module Sequel::Plugins::ClassTableInheritance
Overview¶ ↑
The class_table_inheritance plugin uses the single_table_inheritance plugin, so it supports all of the single_table_inheritance features, but it additionally supports subclasses that have additional columns, which are stored in a separate table with a key referencing the primary table.
Detail¶ ↑
For example, with this hierarchy:
Employee / \ Staff Manager | | Cook Executive | CEO
the following database schema may be used (table - columns):
- employees
-
id, name, kind
- staff
-
id, manager_id
- managers
-
id, num_staff
- executives
-
id, num_managers
The class_table_inheritance plugin assumes that the root table (e.g. employees) has a primary key column (usually autoincrementing), and all other tables have a foreign key of the same name that points to the same column in their superclass's table, which is also the primary key for that table. In this example, the employees table has an id column is a primary key and the id column in every other table is a foreign key referencing employees.id, which is also the primary key of that table.
Additionally, note that other than the primary key column, no subclass table has a column with the same name as any superclass table. This plugin does not support cases where the column names in a subclass table overlap with any column names in a superclass table.
In this example the staff table also stores Cook model objects and the executives table also stores CEO model objects.
When using the class_table_inheritance plugin, subclasses that have additional columns use joined datasets:
Employee.dataset.sql # SELECT * FROM employees Manager.dataset.sql # SELECT employees.id, employees.name, employees.kind, # managers.num_staff # FROM employees # JOIN managers ON (managers.id = employees.id) CEO.dataset.sql # SELECT employees.id, employees.name, employees.kind, # managers.num_staff, executives.num_managers # FROM employees # JOIN managers ON (managers.id = employees.id) # JOIN executives ON (executives.id = managers.id) # WHERE (employees.kind IN ('CEO'))
This allows CEO.all to return instances with all attributes loaded. The plugin overrides the deleting, inserting, and updating in the model to work with multiple tables, by handling each table individually.
Subclass loading¶ ↑
When model objects are retrieved for a superclass the result can contain subclass instances that only have column entries for the columns in the superclass table. Calling the column method on the subclass instance for a column not in the superclass table will cause a query to the database to get the value for that column. If the subclass instance was retreived using Sequel::Dataset#all, the query to the database will attempt to load the column values for all subclass instances that were retrieved. For example:
a = Employee.all # [<#Staff>, <#Manager>, <#Executive>] a.first.values # {:id=>1, name=>'S', :kind=>'Staff'} a.first.manager_id # Loads the manager_id attribute from the database
If you want to get all columns in a subclass instance after loading via the superclass, call Model#refresh.
a = Employee.first a.values # {:id=>1, name=>'S', :kind=>'CEO'} a.refresh.values # {:id=>1, name=>'S', :kind=>'CEO', :num_staff=>4, :num_managers=>2}
You can also load directly from a subclass:
a = Executive.first a.values # {:id=>1, name=>'S', :kind=>'Executive', :num_staff=>4, :num_managers=>2}
Note that when loading from a subclass, because the subclass dataset uses a JOIN, if you are referencing the primary key column, you need to disambiguate the reference by explicitly qualifying it:
a = Executive.where(:id=>1).first # database error a = Executive.where{{executives[:id]=>1}}.first # no error
Usage¶ ↑
# Use the default of storing the class name in the sti_key # column (:kind in this case) class Employee < Sequel::Model plugin :class_table_inheritance, :key=>:kind end # Have subclasses inherit from the appropriate class class Staff < Employee; end # uses staff table class Cook < Staff; end # cooks table doesn't exist so uses staff table class Manager < Employee; end # uses managers table class Executive < Manager; end # uses executives table class CEO < Executive; end # ceos table doesn't exist so uses executives table # Some examples of using these options: # Specifying the tables with a :table_map hash Employee.plugin :class_table_inheritance, :table_map=>{:Employee => :employees, :Staff => :staff, :Cook => :staff, :Manager => :managers, :Executive => :executives, :CEO => :executives } # Using integers to store the class type, with a :model_map hash # and an sti_key of :type Employee.plugin :class_table_inheritance, :type, :model_map=>{1=>:Staff, 2=>:Cook, 3=>:Manager, 4=>:Executive, 5=>:CEO} # Using non-class name strings Employee.plugin :class_table_inheritance, :key=>:type, :model_map=>{'staff'=>:Staff, 'cook staff'=>:Cook, 'supervisor'=>:Manager} # By default the plugin sets the respective column value # when a new instance is created. Cook.create.type == 'cook 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 :class_table_inheritance, :key=>:type, :model_map=>{'cook staff'=>:Cook, 'supervisor'=>:Manager}, :key_chooser=>proc{|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, :key=>:type, :model_map=>proc{|v| v.reverse}, :key_map=>proc{|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, :key=>: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.
Public Class Methods
The class_table_inheritance plugin requires the single_table_inheritance plugin and the lazy_attributes plugin to handle lazily-loaded attributes for subclass instances returned by superclass methods.
# File lib/sequel/plugins/class_table_inheritance.rb, line 176 def self.apply(model, opts = OPTS) model.plugin :single_table_inheritance, nil model.plugin :lazy_attributes end
Initialize the plugin using the following options:
- :key
-
Column symbol that holds the key that identifies the class to use. Necessary if you want to call model methods on a superclass that return subclass instances
- :model_map
-
Hash or proc mapping the key column values to model class names.
- :key_map
-
Hash or proc mapping model class names to key column values. Each value or return is an array of possible key column values.
- :key_chooser
-
proc returning key for the provided model instance
- :table_map
-
Hash with class name symbols keys mapping to table name symbol values Overrides implicit table names
# File lib/sequel/plugins/class_table_inheritance.rb, line 191 def self.configure(model, opts = OPTS) SingleTableInheritance.configure model, opts[:key], opts model.instance_eval do @cti_models = [self] @cti_tables = [table_name] @cti_instance_dataset = @instance_dataset @cti_table_columns = columns @cti_table_map = opts[:table_map] || {} end end