Add migration to set sale/purchase reference_number on entry if nil.
user
```ruby
class AddMissingReferenceNumberOnEntries < ActiveRecord::Migration
  def change
    reversible do |dir|
      dir.up do
        # update null entry reference_number from not null sale reference_number
        execute <<-SQL
          UPDATE journal_entries
          SET reference_number = sales.reference_number
          FROM sales
          WHERE journal_entries.resource_id = sales.id
          AND journal_entries.resource_type = 'Sale'
          AND journal_entries.reference_number IS NULL
          AND sales.reference_number IS NOT NULL
        SQL

        # update null entry reference_number from sale number because 'null sale reference_number'
        execute <<-SQL
          UPDATE journal_entries
          SET reference_number = sales.number
          FROM sales
          WHERE journal_entries.resource_id = sales.id
          AND journal_entries.resource_type = 'Sale'
          AND journal_entries.reference_number IS NULL
          AND sales.reference_number IS NULL
        SQL

        # update null entry reference_number from not null purchase reference_number
        execute <<-SQL
          UPDATE journal_entries
          SET reference_number = purchases.reference_number
          FROM purchases
          WHERE journal_entries.resource_id = purchases.id
          AND journal_entries.resource_type = 'Purchase'
          AND journal_entries.reference_number IS NULL
          AND purchases.reference_number IS NOT NULL
        SQL

        # update null entry reference_number from purchase number because 'null purchase reference_number'
        execute <<-SQL
          UPDATE journal_entries
          SET reference_number = purchases.number
          FROM purchases
          WHERE journal_entries.resource_id = purchases.id
          AND journal_entries.resource_type = 'Purchase'
          AND journal_entries.reference_number IS NULL
          AND purchases.reference_number IS NULL
        SQL
      end
    end
  end
end
```
assistant