Standup 05/30/2008 - Pivotal Labs

Interesting Things

ActiveRecord’s #method_missing takes precedence over private methods, which means you cannot simply mark “private” database-derived attributes.
code:

# File: app/models/rock_star.rb
#
# == Schema Information
# Schema version: 1
#
# Table name: rock_stars
#
#  id            :integer         not null, primary key
#  real_name     :string(255)
#  band_name     :string(255)
#  personal_life :string(255)
#


class RockStar < ActiveRecord::Base
  def method_missing(method, *arguments, &block)
    puts "I see you've sent my #{method} back and my ActiveRecords and they're all scratched"
    super
  end


  private


  def personal_life=(arg)
    puts "Vanish in the air you'll never find me"
    attributes[:personal_life] = arg
  end
end

script/console:

Loading development environment (Rails 2.0.2)
>> sting = RockStar.new(:real_name => 'Gordon Sumner', :band_name => 'The Police')
I see you've sent my real_name= back and my ActiveRecords and they're all scratched
=> #<RockStar id: nil, real_name: "Gordon Sumner", band_name: "The Police", personal_life: nil>
>> sting.personal_life = "I'll be watching you"
I see you've sent my personal_life= back and my ActiveRecords and they're all scratched
=> "I'll be watching you"

Potential solutions:

Convention… name “private” database attributes with leading underscores

Exception:

class RockStar < ActiveRecord::Base
  def personal_life=(arg)
    raise "Protest is futile, nothing seems to get through"
  end
end

Have another? Post a comment.

||= (”or equal”) blows up you have a public “writer”, but a private “reader”; makes sense, but still worth a mention.
code:

class Model < ActiveRecord::Base
  def field_name=(arg)
    @field_name = arg
  end


  private


  def field_name
    @field_name
  end
end

script/console:

Loading development environment (Rails 2.0.2)
>> instance = Model.new
=> #<Model id: nil, field_name: nil>
>> instance.field_name = 'lala'
=> "lala"
>> instance.field_name ||= 'dodo'
NoMethodError: private method `field_name' called for #<Model:0x17df6d0>
  from /Library/Ruby/Gems/1.8/gems/activerecord-2.0.2/lib/active_record/attribute_methods.rb:205:in `method_missing'
  from (irb):4

ActiveRecord writers always return the passed in argument, even if you define some other return value. This also makes sense — necessary for chaining, etc., but what the heck…
code:

class Model < ActiveRecord::Base
  def field_name=(arg)
    @field_name = arg
    return "custom return value"
  end
end

script/console:

Loading development environment (Rails 2.0.2)
>> instance = Model.new
=> #<Model id: nil, field_name: nil>
>> instance.field_name = 'lala'
=> "lala"

5 Comments

coderrr says:

The last example is due to Ruby, not AR.

class X; def x=(a); return :huh?; end; end

>> X.new.x = 5
=> 5

May 30, 2008 at 8:22 pm

coderrr says:

for the private method issue you can do something like this:

class ActiveRecord::Base
alias_method :old_mm, :method_missing
def method_missing m, *a, &b
raise NoMethodError, “its private!” if self.class.private_method_defined? m

old_mm m, *a, &b
end
end

May 30, 2008 at 9:13 pm

Adam says:

Re: ||= (“or equal”) blows up you have a public “writer”, but a private “reader”

The example you show doesn’t work because the caller doesn’t have access to the callee’s private methods. However, the truly annoying thing about ||= for private accessors is that it doesn’t work even when called from within the class. For example:

class Thing
def public_method
value ||= 1 # sets a local, does not call value=
self.value ||= 1 # error: explicit sender
end

private

attr_accessor :value
end

June 29, 2008 at 8:29 am

Adam says:

Nice of my browser to strip out my carriage returns for me.

June 29, 2008 at 8:30 am

Arthur says:

It’s not only ActiveRecord Writers that return the passed argument, it’s all Ruby methods that end in in equals sign and are called “the default way”.

If you use #send to call a method, you will get the correct return value. Weirdness :)

July 4, 2008 at 8:16 am

Interesting Things

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