Set multiplier to 1 for base units

app/models/unit.rb

@@ -1,4 +1,6 @@
 class Unit < ApplicationRecord
+  attribute :multiplier, default: 1
+
   belongs_to :user, optional: true
   # TODO: validate base.user == user
   belongs_to :base, optional: true, class_name: "Unit"
@@ -6,8 +8,8 @@ class Unit < ApplicationRecord
   validates :symbol, presence: true, uniqueness: {scope: :user_id},
     length: {maximum: columns_hash['symbol'].limit}
   validates :name, length: {maximum: columns_hash['name'].limit}
-  validates :multiplier, absence: true, unless: :base
-  validates :multiplier, presence: true, numericality: {other_than: 0}, if: :base
+  validates :multiplier, numericality: {equal_to: 1}, unless: :base
+  validates :multiplier, numericality: {other_than: 0}, if: :base
   validate if: ->{ base&.base.present? } do
     errors.add(:base, :multilevel_nesting)
   end
@@ -16,9 +18,9 @@ class Unit < ApplicationRecord
   scope :ordered, ->{
     parent_symbol = Arel::Nodes::NamedFunction.new(
                       'COALESCE',
-                      [Arel::Table.new(:bases_units)[:symbol], Unit.arel_table[:symbol]]
+                      [Arel::Table.new(:bases_units)[:symbol], arel_table[:symbol]]
                     )
-    left_outer_joins(:base).order(parent_symbol, :multiplier)
+    left_outer_joins(:base).order(parent_symbol, arel_table[:base_id].asc.nulls_first, :multiplier)
   }
 
   before_destroy do