about the built-in support for MySQL, PostgreSQL, and SQLite3.

Logging support for Log4r and Logger.

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[7.2]
  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

Philosophy

Active Record is an implementation of the object-relational mapping (ORM) pattern 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:

github.com/rails/rails/tree/main/activerecord

License

Active Record is released under the MIT license:

opensource.org/licenses/MIT

Support

API documentation is at:

api.rubyonrails.org

Bug reports for the Ruby on Rails project can be filed here:

github.com/rails/rails/issues

Feature requests should be discussed on the rails-core mailing list here:

discuss.rubyonrails.org/c/rubyonrails-core

Validation error class to wrap association records’ errors, with index_errors support.

Namespace

MODULE ActiveRecord::Aggregations
MODULE ActiveRecord::Assertions
MODULE ActiveRecord::Associations
MODULE ActiveRecord::AttributeAssignment
MODULE ActiveRecord::AttributeMethods
MODULE ActiveRecord::Attributes
MODULE ActiveRecord::AutosaveAssociation
MODULE ActiveRecord::Batches
MODULE ActiveRecord::Calculations
MODULE ActiveRecord::Callbacks
MODULE ActiveRecord::Coders
MODULE ActiveRecord::ConnectionAdapters
MODULE ActiveRecord::ConnectionHandling
MODULE ActiveRecord::Core
MODULE ActiveRecord::CounterCache
MODULE ActiveRecord::DelegatedType
MODULE ActiveRecord::DynamicMatchers
MODULE ActiveRecord::Encryption
MODULE ActiveRecord::Enum
MODULE ActiveRecord::Explain
MODULE ActiveRecord::FinderMethods
MODULE ActiveRecord::Inheritance
MODULE ActiveRecord::Integration
MODULE ActiveRecord::Locking
MODULE ActiveRecord::Marshalling
MODULE ActiveRecord::MessagePack
MODULE ActiveRecord::Middleware
MODULE ActiveRecord::ModelSchema
MODULE ActiveRecord::NestedAttributes
MODULE ActiveRecord::NoTouching
MODULE ActiveRecord::Normalization
MODULE ActiveRecord::Persistence
MODULE ActiveRecord::QueryLogs
MODULE ActiveRecord::QueryMethods
MODULE ActiveRecord::Querying
MODULE ActiveRecord::ReadonlyAttributes
MODULE ActiveRecord::Reflection
MODULE ActiveRecord::Sanitization
MODULE ActiveRecord::Scoping
MODULE ActiveRecord::SecurePassword
MODULE ActiveRecord::SecureToken
MODULE ActiveRecord::Serialization
MODULE ActiveRecord::SignedId
MODULE ActiveRecord::SpawnMethods
MODULE ActiveRecord::Store
MODULE ActiveRecord::Suppressor
MODULE ActiveRecord::Tasks
MODULE ActiveRecord::TestFixtures
MODULE ActiveRecord::Timestamp
MODULE ActiveRecord::TokenFor
MODULE ActiveRecord::Transactions
MODULE ActiveRecord::Translation
MODULE ActiveRecord::Type
MODULE ActiveRecord::VERSION
MODULE ActiveRecord::Validations
CLASS ActiveRecord::ActiveRecordError
CLASS ActiveRecord::AdapterError
CLASS ActiveRecord::AdapterNotFound
CLASS ActiveRecord::AdapterNotSpecified
CLASS ActiveRecord::AdapterTimeout
CLASS ActiveRecord::AssociationTypeMismatch
CLASS ActiveRecord::AsynchronousQueryInsideTransactionError
CLASS ActiveRecord::AttributeAssignmentError
CLASS ActiveRecord::Base
CLASS ActiveRecord::ConfigurationError
CLASS ActiveRecord::ConnectionFailed
CLASS ActiveRecord::ConnectionNotEstablished
CLASS ActiveRecord::ConnectionTimeoutError
CLASS ActiveRecord::DangerousAttributeError
CLASS ActiveRecord::DatabaseAlreadyExists
CLASS ActiveRecord::DatabaseConfigurations
CLASS ActiveRecord::DatabaseConnectionError
CLASS ActiveRecord::DatabaseVersionError
CLASS ActiveRecord::Deadlocked
CLASS ActiveRecord::DestroyAssociationAsyncError
CLASS ActiveRecord::DestroyAssociationAsyncJob
CLASS ActiveRecord::EagerLoadPolymorphicError
CLASS ActiveRecord::EnvironmentMismatchError
CLASS ActiveRecord::ExclusiveConnectionTimeoutError
CLASS ActiveRecord::FixtureSet
CLASS ActiveRecord::FutureResult
CLASS ActiveRecord::InvalidForeignKey
CLASS ActiveRecord::IrreversibleMigration
CLASS ActiveRecord::IrreversibleOrderError
CLASS ActiveRecord::LockWaitTimeout
CLASS ActiveRecord::LogSubscriber
CLASS ActiveRecord::Migration
CLASS ActiveRecord::MigrationContext
CLASS ActiveRecord::MismatchedForeignKey
CLASS ActiveRecord::MultiparameterAssignmentErrors
CLASS ActiveRecord::NoDatabaseError
CLASS ActiveRecord::NotNullViolation
CLASS ActiveRecord::PreparedStatementCacheExpired
CLASS ActiveRecord::PreparedStatementInvalid
CLASS ActiveRecord::Promise
CLASS ActiveRecord::QueryAborted
CLASS ActiveRecord::QueryCache
CLASS ActiveRecord::QueryCanceled
CLASS ActiveRecord::RangeError
CLASS ActiveRecord::ReadOnlyError
CLASS ActiveRecord::ReadOnlyRecord
CLASS ActiveRecord::ReadonlyAttributeError
CLASS ActiveRecord::RecordInvalid
CLASS ActiveRecord::RecordNotDestroyed
CLASS ActiveRecord::RecordNotFound
CLASS ActiveRecord::RecordNotSaved
CLASS ActiveRecord::RecordNotUnique
CLASS ActiveRecord::Relation
CLASS ActiveRecord::Result
CLASS ActiveRecord::Rollback
CLASS ActiveRecord::SQLWarning
CLASS ActiveRecord::Schema
CLASS ActiveRecord::SerializationFailure
CLASS ActiveRecord::SerializationTypeMismatch
CLASS ActiveRecord::SoleRecordExceeded
CLASS ActiveRecord::StaleObjectError
CLASS ActiveRecord::StatementCache
CLASS ActiveRecord::StatementInvalid
CLASS ActiveRecord::StatementTimeout
CLASS ActiveRecord::StrictLoadingViolationError
CLASS ActiveRecord::SubclassNotFound
CLASS ActiveRecord::TableNotSpecified
CLASS ActiveRecord::Transaction
CLASS ActiveRecord::TransactionIsolationError
CLASS ActiveRecord::TransactionRollbackError
CLASS ActiveRecord::UnknownAttributeError
CLASS ActiveRecord::UnknownAttributeReference
CLASS ActiveRecord::UnknownPrimaryKey
CLASS ActiveRecord::UnmodifiableRelation
CLASS ActiveRecord::ValueTooLong
CLASS ActiveRecord::WrappedDatabaseException

Methods

A

action_on_strict_loading_violation,
after_all_transactions_commit,
allow_deprecated_singular_associations_name,
allow_deprecated_singular_associations_name=,
async_query_executor

C

commit_transaction_on_non_local_return,
commit_transaction_on_non_local_return=

D

db_warnings_action,
db_warnings_action=,
db_warnings_ignore,
default_timezone=,
disconnect_all!,
dump_schema_after_migration,
dump_schemas

E

eager_load!,
error_on_ignored_order

G

gem_version,
generate_secure_token_on,
global_executor_concurrency=

L

lazily_load_schema_cache,
legacy_connection_handling=

M

marshalling_format_version,
marshalling_format_version=,
migration_strategy

P

permanent_connection_checkout=,
protocol_adapters

Q

queues

R

raise_int_wider_than_64bit

S

schema_cache_ignored_tables,
schema_format

T

timestamped_migrations

U

use_yaml_unsafe_load

V

validate_migration_timestamps,
verbose_query_logs,
verify_foreign_keys_for_fixtures,
version

W

warn_on_records_fetched_greater_than

Y

yaml_column_permitted_classes

Included Modules

ActiveSupport::Deprecation::DeprecatedConstantAccessor

Constants

MigrationProxy	=	Struct.new(:name, :version, :filename, :scope) do def initialize(name, version, filename, scope) super @migration = nil end def basename File.basename(filename) end delegate :migrate, :announce, :write, :disable_ddl_transaction, to: :migration private def migration @migration \|\|= load_migration end def load_migration Object.send(:remove_const, name) rescue nil load(File.expand_path(filename)) name.constantize.new(name, version) end end
	`MigrationProxy` is used to defer loading of the actual migration classes until they are needed
Point	=	Struct.new(:x, :y)

UnknownAttributeError	=	ActiveModel::UnknownAttributeError
	Active Model UnknownAttributeError Raised when unknown attributes are supplied via mass assignment. `class Person include ActiveModel::AttributeAssignment include ActiveModel::Validations end person = Person.new person.assign_attributes(name: 'Gorby') # => ActiveModel::UnknownAttributeError: unknown attribute 'name' for Person.`

Attributes

[RW]	application_record_class
[RW]	before_committed_on_all_records
[RW]	belongs_to_required_validates_foreign_key
[R]	default_timezone
[RW]	disable_prepared_statements
[RW]	index_nested_attribute_errors
[RW]	maintain_test_schema
[R]	permanent_connection_checkout
[RW]	query_transformers
[RW]	raise_on_assign_to_attr_readonly
[RW]	reading_role
[RW]	run_after_transaction_callbacks_in_order_defined
[RW]	writing_role

Class Public methods

action_on_strict_loading_violation Link

Set the application to log or raise when an association violates strict loading. Defaults to :raise.

Module ActiveRecord

Active Record – Object-relational mapping in Rails

Philosophy

Download and installation

License

Support

Constants

Active Model UnknownAttributeError

Attributes

Class Public methods

action_on_strict_loading_violation Link

after_all_transactions_commit(&block) Link

allow_deprecated_singular_associations_name() Link

allow_deprecated_singular_associations_name=(value) Link

async_query_executor Link

commit_transaction_on_non_local_return() Link

commit_transaction_on_non_local_return=(value) Link

db_warnings_action Link

db_warnings_action=(action) Link

db_warnings_ignore Link

default_timezone=(default_timezone) Link

disconnect_all!() Link

dump_schema_after_migration Link

dump_schemas Link

eager_load!() Link

error_on_ignored_order Link

gem_version() Link

generate_secure_token_on Link

global_executor_concurrency=(global_executor_concurrency) Link

lazily_load_schema_cache Link

legacy_connection_handling=(_) Link

marshalling_format_version() Link

marshalling_format_version=(value) Link

migration_strategy Link

permanent_connection_checkout=(value) Link

protocol_adapters Link

Example

queues Link

raise_int_wider_than_64bit Link

schema_cache_ignored_tables Link

schema_format Link

timestamped_migrations Link

use_yaml_unsafe_load Link

validate_migration_timestamps Link

verbose_query_logs Link

verify_foreign_keys_for_fixtures Link

version() Link

warn_on_records_fetched_greater_than Link

yaml_column_permitted_classes Link