Fix warnings in Rails 5.1 about using dirty tracking

lib/ancestry/instance_methods.rb

@@ -78,7 +78,13 @@ def child_ancestry
       # New records cannot have children
       raise Ancestry::AncestryException.new('No child ancestry for new record. Save record before performing tree operations.') if new_record?
 
-      if self.send("#{self.ancestry_base_class.ancestry_column}_was").blank? then id.to_s else "#{self.send "#{self.ancestry_base_class.ancestry_column}_was"}/#{id}" end
+      if ActiveRecord::VERSION::STRING >= '5.1.0'
+        ancestry_column_was = self.attribute_in_database(self.ancestry_base_class.ancestry_column)
+      else
+        ancestry_column_was = self.send("#{self.ancestry_base_class.ancestry_column}_was")
+      end
+
+      ancestry_column_was.blank? ? id.to_s : "#{ancestry_column_was}/#{id}"
     end
 
     # Ancestors