Is there a way to override the << operator in Ruby?

Question

I'm trying to do something like:

account.users << User.new

But I need users to be a method on an account. So I've tried things like:

def users<<(obj)

But I've had no luck with that. Is this even possible to do in Ruby? I would assume so because the ActiveRecord relationships seem to work this way in Rails.

Is this `users` Rails association? – lulalala Dec 14 '12 at 06:22 — lulalala, Dec 14 '12 at 06:22

score 10 · Answer 1 · edited May 23 '17 at 11:54

10

Check this answer: Rails: Overriding ActiveRecord association method

[this code is completely from the other answer, here for future searchers]

has_many :tags, :through => :taggings, :order => :name do
    def << (value)
      "overriden" #your code here
    end     
  end

edited May 23 '17 at 11:54

Community

1
1

answered Mar 02 '12 at 16:55

Jesse Wolgamott

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score 5 · Answer 2 · answered Mar 02 '12 at 16:37

5

It seems like you might not be describing your actual problem, but to answer your question -- yes you can override the << operator:

class Foo
  def <<(x)
    puts "hi! #{x}"
  end
end

f = Foo.new
=> #<Foo:0x00000009b389f0>
> f << "there"
hi! there

answered Mar 02 '12 at 16:37

muffinista

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I don't want to use the Foo class though. I want to user foo.users << x – Matthew Stopa Mar 02 '12 at 16:49
@JohnBaker, `foo.users << obj` is equivalent to `foo.users.send :< – Matheus Moreira Mar 02 '12 at 17:42

Miscreant · Answer 3 · 2012-12-18T21:54:44.990

I assume you have a model like this:

class Account < ActiveRecord::Base
  has_and_belongs_to_many :users
end

To override Account#users<<, you need to define it in a block that you pass to has_and_belongs_to_many:

class Account < ActiveRecord::Base
  has_and_belongs_to_many :users do
    def <<(user)
      # ...
    end
  end
end

You can access the appropriate Account object by referring to proxy_association.owner:

def <<(user)
  account = proxy_association.owner
end

To call the original Account#users<<, call Account#users.concat:

def <<(user)
  account = proxy_association.owner
  # user = do_something(user)
  account.users.concat(user)
end

For more details, see this page: Association extensions - ActiveRecord

score 1 · Answer 4 · answered Mar 02 '12 at 16:36

In this case it's the << of you class of you User. So can be an Array or a AssociationProxy.

The must simplest is create a new method to do what you want.

You can override the method by instance instead.

account.users.instance_eval do
  def <<(x)
    put 'add'
  end
end

account.users << User.new
# add

But you need do that all the time before you add by <<

score 0 · Answer 5 · answered Mar 02 '12 at 16:37

0

users would return an object that has overridden << operator like Array, IO, String, or any type you create. You override like this:

class SomeType
  def <<(obj)
    puts "Appending #{obj}"
  end
end

answered Mar 02 '12 at 16:37

Linuxios

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score 0 · Answer 6 · answered Mar 03 '12 at 02:19

If you are trying to perform an action upon adding an User to the users collection, you can use association callbacks instead of over-riding <<(as there are many ways to add an object to an association).

class Account
  has_many :users, :after_add => :on_user_add

  def on_user_add(user)
    p "Added user : #{user.name} to the account: #{name}"
  end
end

Is there a way to override the << operator in Ruby?

6 Answers6