Order attributes by association if it exists

Adding an order clause to the relation for an associated
attribute was having no effect on the dashboard's sorting.
When the order is applied, the current relation will check
if the attribute is an association and reorder by count
on the attribute's id column or on the relation's
{attribute}_id column following the Rails convention
of naming FK columns. The relation order method was
changed to reorder to override any default scoping
that may exist for the relation.

thoughtbot#471

lib/administrate/order.rb

@@ -6,11 +6,13 @@ def initialize(attribute = nil, direction = nil)
     end
 
     def apply(relation)
-      if relation.columns_hash.keys.include?(attribute.to_s)
-        relation.order("#{attribute} #{direction}")
-      else
-        relation
-      end
+      return order_by_association(relation) if
+        !reflect_association(relation).nil?
+
+      return relation.reorder("#{attribute} #{direction}") if
+        relation.columns_hash.keys.include?(attribute.to_s)
+
+      relation
     end
 
     def ordered_by?(attr)
@@ -41,5 +43,36 @@ def reversed_direction_param_for(attr)
     def opposite_direction
       direction.to_sym == :asc ? :desc : :asc
     end
+
+    def order_by_association(relation)
+      return order_by_count(relation) if has_many_attribute?(relation)
+
+      return order_by_id(relation) if belongs_to_attribute?(relation)
+
+      relation
+    end
+
+    def order_by_count(relation)
+      relation.
+        left_joins(attribute.to_sym).
+        group(:id).
+        reorder("COUNT(#{attribute}.id) #{direction}")
+    end
+
+    def order_by_id(relation)
+      relation.reorder("#{attribute}_id #{direction}")
+    end
+
+    def has_many_attribute?(relation)
+      reflect_association(relation).macro == :has_many
+    end
+
+    def belongs_to_attribute?(relation)
+      reflect_association(relation).macro == :belongs_to
+    end
+
+    def reflect_association(relation)
+      relation.klass.reflect_on_association(attribute.to_s)
+    end
   end
 end