List units ordered, not using nested_set

app/models/unit.rb

@@ -15,10 +15,13 @@ class Unit < ApplicationRecord
     order_column: :multiplier
 
   scope :defaults, ->{ where(user: nil) }
-
-  after_save if: :base do |record|
-    record.move_to_ordered_child_of(record.base, :multiplier)
-  end
+  scope :ordered, ->{
+    parent_symbol = Arel::Nodes::NamedFunction.new(
+                      'COALESCE',
+                      [Arel::Table.new(:bases_units)[:symbol], Unit.arel_table[:symbol]]
+                    )
+    left_outer_joins(:base).order(parent_symbol, :multiplier)
+  }
 
   before_destroy do
     # TODO: disallow destruction if any object depends on this unit

app/models/user.rb

@@ -11,7 +11,7 @@ class User < ApplicationRecord
     disabled: 0,    # administratively disallowed to sign in
   }, default: :active
 
-  has_many :units, -> { order :lft }, dependent: :destroy
+  has_many :units, -> { ordered }, dependent: :destroy
 
   def at_least(status)
     User.statuses[self.status] >= User.statuses[status]