Find successive Unit according to sort order

This will allow to insert Units into table without refresh of all
items/removal of forms

app/models/unit.rb

@@ -75,11 +75,16 @@ class Unit < ApplicationRecord
       .from(units).group(:base_id, :symbol)
   }
   scope :ordered, ->{
-    left_outer_joins(:base)
+    left_outer_joins(:base).order(ordering)
-      .order(arel_table.coalesce(Arel::Table.new(:bases_units)[:symbol], arel_table[:symbol]),
-             arel_table[:base_id].not_eq(nil), :multiplier, :symbol)
   }
 
+  def self.ordering
+    [arel_table.coalesce(Arel::Table.new(:bases_units)[:symbol], arel_table[:symbol]),
+     arel_table[:base_id].not_eq(nil),
+     :multiplier,
+     :symbol]
+  end
+
   before_destroy do
     # TODO: disallow destruction if any object depends on this unit
     nil
@@ -104,4 +109,14 @@ class Unit < ApplicationRecord
     Unit.find_or_initialize_by(user: recipient, symbol: symbol)
       .update!(base: recipient_base, **params)
   end
+
+  def successive
+    units = Unit.arel_table
+    lead = Arel::Nodes::NamedFunction.new('LAG', [units[:id]])
+    window = Arel::Nodes::Window.new.order(*Unit.ordering)
+    lag_id = lead.over(window).as('lag_id')
+    Unit.with(
+      units: user.units.left_outer_joins(:base).select(units[Arel.star], lag_id)
+    ).where(units[:lag_id].eq(id)).first
+  end
 end