module ActiveRecord::Inheritance::ClassMethods

Attributes

abstract_class[RW]

Set this to true if this is an abstract class (see abstract_class?). If you are using inheritance with Active Record and don't want a class to be considered as part of the STI hierarchy, you must set this to true. ApplicationRecord, for example, is generated as an abstract class.

Consider the following default behaviour:

Shape = Class.new(ActiveRecord::Base)
Polygon = Class.new(Shape)
Square = Class.new(Polygon)

Shape.table_name   # => "shapes"
Polygon.table_name # => "shapes"
Square.table_name  # => "shapes"
Shape.create!      # => #<Shape id: 1, type: nil>
Polygon.create!    # => #<Polygon id: 2, type: "Polygon">
Square.create!     # => #<Square id: 3, type: "Square">

However, when using abstract_class, Shape is omitted from the hierarchy:

class Shape < ActiveRecord::Base
  self.abstract_class = true
end
Polygon = Class.new(Shape)
Square = Class.new(Polygon)

Shape.table_name   # => nil
Polygon.table_name # => "polygons"
Square.table_name  # => "polygons"
Shape.create!      # => NotImplementedError: Shape is an abstract class and cannot be instantiated.
Polygon.create!    # => #<Polygon id: 1, type: nil>
Square.create!     # => #<Square id: 2, type: "Square">

Note that in the above example, to disallow the creation of a plain Polygon, you should use validates :type, presence: true, instead of setting it as an abstract class. This way, Polygon will stay in the hierarchy, and Active Record will continue to correctly derive the table name.

Public Instance Methods

abstract_class?() click to toggle source

Returns whether this class is an abstract class or not.

# File lib/active_record/inheritance.rb, line 163
def abstract_class?
  defined?(@abstract_class) && @abstract_class == true
end
base_class() click to toggle source

Returns the class descending directly from ActiveRecord::Base, or an abstract class, if any, in the inheritance hierarchy.

If A extends ActiveRecord::Base, A.base_class will return A. If B descends from A through some arbitrarily deep hierarchy, B.base_class will return A.

If B < A and C < B and if A is an abstract_class then both B.base_class and C.base_class would return B as the answer since A is an abstract_class.

# File lib/active_record/inheritance.rb, line 101
def base_class
  unless self < Base
    raise ActiveRecordError, "#{name} doesn't belong in a hierarchy descending from ActiveRecord"
  end

  if superclass == Base || superclass.abstract_class?
    self
  else
    superclass.base_class
  end
end
base_class?() click to toggle source

Returns whether the class is a base class. See base_class for more information.

# File lib/active_record/inheritance.rb, line 115
def base_class?
  base_class == self
end
descends_from_active_record?() click to toggle source

Returns true if this does not need STI type condition. Returns false if STI type condition needs to be applied.

# File lib/active_record/inheritance.rb, line 78
def descends_from_active_record?
  if self == Base
    false
  elsif superclass.abstract_class?
    superclass.descends_from_active_record?
  else
    superclass == Base || !columns_hash.include?(inheritance_column)
  end
end
inherited(subclass) click to toggle source
Calls superclass method
# File lib/active_record/inheritance.rb, line 205
def inherited(subclass)
  subclass.instance_variable_set(:@_type_candidates_cache, Concurrent::Map.new)
  super
end
new(attributes = nil, &block) click to toggle source

Determines if one of the attributes passed in is the inheritance column, and if the inheritance column is attr accessible, it initializes an instance of the given subclass instead of the base class.

Calls superclass method
# File lib/active_record/inheritance.rb, line 52
def new(attributes = nil, &block)
  if abstract_class? || self == Base
    raise NotImplementedError, "#{self} is an abstract class and cannot be instantiated."
  end

  if _has_attribute?(inheritance_column)
    subclass = subclass_from_attributes(attributes)

    if subclass.nil? && scope_attributes = current_scope&.scope_for_create
      subclass = subclass_from_attributes(scope_attributes)
    end

    if subclass.nil? && base_class?
      subclass = subclass_from_attributes(column_defaults)
    end
  end

  if subclass && subclass != self
    subclass.new(attributes, &block)
  else
    super
  end
end
polymorphic_class_for(name) click to toggle source

Returns the class for the provided name.

It is used to find the class correspondent to the value stored in the polymorphic type column.

# File lib/active_record/inheritance.rb, line 197
def polymorphic_class_for(name)
  if store_full_class_name
    ActiveSupport::Dependencies.constantize(name)
  else
    compute_type(name)
  end
end
polymorphic_name() click to toggle source

Returns the value to be stored in the polymorphic type column for Polymorphic Associations.

# File lib/active_record/inheritance.rb, line 190
def polymorphic_name
  store_full_class_name ? base_class.name : base_class.name.demodulize
end
sti_class_for(type_name) click to toggle source

Returns the class for the provided type_name.

It is used to find the class correspondent to the value stored in the inheritance column.

# File lib/active_record/inheritance.rb, line 175
def sti_class_for(type_name)
  if store_full_sti_class && store_full_class_name
    ActiveSupport::Dependencies.constantize(type_name)
  else
    compute_type(type_name)
  end
rescue NameError
  raise SubclassNotFound,
    "The single-table inheritance mechanism failed to locate the subclass: '#{type_name}'. " \
    "This error is raised because the column '#{inheritance_column}' is reserved for storing the class in case of inheritance. " \
    "Please rename this column if you didn't intend it to be used for storing the inheritance class " \
    "or overwrite #{name}.inheritance_column to use another column for that information."
end
sti_name() click to toggle source

Returns the value to be stored in the inheritance column for STI.

# File lib/active_record/inheritance.rb, line 168
def sti_name
  store_full_sti_class && store_full_class_name ? name : name.demodulize
end

Protected Instance Methods

compute_type(type_name) click to toggle source

Returns the class type of the record using the current module as a prefix. So descendants of MyApp::Business::Account would appear as MyApp::Business::AccountSubclass.

# File lib/active_record/inheritance.rb, line 213
def compute_type(type_name)
  if type_name.start_with?("::")
    # If the type is prefixed with a scope operator then we assume that
    # the type_name is an absolute reference.
    ActiveSupport::Dependencies.constantize(type_name)
  else
    type_candidate = @_type_candidates_cache[type_name]
    if type_candidate && type_constant = ActiveSupport::Dependencies.safe_constantize(type_candidate)
      return type_constant
    end

    # Build a list of candidates to search for
    candidates = []
    name.scan(/::|$/) { candidates.unshift "#{$`}::#{type_name}" }
    candidates << type_name

    candidates.each do |candidate|
      constant = ActiveSupport::Dependencies.safe_constantize(candidate)
      if candidate == constant.to_s
        @_type_candidates_cache[type_name] = candidate
        return constant
      end
    end

    raise NameError.new("uninitialized constant #{candidates.first}", candidates.first)
  end
end

Private Instance Methods

discriminate_class_for_record(record) click to toggle source

Called by instantiate to decide which class to use for a new record instance. For single-table inheritance, we check the record for a type column and return the corresponding class.

Calls superclass method
# File lib/active_record/inheritance.rb, line 245
def discriminate_class_for_record(record)
  if using_single_table_inheritance?(record)
    find_sti_class(record[inheritance_column])
  else
    super
  end
end
find_sti_class(type_name) click to toggle source
# File lib/active_record/inheritance.rb, line 257
def find_sti_class(type_name)
  type_name = base_class.type_for_attribute(inheritance_column).cast(type_name)
  subclass = sti_class_for(type_name)

  unless subclass == self || descendants.include?(subclass)
    raise SubclassNotFound, "Invalid single-table inheritance type: #{subclass.name} is not a subclass of #{name}"
  end

  subclass
end
subclass_from_attributes(attrs) click to toggle source

Detect the subclass from the inheritance column of attrs. If the inheritance column value is not self or a valid subclass, raises ActiveRecord::SubclassNotFound

# File lib/active_record/inheritance.rb, line 277
def subclass_from_attributes(attrs)
  attrs = attrs.to_h if attrs.respond_to?(:permitted?)
  if attrs.is_a?(Hash)
    subclass_name = attrs[inheritance_column] || attrs[inheritance_column.to_sym]

    if subclass_name.present?
      find_sti_class(subclass_name)
    end
  end
end
type_condition(table = arel_table) click to toggle source
# File lib/active_record/inheritance.rb, line 268
def type_condition(table = arel_table)
  sti_column = table[inheritance_column]
  sti_names  = ([self] + descendants).map(&:sti_name)

  predicate_builder.build(sti_column, sti_names)
end
using_single_table_inheritance?(record) click to toggle source
# File lib/active_record/inheritance.rb, line 253
def using_single_table_inheritance?(record)
  record[inheritance_column].present? && _has_attribute?(inheritance_column)
end