How to declare a class instance variable in Ruby?

Question

I need a class variable which does not get inherited, so I decided to use a class instance variable. Currently I have this code:

class A
  def self.symbols
    history_symbols
  end

  private

  def self.history_tables
    @@history_tables ||= ActiveRecord::Base.connection.tables.select do |x|
      x.starts_with?(SOME_PREFIX)
    end
  end

  def self.history_symbols
    Rails.cache.fetch('history_symbols', expires_in: 10.minutes) do
      history_tables.map { |x| x.sub(SOME_PREFIX, '') }
    end
  end
end

Can I safely transform @@history_tables to @history_tables without braking anything? Currently all my tests pass, but I am still not sure if this could be done just like that.

score 1 · Answer 1 · answered Feb 07 '14 at 14:09

1

Since you wish to use the instance variable, you shall use instance of the class, instead of the singleton methods:

class A
  def symbols
    history_symbols    
  end

  private

  def history_tables
    @history_tables ||= ActiveRecord::Base.connection.tables.select do |x|
      x.starts_with?(SOME_PREFIX)
    end
  end

  def history_symbols
    Rails.cache.fetch('history_symbols', expires_in: 10.minutes) do
      history_tables.map { |x| x.sub(SOME_PREFIX, '') }
    end
  end
end

A.new.symbols

instead of:

A.symbols

answered Feb 07 '14 at 14:09

Малъ Скрылевъ

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Yes, exactly, that was what I was worried about. How do I continue to use the class but still prevent inheritance? – Alexander Popov Feb 07 '14 at 14:10
why prevents? you can do `class B < A; end` – Малъ Скрылевъ Feb 07 '14 at 14:13
If `history_tables` is a class variable and I have `B < A` if I change the value of `history_tables` in B, it would also change in A. I don't want that. – Alexander Popov Feb 07 '14 at 14:15
1

@AlexPopov No, since you have to create a new instance you can't replace it with another. `A.new` you can't change to `B`. but then `B.new` you still can to do `B.new.symbols`, with a new instance variable `history_tables` – Малъ Скрылевъ Feb 07 '14 at 14:20

How to declare a class instance variable in Ruby?

1 Answers1