Add Units

app/models/unit.rb

@@ -0,0 +1,26 @@
+class Unit < ApplicationRecord
+  attribute :multiplier, default: 1
+
+  belongs_to :user, optional: true
+  belongs_to :base, optional: true, class_name: "Unit"
+
+  validates :symbol, presence: true, uniqueness: {scope: :user_id}
+  validates :multiplier, numericality: {equal_to: 1}, unless: :base
+  validates :multiplier, numericality: {other_than: 1}, if: :base
+  validate if: -> { base.present? } do
+    errors.add(:base, :only_top_level_base_units) unless base.base.nil?
+  end
+
+  acts_as_nested_set parent_column: :base_id, scope: :user, dependent: :destroy, order_column: :multiplier
+
+  scope :defaults, -> { where(user: nil) }
+
+  after_save if: :base do |record|
+    record.move_to_ordered_child_of(record.base, :multiplier)
+  end
+
+  before_destroy do
+    # TODO: disallow destruction if any object depends on this unit
+    nil
+  end
+end