You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: 20230505165821_variant_options_columns.rb
    path of file: ./repos/openfoodnetwork/db/migrate
    
    the code of the file until where you have to start completion: class VariantOptionsColumns < ActiveRecord::Migration[7.0]
  def up
    add_column :spree_variants, :variant_unit, :
    
  def migrate_line_item_presentation
    ActiveRecord::Base.connection.execute(<<-SQL
      UPDATE spree_line_items
      SET unit_presentation = option_values.presentation
      FROM (
        SELECT 
          DISTINCT ON (spree_option_values_line_items.line_item_id) line_item_id,
          spree_option_values.presentation AS presentation
        FROM spree_option_values_line_items
          LEFT JOIN spree_option_values ON spree_option_values.id = spree_option_values_line_items.option_value_id
      ) option_values
      WHERE spree_line_items.id = option_values.line_item_id
    SQL
    )
  end