# File activerecord/lib/active_record/attributes.rb, line 208
def attribute(name, cast_type = nil, default: NO_DEFAULT_PROVIDED, **options)
  name = name.to_s
  name = attribute_aliases[name] || name

  reload_schema_from_cache

  case cast_type
  when Symbol
    cast_type = Type.lookup(cast_type, **options, adapter: Type.adapter_name_from(self))
  when nil
    if (prev_cast_type, prev_default = attributes_to_define_after_schema_loads[name])
      default = prev_default if default == NO_DEFAULT_PROVIDED
    else
      prev_cast_type = -> subtype { subtype }
    end

    cast_type = if block_given?
      -> subtype { yield Proc === prev_cast_type ? prev_cast_type[subtype] : prev_cast_type }
    else
      prev_cast_type
    end
  end

  self.attributes_to_define_after_schema_loads =
    attributes_to_define_after_schema_loads.merge(name => [cast_type, default])
end

Defines an attribute with a type on this model. It will override the type of existing attributes if needed. This allows control over how values are converted to and from SQL when assigned to a model. It also changes the behavior of values passed to ActiveRecord::Base.where. This will let you use your domain objects across much of Active Record, without having to rely on implementation details or monkey patching.

name The name of the methods to define attribute methods for, and the column which this will persist to.

cast_type A symbol such as :string or :integer, or a type object to be used for this attribute. See the examples below for more information about providing custom type objects.

Options

The following options are accepted:

default The default value to use when no value is provided. If this option is not passed, the previous default value (if any) will be used. Otherwise, the default will be nil.

array (PostgreSQL only) specifies that the type should be an array (see the examples below).

range (PostgreSQL only) specifies that the type should be a range (see the examples below).

When using a symbol for cast_type, extra options are forwarded to the constructor of the type object.

Examples

The type detected by Active Record can be overridden.

# db/schema.rb
create_table :store_listings, force: true do |t|
  t.decimal :price_in_cents
end

# app/models/store_listing.rb
class StoreListing < ActiveRecord::Base
end

store_listing = StoreListing.new(price_in_cents: '10.1')

# before
store_listing.price_in_cents # => BigDecimal(10.1)

class StoreListing < ActiveRecord::Base
  attribute :price_in_cents, :integer
end

# after
store_listing.price_in_cents # => 10

A default can also be provided.

# db/schema.rb
create_table :store_listings, force: true do |t|
  t.string :my_string, default: "original default"
end

StoreListing.new.my_string # => "original default"

# app/models/store_listing.rb
class StoreListing < ActiveRecord::Base
  attribute :my_string, :string, default: "new default"
end

StoreListing.new.my_string # => "new default"

class Product < ActiveRecord::Base
  attribute :my_default_proc, :datetime, default: -> { Time.now }
end

Product.new.my_default_proc # => 2015-05-30 11:04:48 -0600
sleep 1
Product.new.my_default_proc # => 2015-05-30 11:04:49 -0600

Attributes do not need to be backed by a database column.

# app/models/my_model.rb
class MyModel < ActiveRecord::Base
  attribute :my_string, :string
  attribute :my_int_array, :integer, array: true
  attribute :my_float_range, :float, range: true
end

model = MyModel.new(
  my_string: "string",
  my_int_array: ["1", "2", "3"],
  my_float_range: "[1,3.5]",
)
model.attributes
# =>
  {
    my_string: "string",
    my_int_array: [1, 2, 3],
    my_float_range: 1.0..3.5
  }

Passing options to the type constructor

# app/models/my_model.rb
class MyModel < ActiveRecord::Base
  attribute :small_int, :integer, limit: 2
end

MyModel.create(small_int: 65537)
# => Error: 65537 is out of range for the limit of two bytes

Creating Custom Types

Users may also define their own custom types, as long as they respond to the methods defined on the value type. The method deserialize or cast will be called on your type object, with raw input from the database or from your controllers. See ActiveModel::Type::Value for the expected API. It is recommended that your type objects inherit from an existing type, or from ActiveRecord::Type::Value

class MoneyType < ActiveRecord::Type::Integer
  def cast(value)
    if !value.kind_of?(Numeric) && value.include?('$')
      price_in_dollars = value.gsub(/\$/, '').to_f
      super(price_in_dollars * 100)
    else
      super
    end
  end
end

# config/initializers/types.rb
ActiveRecord::Type.register(:money, MoneyType)

# app/models/store_listing.rb
class StoreListing < ActiveRecord::Base
  attribute :price_in_cents, :money
end

store_listing = StoreListing.new(price_in_cents: '$10.00')
store_listing.price_in_cents # => 1000

For more details on creating custom types, see the documentation for ActiveModel::Type::Value. For more details on registering your types to be referenced by a symbol, see ActiveRecord::Type.register. You can also pass a type object directly, in place of a symbol.

Querying

When ActiveRecord::Base.where is called, it will use the type defined by the model class to convert the value to SQL, calling serialize on your type object. For example:

class Money < Struct.new(:amount, :currency)
end

class MoneyType < ActiveRecord::Type::Value
  def initialize(currency_converter:)
    @currency_converter = currency_converter
  end

  # value will be the result of +deserialize+ or
  # +cast+. Assumed to be an instance of +Money+ in
  # this case.
  def serialize(value)
    value_in_bitcoins = @currency_converter.convert_to_bitcoins(value)
    value_in_bitcoins.amount
  end
end

# config/initializers/types.rb
ActiveRecord::Type.register(:money, MoneyType)

# app/models/product.rb
class Product < ActiveRecord::Base
  currency_converter = ConversionRatesFromTheInternet.new
  attribute :price_in_bitcoins, :money, currency_converter: currency_converter
end

Product.where(price_in_bitcoins: Money.new(5, "USD"))
# => SELECT * FROM products WHERE price_in_bitcoins = 0.02230

Product.where(price_in_bitcoins: Money.new(5, "GBP"))
# => SELECT * FROM products WHERE price_in_bitcoins = 0.03412

Dirty Tracking

The type of an attribute is given the opportunity to change how dirty tracking is performed. The methods changed? and changed_in_place? will be called from ActiveModel::Dirty. See the documentation for those methods in ActiveModel::Type::Value for more details.

# File activerecord/lib/active_record/attributes.rb, line 253
def define_attribute(
  name,
  cast_type,
  default: NO_DEFAULT_PROVIDED,
  user_provided_default: true
)
  attribute_types[name] = cast_type
  define_default_attribute(name, default, cast_type, from_user: user_provided_default)
end

This is the low level API which sits beneath attribute. It only accepts type objects, and will do its work immediately instead of waiting for the schema to load. Automatic schema detection and ClassMethods#attribute both call this under the hood. While this method is provided so it can be used by plugin authors, application code should probably use ClassMethods#attribute.

name The name of the attribute being defined. Expected to be a String.

cast_type The type object to use for this attribute.

default The default value to use when no value is provided. If this option is not passed, the previous default value (if any) will be used. Otherwise, the default will be nil. A proc can also be passed, and will be called once each time a new value is needed.

user_provided_default Whether the default value should be cast using cast or deserialize.