More helpful error message when instantiating an abstract class

Calling a literal ActiveRecord::Base.new raises NoMethodError,
since it ends up calling Class.abstract_class? which does not exist.
Similarly, instantiating an actual abstract class hits the database,
when conventionally it should immediately throw NotImplementedError.

ActiveRecord::Base can't be made abstract without breaking many,
many things, so check for it separately.

activerecord/CHANGELOG.md

@@ -1,5 +1,9 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Throw `NotImplementedError` when trying to instantiate Base or an abstract class.
+
+    *Aaron Weiner*
+
 *   Postgresql timestamp with time zone (timestamptz) datatype now returns a
     ActiveSupport::TimeWithZone instance instead of a string
 

activerecord/lib/active_record/inheritance.rb

@@ -15,6 +15,9 @@ module ActiveRecord
       # and if the inheritance column is attr accessible, it initializes an
       # instance of the given subclass instead of the base class
       def new(*args, &block)
+        if abstract_class? || self == Base
+          raise NotImplementedError, "#{self} is an abstract class and can not be instantiated."
+        end
         if (attrs = args.first).is_a?(Hash)
           if subclass = subclass_from_attrs(attrs)
             return subclass.new(*args, &block)

activerecord/test/cases/base_test.rb

@@ -307,6 +307,20 @@ class BasicsTest < ActiveRecord::TestCase
     assert_equal("last_read", ex.errors[0].attribute)
   end
 
+  def test_initialize_abstract_class
+    e = assert_raises(NotImplementedError) do
+      FirstAbstractClass.new
+    end
+    assert_equal("FirstAbstractClass is an abstract class and can not be instantiated.", e.message)
+  end
+
+  def test_initialize_base
+    e = assert_raises(NotImplementedError) do
+      ActiveRecord::Base.new
+    end
+    assert_equal("ActiveRecord::Base is an abstract class and can not be instantiated.", e.message)
+  end
+
   def test_create_after_initialize_without_block
     cb = CustomBulb.create(:name => 'Dude')
     assert_equal('Dude', cb.name)