Simplify object and association checks

* check for <object> instead of <object>.nil?
* check for <association>_id? instead of <association>.nil? (avoids
  record loading)

app/models/quantity.rb

@@ -7,8 +7,8 @@ class Quantity < ApplicationRecord
     dependent: :restrict_with_error
 
   validate if: ->{ parent.present? } do
-    errors.add(:parent, :user_mismatch) unless user == parent.user
-    errors.add(:parent, :self_reference) if self == parent
+    errors.add(:parent, :user_mismatch) unless user_id == parent.user_id
+    errors.add(:parent, :self_reference) if id == parent_id
   end
   validate if: ->{ parent.present? }, on: :update do
     errors.add(:parent, :descendant_reference) if ancestor_of?(parent)
@@ -68,7 +68,7 @@ class Quantity < ApplicationRecord
       numbered.project(
         numbered[Arel.star],
         numbered.cast(numbered[:child_number], 'BINARY').as('path'),
-      ).where(root.nil? ? numbered[:parent_id].eq(nil) : numbered[:id].eq(root.id)),
+      ).where(root ? numbered[:id].eq(root.id) : numbered[:parent_id].eq(nil)),
       numbered.project(
         numbered[Arel.star],
         arel_table[:path].concat(numbered[:child_number]),

app/models/unit.rb

@@ -6,9 +6,9 @@ class Unit < ApplicationRecord
   has_many :subunits, class_name: "Unit", inverse_of: :base, dependent: :restrict_with_error
 
   validate if: ->{ base.present? } do
-    errors.add(:base, :user_mismatch) unless user == base.user
-    errors.add(:base, :self_reference) if self == base
-    errors.add(:base, :multilevel_nesting) if base.base.present?
+    errors.add(:base, :user_mismatch) unless user_id == base.user_id
+    errors.add(:base, :self_reference) if id == base_id
+    errors.add(:base, :multilevel_nesting) if base.base_id?
   end
   validates :symbol, presence: true, uniqueness: {scope: :user_id},
     length: {maximum: type_for_attribute(:symbol).limit}