adds ActiveRecord 5.0.0.beta1 support for tree roots; fixes #195

AR 5 now defaults a `belongs_to` relation to be required.  This does
not work in a tree where the root will not have a `belongs_to :parent` relationship.
 To manage this change, AR 5 adds an `optional: true` key, which I have added here.
 However, this key is not allowed in versions prior to AR 5 and will raise an exception,
 so I've added a method that checks the version and strips the `optional` key if necessary.

This will maintain backwards compatibility with AR 4 without changing any
functionality.

Author: jayfredlund

lib/closure_tree/model.rb

@@ -5,11 +5,13 @@ module Model
     extend ActiveSupport::Concern
 
     included do
-      belongs_to :parent,
-                 class_name: _ct.model_class.to_s,
-                 foreign_key: _ct.parent_column_name,
-                 inverse_of: :children,
-                 touch: _ct.options[:touch]
+
+      belongs_to :parent, nil, *_ct.belongs_to_with_optional_option(
+        class_name: _ct.model_class.to_s,
+        foreign_key: _ct.parent_column_name,
+        inverse_of: :children,
+        touch: _ct.options[:touch],
+        optional: true)
 
       order_by_generations = "#{_ct.quoted_hierarchy_table_name}.generations asc"
 

lib/closure_tree/support.rb

@@ -76,6 +76,10 @@ def scope_with_order(scope, additional_order_by = nil)
       end
     end
 
+    def belongs_to_with_optional_option(opts)
+      [ActiveRecord::VERSION::MAJOR < 5 ? opts.except(:optional) : opts]
+    end
+
     # lambda-ize the order, but don't apply the default order_option
     def has_many_without_order_option(opts)
         [lambda { order(opts[:order]) }, opts.except(:order)]