module ActiveRecord::Inheritance::ClassMethods

Attributes

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 behavior:

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.

base_class [R]

Returns the first class in the inheritance hierarchy that descends from either an abstract class or from ActiveRecord::Base.

Consider the following behaviour:

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

ApplicationRecord.base_class # => ApplicationRecord
Shape.base_class # => Shape
Polygon.base_class # => Polygon
Square.base_class # => Polygon

Public Instance Methods

abstract_class? ()

Returns whether this class is an abstract class or not.

# File activerecord/lib/active_record/inheritance.rb, line 167
def abstract_class?
  @abstract_class == true
end

base_class? ()

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

# File activerecord/lib/active_record/inheritance.rb, line 119
def base_class?
  base_class == self
end

descends_from_active_record? ()

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

# File activerecord/lib/active_record/inheritance.rb, line 82
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

new (attributes = nil, &block)

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 activerecord/lib/active_record/inheritance.rb, line 56
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)

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 activerecord/lib/active_record/inheritance.rb, line 218
def polymorphic_class_for(name)
  if store_full_class_name
    name.constantize
  else
    compute_type(name)
  end
end

polymorphic_name ()

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

# File activerecord/lib/active_record/inheritance.rb, line 211
def polymorphic_name
  store_full_class_name ? base_class.name : base_class.name.demodulize
end

primary_abstract_class ()

Sets the application record class for Active Record

This is useful if your application uses a different class than ApplicationRecord for your primary abstract class. This class will share a database connection with Active Record. It is the class that connects to your primary database.

# File activerecord/lib/active_record/inheritance.rb, line 177
def primary_abstract_class
  if ActiveRecord.application_record_class && ActiveRecord.application_record_class.name != name
    raise ArgumentError, "The `primary_abstract_class` is already set to #{ActiveRecord.application_record_class.inspect}. There can only be one `primary_abstract_class` in an application."
  end

  self.abstract_class = true
  ActiveRecord.application_record_class = self
end

sti_class_for (type_name)

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 activerecord/lib/active_record/inheritance.rb, line 194
def sti_class_for(type_name)
  if store_full_sti_class && store_full_class_name
    type_name.constantize
  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. " \
    "If you wish to disable single-table inheritance for #{name} set " \
    "#{name}.inheritance_column to nil"
end

sti_name ()

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

# File activerecord/lib/active_record/inheritance.rb, line 187
def sti_name
  store_full_sti_class && store_full_class_name ? name : name.demodulize
end

Protected Instance Methods

compute_type (type_name)

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 activerecord/lib/active_record/inheritance.rb, line 242
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.
    type_name.constantize
  else
    type_candidate = @_type_candidates_cache[type_name]
    if type_candidate && type_constant = type_candidate.safe_constantize
      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 = candidate.safe_constantize
      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