Delete Units respecting foreign key constraint

db/seeds.rb

@@ -20,6 +20,4 @@ end
 # Formulas will be deleted as dependent on Quantities
 #[Source, Quantity, Unit].each { |model| model.defaults.delete_all }
 
-# To clear contents of the table, use #truncate instead of #delete_all. This
-# avoids foreign_key constraints errors.
 require_relative 'seeds/units.rb'

db/seeds/templates/units.erb

@@ -1,5 +1,5 @@
 Unit.transaction do
-  ActiveRecord::Base.connection.truncate(Unit.table_name)
+  Unit.defaults.order(Unit.arel_table[:base_id].eq(nil)).delete_all
   units = {}
 <% Unit.defaults.ordered.each do |unit| %>
 <%= "\n" if unit.base.nil? %>

db/seeds/units.rb

@@ -1,5 +1,6 @@
 Unit.transaction do
-  ActiveRecord::Base.connection.truncate(Unit.table_name)
+  Unit.defaults.order(Unit.arel_table[:base_id].eq(nil)).delete_all
+
   units = {}
 
   units['1'] =