The single_table_inheritance plugin allows storing all objects in the same class hierarchy in the same table. It makes it so subclasses of this model only load rows related to the subclass, and when you retrieve rows from the main class, you get instances of the subclasses (if the rows should use the subclasses‘s class).

By default, the plugin assumes that the sti_key column (the first argument to the plugin) holds the class name as a string. However, you can override this by using the :model_map option and/or the :key_map option.

You should only load this plugin in the parent class, not in the subclasses.

You shouldn‘t call set_dataset in the model after applying this plugin, otherwise subclasses might use the wrong dataset. You should make sure this plugin is loaded before the subclasses. Note that since you need to load the plugin before the subclasses are created, you can‘t use direct class references in the plugin class. You should specify subclasses in the plugin call using class name strings or symbols, see usage below.

Usage:

  # Use the default of storing the class name in the sti_key
  # column (:kind in this case)
  class Employee < Sequel::Model
    plugin :single_table_inheritance, :kind
  end

  # Have subclasses inherit from the appropriate class
  class Staff < Employee; end
  class Manager < Employee; end

  # You can also use many different options to configure the plugin:

  # Using integers to store the class type, with a :model_map hash
  # and an sti_key of :type
  Employee.plugin :single_table_inheritance, :type,
    model_map: {1=>:Staff, 2=>:Manager}

  # Using non-class name strings
  Employee.plugin :single_table_inheritance, :type,
    model_map: {'line staff'=>:Staff, 'supervisor'=>:Manager}

  # By default the plugin sets the respective column value
  # when a new instance is created.
  Staff.create.type == 'line staff'
  Manager.create.type == 'supervisor'

  # You can customize this behavior with the :key_chooser option.
  # This is most useful when using a non-bijective mapping.
  Employee.plugin :single_table_inheritance, :type,
    model_map: {'line staff'=>:Staff, 'supervisor'=>:Manager},
    key_chooser: lambda{|instance| instance.model.sti_key_map[instance.model.to_s].first || 'stranger'}

  # Using custom procs, with :model_map taking column values
  # and yielding either a class, string, symbol, or nil,
  # and :key_map taking a class object and returning the column
  # value to use
  Employee.plugin :single_table_inheritance, :type,
    model_map: :reverse.to_proc,
    key_map: lambda{|klass| klass.name.reverse}

  # You can use the same class for multiple values.
  # This is mainly useful when the sti_key column contains multiple values
  # which are different but do not require different code.
  Employee.plugin :single_table_inheritance, :type,
    model_map: {'staff' => "Staff",
                 'manager' => "Manager",
                 'overpayed staff' => "Staff",
                 'underpayed staff' => "Staff"}

One minor issue to note is that if you specify the :key_map option as a hash, instead of having it inferred from the :model_map, you should only use class name strings as keys, you should not use symbols as keys.