Ensure that associations have a symbol argument.

Fixes #7418.

activerecord/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Ensure that associations take a symbol argument. *Steve Klabnik*
+
 *   Fix dirty attribute checks for `TimeZoneConversion` with nil and blank
     datetime attributes. Setting a nil datetime to a blank string should not
     result in a change being flagged. Fix #8310

activerecord/lib/active_record/associations/builder/association.rb

@@ -13,6 +13,8 @@ def self.build(*args, &block)
     end
 
     def initialize(model, name, scope, options)
+      raise ArgumentError, "association names must be a Symbol" unless name.kind_of?(Symbol)
+
       @model   = model
       @name    = name
 

activerecord/test/cases/associations_test.rb

@@ -289,6 +289,14 @@ def test_has_one_association_redefinition_reflections_should_differ_and_not_inhe
       DifferentPeopleList.reflect_on_association(:has_one)
     )
   end
+
+  def test_requires_symbol_argument
+    assert_raises ArgumentError do 
+      Class.new(Post) do
+        belongs_to "author"
+      end
+    end
+  end
 end
 
 class GeneratedMethodsTest < ActiveRecord::TestCase