ActiveRecord::AttributeMethods::Serialization::ClassMethods

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.

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 name of the attribute to serialize.

• coder The serializer implementation to use, e.g. JSON.

  • The attribute value will be serialized using the coder’s dump(value) method, and will be deserialized using the coder’s load(string) method. The dump method may return nil to serialize the value as NULL.

• type - Optional. What the type of the serialized object should be.

  • Attempting to serialize another type will raise an ActiveRecord::SerializationTypeMismatch error.

  • If the column is NULL or starting from a new record, the default value will set to type.new

• yaml - Optional. Yaml specific options. The allowed config is:

  • :permitted_classes - Array with the permitted classes.

  • :unsafe_load - Unsafely load YAML blobs, allow YAML to load any class.

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.

Choosing a serializer

While any serialization format can be used, it is recommended to carefully evaluate the properties of a serializer before using it, as migrating to another format later on can be difficult.

Avoid accepting arbitrary types

When serializing data in a column, it is heavily recommended to make sure only expected types will be serialized. For instance some serializer like Marshal or YAML are capable of serializing almost any Ruby object.

This can lead to unexpected types being serialized, and it is important that type serialization remains backward and forward compatible as long as some database records still contain these serialized types.

class Address
  def initialize(line, city, country)
    @line, @city, @country = line, city, country
  end
end

In the above example, if any of the Address attributes is renamed, instances that were persisted before the change will be loaded with the old attributes. This problem is even worse when the serialized type comes from a dependency which doesn’t expect to be serialized this way and may change its internal representation without notice.

As such, it is heavily recommended to instead convert these objects into primitives of the serialization format, for example:

class Address
  attr_reader :line, :city, :country

  def self.load(payload)
    data = YAML.safe_load(payload)
    new(data["line"], data["city"], data["country"])
  end

  def self.dump(address)
    YAML.safe_dump(
      "line" => address.line,
      "city" => address.city,
      "country" => address.country,
    )
  end

  def initialize(line, city, country)
    @line, @city, @country = line, city, country
  end
end

class User < ActiveRecord::Base
  serialize :address, coder: Address
end

This pattern allows to be more deliberate about what is serialized, and to evolve the format in a backward compatible way.

Ensure serialization stability

Some serialization methods may accept some types they don’t support by silently casting them to other types. This can cause bugs when the data is deserialized.

For instance the JSON serializer provided in the standard library will silently cast unsupported types to String:

>> JSON.parse(JSON.dump(Struct.new(:foo)))
=> "#<Class:0x000000013090b4c0>"

Examples

Serialize the preferences attribute using YAML

class User < ActiveRecord::Base
  serialize :preferences, coder: YAML
end

Serialize the preferences attribute using JSON

class User < ActiveRecord::Base
  serialize :preferences, coder: JSON
end

Serialize the preferences Hash using YAML

class User < ActiveRecord::Base
  serialize :preferences, type: Hash, coder: YAML
end

Serializes preferences to YAML, permitting select classes

class User < ActiveRecord::Base
  serialize :preferences, coder: YAML, yaml: { permitted_classes: [Symbol, Time] }
end

Serialize the preferences attribute using a custom coder

class Rot13JSON
  def self.rot13(string)
    string.tr("a-zA-Z", "n-za-mN-ZA-M")
  end

  # Serializes an attribute value to a string that will be stored in the database.
  def self.dump(value)
    rot13(ActiveSupport::JSON.dump(value))
  end

  # Deserializes a string from the database to an attribute value.
  def self.load(string)
    ActiveSupport::JSON.load(rot13(string))
  end
end

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