Class: ActiveRecord::Base

Active Record objects don‘t specify their attributes directly, but rather infer them from the table definition with which they‘re linked. Adding, removing, and changing attributes and their type is done directly in the database. Any change is instantly reflected in the Active Record objects. The mapping that binds a given Active Record class to a certain database table will happen automatically in most common cases, but can be overwritten for the uncommon ones.

See the mapping rules in table_name and the full example in files/README.html for more insight.

Creation

Active Records accept constructor parameters either in a hash or as a block. The hash method is especially useful when you‘re receiving the data from somewhere else, like an HTTP request. It works like this:

  user = User.new(:name => "David", :occupation => "Code Artist")
  user.name # => "David"

You can also use block initialization:

  user = User.new do |u|
    u.name = "David"
    u.occupation = "Code Artist"
  end

And of course you can just create a bare object and specify the attributes after the fact:

  user = User.new
  user.name = "David"
  user.occupation = "Code Artist"

Conditions

Conditions can either be specified as a string, array, or hash representing the WHERE-part of an SQL statement. The array form is to be used when the condition input is tainted and requires sanitization. The string form can be used for statements that don‘t involve tainted data. The hash form works much like the array form, except only equality and range is possible. Examples:

  class User < ActiveRecord::Base
    def self.authenticate_unsafely(user_name, password)
      find(:first, :conditions => "user_name = '#{user_name}' AND password = '#{password}'")
    end

    def self.authenticate_safely(user_name, password)
      find(:first, :conditions => [ "user_name = ? AND password = ?", user_name, password ])
    end

    def self.authenticate_safely_simply(user_name, password)
      find(:first, :conditions => { :user_name => user_name, :password => password })
    end
  end

The authenticate_unsafely method inserts the parameters directly into the query and is thus susceptible to SQL-injection attacks if the user_name and password parameters come directly from an HTTP request. The authenticate_safely and authenticate_safely_simply both will sanitize the user_name and password before inserting them in the query, which will ensure that an attacker can‘t escape the query and fake the login (or worse).

When using multiple parameters in the conditions, it can easily become hard to read exactly what the fourth or fifth question mark is supposed to represent. In those cases, you can resort to named bind variables instead. That‘s done by replacing the question marks with symbols and supplying a hash with values for the matching symbol keys:

  Company.find(:first, :conditions => [
    "id = :id AND name = :name AND division = :division AND created_at > :accounting_date",
    { :id => 3, :name => "37signals", :division => "First", :accounting_date => '2005-01-01' }
  ])

Similarly, a simple hash without a statement will generate conditions based on equality with the SQL AND operator. For instance:

  Student.find(:all, :conditions => { :first_name => "Harvey", :status => 1 })
  Student.find(:all, :conditions => params[:student])

A range may be used in the hash to use the SQL BETWEEN operator:

  Student.find(:all, :conditions => { :grade => 9..12 })

Overwriting default accessors

All column values are automatically available through basic accessors on the Active Record object, but sometimes you want to specialize this behavior. This can be done by overwriting the default accessors (using the same name as the attribute) and calling read_attribute(attr_name) and write_attribute(attr_name, value) to actually change things. Example:

  class Song < ActiveRecord::Base
    # Uses an integer of seconds to hold the length of the song

    def length=(minutes)
      write_attribute(:length, minutes * 60)
    end

    def length
      read_attribute(:length) / 60
    end
  end

You can alternatively use self[:attribute]=(value) and self[:attribute] instead of write_attribute(:attribute, value) and read_attribute(:attribute) as a shorter form.

Attribute query methods

In addition to the basic accessors, query methods are also automatically available on the Active Record object. Query methods allow you to test whether an attribute value is present.

For example, an Active Record User with the name attribute has a name? method that you can call to determine whether the user has a name:

  user = User.new(:name => "David")
  user.name? # => true

  anonymous = User.new(:name => "")
  anonymous.name? # => false

Accessing attributes before they have been typecasted

Sometimes you want to be able to read the raw attribute data without having the column-determined typecast run its course first. That can be done by using the <attribute>_before_type_cast accessors that all attributes have. For example, if your Account model has a balance attribute, you can call account.balance_before_type_cast or account.id_before_type_cast.

This is especially useful in validation situations where the user might supply a string for an integer field and you want to display the original string back in an error message. Accessing the attribute normally would typecast the string to 0, which isn‘t what you want.

Dynamic attribute-based finders

Dynamic attribute-based finders are a cleaner way of getting (and/or creating) objects by simple queries without turning to SQL. They work by appending the name of an attribute to find_by_ or find_all_by_, so you get finders like Person.find_by_user_name, Person.find_all_by_last_name, Payment.find_by_transaction_id. So instead of writing Person.find(:first, :conditions => ["user_name = ?", user_name]), you just do Person.find_by_user_name(user_name). And instead of writing Person.find(:all, :conditions => ["last_name = ?", last_name]), you just do Person.find_all_by_last_name(last_name).

It‘s also possible to use multiple attributes in the same find by separating them with "and", so you get finders like Person.find_by_user_name_and_password or even Payment.find_by_purchaser_and_state_and_country. So instead of writing Person.find(:first, :conditions => ["user_name = ? AND password = ?", user_name, password]), you just do Person.find_by_user_name_and_password(user_name, password).

It‘s even possible to use all the additional parameters to find. For example, the full interface for Payment.find_all_by_amount is actually Payment.find_all_by_amount(amount, options). And the full interface to Person.find_by_user_name is actually Person.find_by_user_name(user_name, options). So you could call Payment.find_all_by_amount(50, :order => "created_on").

The same dynamic finder style can be used to create the object if it doesn‘t already exist. This dynamic finder is called with find_or_create_by_ and will return the object if it already exists and otherwise creates it, then returns it. Protected attributes won‘t be set unless they are given in a block. For example:

  # No 'Summer' tag exists
  Tag.find_or_create_by_name("Summer") # equal to Tag.create(:name => "Summer")

  # Now the 'Summer' tag does exist
  Tag.find_or_create_by_name("Summer") # equal to Tag.find_by_name("Summer")

  # Now 'Bob' exist and is an 'admin'
  User.find_or_create_by_name('Bob', :age => 40) { |u| u.admin = true }

Use the find_or_initialize_by_ finder if you want to return a new record without saving it first. Protected attributes won‘t be setted unless they are given in a block. For example:

  # No 'Winter' tag exists
  winter = Tag.find_or_initialize_by_name("Winter")
  winter.new_record? # true

To find by a subset of the attributes to be used for instantiating a new object, pass a hash instead of a list of parameters. For example:

  Tag.find_or_create_by_name(:name => "rails", :creator => current_user)

That will either find an existing tag named "rails", or create a new one while setting the user that created it.

Saving arrays, hashes, and other non-mappable objects in text columns

Active Record can serialize any object in text columns using YAML. To do so, you must specify this with a call to the class method serialize. This makes it possible to store arrays, hashes, and other non-mappable objects without doing any additional work. Example:

  class User < ActiveRecord::Base
    serialize :preferences
  end

  user = User.create(:preferences => { "background" => "black", "display" => large })
  User.find(user.id).preferences # => { "background" => "black", "display" => large }

You can also specify a class option as the second parameter that‘ll raise an exception if a serialized object is retrieved as a descendent of a class not in the hierarchy. Example:

  class User < ActiveRecord::Base
    serialize :preferences, Hash
  end

  user = User.create(:preferences => %w( one two three ))
  User.find(user.id).preferences    # raises SerializationTypeMismatch

Single table inheritance

Active Record allows inheritance by storing the name of the class in a column that by default is named "type" (can be changed by overwriting Base.inheritance_column). This means that an inheritance looking like this:

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

When you do Firm.create(:name => "37signals"), this record will be saved in the companies table with type = "Firm". You can then fetch this row again using Company.find(:first, "name = ‘37signals’") and it will return a Firm object.

If you don‘t have a type column defined in your table, single-table inheritance won‘t be triggered. In that case, it‘ll work just like normal subclasses with no special magic for differentiating between them or reloading the right type with find.

Note, all the attributes for all the cases are kept in the same table. Read more: www.martinfowler.com/eaaCatalog/singleTableInheritance.html

Connection to multiple databases in different models

Connections are usually created through ActiveRecord::Base.establish_connection and retrieved by ActiveRecord::Base.connection. All classes inheriting from ActiveRecord::Base will use this connection. But you can also set a class-specific connection. For example, if Course is an ActiveRecord::Base, but resides in a different database, you can just say Course.establish_connection and Course *and all its subclasses* will use this connection instead.

This feature is implemented by keeping a connection pool in ActiveRecord::Base that is a Hash indexed by the class. If a connection is requested, the retrieve_connection method will go up the class-hierarchy until a connection is found in the connection pool.

Exceptions

ActiveRecordError — generic error class and superclass of all other errors raised by Active Record
AdapterNotSpecified — the configuration hash used in establish_connection didn‘t include an :adapter key.
AdapterNotFound — the :adapter key used in establish_connection specified a non-existent adapter (or a bad spelling of an existing one).
AssociationTypeMismatch — the object assigned to the association wasn‘t of the type specified in the association definition.
SerializationTypeMismatch — the serialized object wasn‘t of the class specified as the second parameter.
ConnectionNotEstablished — no connection has been established. Use establish_connection before querying.
RecordNotFound — no record responded to the find* method. Either the row with the given ID doesn‘t exist or the row didn‘t meet the additional restrictions.
StatementInvalid — the database server rejected the SQL statement. The precise error is added in the message. Either the record with the given ID doesn‘t exist or the record didn‘t meet the additional restrictions.
MultiparameterAssignmentErrors — collection of errors that occurred during a mass assignment using the +attributes=+ method. The errors property of this exception contains an array of AttributeAssignmentError objects that should be inspected to determine which attributes triggered the errors.
AttributeAssignmentError — an error occurred while doing a mass assignment through the +attributes=+ method. You can inspect the attribute property of the exception object to determine which attribute triggered the error.

Note: The attributes listed are class-level attributes (accessible from both the class and instance level). So it‘s possible to assign a logger to the class through Base.logger= which will then be used by all instances in the current object space.

Child modules and classes

Class ActiveRecord::Base::ConnectionSpecification

Constants

Name Value

VALID_FIND_OPTIONS [ :conditions, :include, :joins, :limit, :offset, :order, :select, :readonly, :group, :from, :lock ]

Name	Value
VALID_FIND_OPTIONS	[ :conditions, :include, :joins, :limit, :offset, :order, :select, :readonly, :group, :from, :lock ]

Aliases

Old name New name

set_table_name table_name=

set_primary_key primary_key=

set_inheritance_column inheritance_column=

set_sequence_name sequence_name=

sanitize_sql_for_conditions sanitize_sql

sanitize_sql_hash_for_conditions sanitize_sql_hash

sanitize_sql sanitize_conditions

Old name	New name
set_table_name	table_name=
set_primary_key	primary_key=
set_inheritance_column	inheritance_column=
set_sequence_name	sequence_name=
sanitize_sql_for_conditions	sanitize_sql
sanitize_sql_hash_for_conditions	sanitize_sql_hash
sanitize_sql	sanitize_conditions

Attributes

Name Read/write?

abstract_class RW

Name	Read/write?
abstract_class	RW

Public Class Methods

=== (object)

Overwrite the default class equality method to provide support for association proxies.

Class: ActiveRecord::Base

Child of

Defined in

Methods

All classes

All files

All methods

Class: ActiveRecord::Base

Creation

Conditions

Overwriting default accessors

Attribute query methods

Accessing attributes before they have been typecasted

Dynamic attribute-based finders

Saving arrays, hashes, and other non-mappable objects in text columns

Single table inheritance

Connection to multiple databases in different models

Exceptions

Child modules and classes

Constants

Aliases

Attributes

Public Class Methods

Options

Examples

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Example

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Examples

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Protected Class Methods