# File activerecord/lib/active_record/attribute_methods/serialization.rb, line 119
def serialize(attr_name, class_name_or_coder = Object, **options)
  # When ::JSON is used, force it to go through the Active Support JSON encoder
  # to ensure special objects (e.g. Active Record models) are dumped correctly
  # using the #as_json hook.
  coder = if class_name_or_coder == ::JSON
    Coders::JSON
  elsif [:load, :dump].all? { |x| class_name_or_coder.respond_to?(x) }
    class_name_or_coder
  else
    Coders::YAMLColumn.new(attr_name, class_name_or_coder)
  end

  attribute(attr_name, **options) do |cast_type|
    if type_incompatible_with_serialize?(cast_type, class_name_or_coder)
      raise ColumnNotSerializableError.new(attr_name, cast_type)
    end

    cast_type = cast_type.subtype if Type::Serialized === cast_type
    Type::Serialized.new(cast_type, coder)
  end
end

If you have an attribute that needs to be saved to the database as a serialized object, and retrieved by deserializing into the same object, then specify the name of that attribute using this method and serialization will be handled automatically.

The serialization format may be YAML, JSON, or any custom format using a custom coder class.

formats

serialize attr_name [, class_name_or_coder]

                     |                           |  database storage   |
class_name_or_coder  | attribute read/write type | serialized | NULL   |
---------------------+---------------------------+------------+--------+
  <not given>        | any value that supports   |    YAML    |        |
                     |   .to_yaml                |            |        |
                     |                           |            |        |
Array                | Array **                  |    YAML    |  []    |
                     |                           |            |        |
Hash                 | Hash **                   |    YAML    |  {}    |
                     |                           |            |        |
JSON                 | any value that supports   |    JSON    |        |
                     |   .to_json                |            |        |
                     |                           |            |        |
<custom coder class> | any value supported by    |   custom   | custom |
                     | the custom coder class    |            |        |

** If class_name_or_coder is Array or Hash, values retrieved will always be of that type, and any value assigned must be of that type or SerializationTypeMismatch will be raised.

Custom coders

A custom coder class or module may be given. This must have self.load and self.dump class/module methods. self.dump(object) will be called to serialize an object and should return the serialized value to be stored in the database (nil to store as NULL). self.load(string) will be called to reverse the process and load (unserialize) from the database.

Keep in mind that database adapters handle certain serialization tasks for you. For instance: json and jsonb types in PostgreSQL will be converted between JSON object/array syntax and Ruby Hash or Array objects transparently. There is no need to use serialize in this case.

For more complex cases, such as conversion to or from your application domain objects, consider using the ActiveRecord::Attributes API.

Parameters

• attr_name - The field name that should be serialized.

• class_name_or_coder - Optional, may be be Array or Hash or JSON or a custom coder class or module which responds to .load and .dump. See table above.

Options

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.

Example

# Serialize a preferences attribute using YAML coder.
class User < ActiveRecord::Base
  serialize :preferences
end

# Serialize preferences using JSON as coder.
class User < ActiveRecord::Base
  serialize :preferences, JSON
end

# Serialize preferences as Hash using YAML coder.
class User < ActiveRecord::Base
  serialize :preferences, Hash
end

# Serialize preferences using a custom coder.
class Rot13JSON
  def self.rot13(string)
    string.tr("a-zA-Z", "n-za-mN-ZA-M")
  end

  # returns serialized string that will be stored in the database
  def self.dump(object)
    ActiveSupport::JSON.encode(object).rot13
  end

  # reverses the above, turning the serialized string from the database
  # back into its original value
  def self.load(string)
    ActiveSupport::JSON.decode(string.rot13)
  end
end

class User < ActiveRecord::Base
  serialize :preferences, Rot13JSON
end