What Is Mattr_Accessor in a Rails Module

What is mattr_accessor in a Rails module?

Rails extends Ruby with both mattr_accessor (Module accessor) and cattr_accessor (as well as _reader/_writer versions). As Ruby's attr_accessor generates getter/setter methods for instances, cattr/mattr_accessor provide getter/setter methods at the class or module level. Thus:

module Config
  mattr_accessor :hostname
  mattr_accessor :admin_email
end

is short for:

module Config
  def self.hostname
    @hostname
  end
  def self.hostname=(hostname)
    @hostname = hostname
  end
  def self.admin_email
    @admin_email
  end
  def self.admin_email=(admin_email)
    @admin_email = admin_email
  end
end

Both versions allow you to access the module-level variables like so:

>> Config.hostname = "example.com"
>> Config.admin_email = "admin@example.com"
>> Config.hostname # => "example.com"
>> Config.admin_email # => "admin@example.com"

Difference between mattr_accessor and cattr_accessor in ActiveSupport?

Module is the superclass of the class Class so if a suitably generic name could be thought of then the methods for defining accessors could be put on Module and it would work for modules and classes. Notice that the follow works:

class Bar
  mattr_accessor :test
end

but

module Foo
  cattr_accessor :test
end

wouldn't work.

Having a c prefix on the methods that should be used inside classes and an m prefix on the methods for use inside modules just helps to make your code a bit clearer.

Rails plugin: undefined method 'mattr_accessor' for Sentry:Module

I'm afraid this resolved itself while I was working on other issues. I'm not sure exactly what the problem was, but I believe it had to do with outdated/conflicting versions of plugins and gems.

Rails: thread_mattr_accessor randomly becoming `nil` in development

Module Test

module Test
  thread_mattr_accessor :max_installments
  self.max_installments = 12
end

Normal use

puts "max_installments is: #{Test.max_installments.inspect}"
# max_installments is: 12

In Thread use

Thread.new { puts "max_installments in thread is #{Test.max_installments.inspect}" }
# max_installments in thread is nil

Set value inside the thread

Thread.new do
  Test.max_installments = 99
  puts "max_installments in thread now is #{Test.max_installments.inspect}"
end
# max_installments in thread now is 99

Thread.new { puts "max_installments in another thread is #{Test.max_installments.inspect}" }
# max_installments in another thread is nil

puts "max_installments is: #{Test.max_installments}"
# max_installments is: 12

ASAP this commit must be released and we can set a default value to use in threads like this

module Test
  thread_mattr_accessor :max_installments, default: 12
  self.max_installments = 12
end

For now, if you go use inside threads, you need set value before to avoid get nil.

Install generator won't see a method I defined

Turns out when you try requiring mongoid_forums in pry, you'll see that an error involving decorators occurs. The issue is fixed here in my pull request to decorators: parndt/decorators#13

It's because of the way files are required and how load! is called over there.

Waiting on PR status, that is the same version that radar/forem uses as well.

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