ModelAttribute: Make `@@all` a class instance variable

Robert argues that the class variable is not ideal because its value
gets shared with subclasses, violating the Liskov substitution
principle:

    17:44:23 < robert> ```133] pry(main)> class A; @@greeting = :hello end
    17:44:23 < robert> => :hello
    17:44:23 < robert> [134] pry(main)> class B < A; end
    17:44:23 < robert> => nil
    17:44:23 < robert> [135] pry(main)> B.class_variable_get '@@greeting'
    17:44:23 < robert> => :hello

Instead, go with his suggestion to use a class instance variable
instead, which gives us the same amount of availability of the `all`
list while not sharing it indiscriminately and making it easier to test.

Refs #4401

1 files changed, 5 insertions, 5 deletions

diff --git a/lib/model_attribute.rb b/lib/model_attribute.rb
index 22c366b37..98bb54b67 100644
--- a/lib/model_attribute.rb
+++ b/lib/model_attribute.rb
@@ -1,12 +1,12 @@
 class ModelAttribute
-  cattr_reader :all
-
-  @@all = []
-
   attr_reader :klass, :name, :data_type
 
+  def self.all
+    @__all__ ||= []
+  end
+
   def self.define(klass, name, data_type)
-    @@all << new(klass, name, data_type)
+    all << new(klass, name, data_type)
   end
 
   def initialize(klass, name, data_type)