mattr_accessor

mattr_accessor(*syms, &blk)

Instance Public methods

Defines both class and instance accessors for class attributes.

module HairColors
  mattr_accessor :hair_colors
end
 
class Person
  include HairColors
end
 
Person.hair_colors = [:brown, :black, :blonde, :red]
Person.hair_colors     # => [:brown, :black, :blonde, :red]
Person.new.hair_colors # => [:brown, :black, :blonde, :red]

If a subclass changes the value then that would also change the value for parent class. Similarly if parent class changes the value then that would change the value of subclasses too.

class Male < Person
end
 
Male.hair_colors << :blue
Person.hair_colors # => [:brown, :black, :blonde, :red, :blue]

To opt out of the instance writer method, pass instance_writer: false. To opt out of the instance reader method, pass instance_reader: false.

module HairColors
  mattr_accessor :hair_colors, instance_writer: false, instance_reader: false
end
 
class Person
  include HairColors
end
 
Person.new.hair_colors = [:brown]  # => NoMethodError
Person.new.hair_colors             # => NoMethodError

Or pass instance_accessor: false, to opt out both instance methods.

module HairColors
  mattr_accessor :hair_colors, instance_accessor: false
end
 
class Person
  include HairColors
end
 
Person.new.hair_colors = [:brown]  # => NoMethodError
Person.new.hair_colors             # => NoMethodError

Also you can pass a block to set up the attribute with a default value.

module HairColors
  mattr_accessor :hair_colors do
    [:brown, :black, :blonde, :red]
  end
end
 
class Person
  include HairColors
end
 
Person.class_variable_get("@@hair_colors") #=> [:brown, :black, :blonde, :red]

cattr_accessor

Links:

https://github.com/rails/rails/blob/c758093eca303704f52b45fd0660e13600b9b315/activesupport/lib/active_support/core_ext/module/attribute_accessors.rb#L207

doc_ruby_on_rails

2025-01-10 15:47:30

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