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: 20230305204742_add_observation_user_foreign_key.rb
    path of file: ./repos/falling-fruit/db/migrate
    
    the code of the file until where you have to start completion: class AddObservationUserForeignKey < ActiveRecord::Migration
  def up
    execute <<-SQL
      alter table obser
    
  def up
    execute <<-SQL
      alter table observations drop constraint if exists observations_user_id_fkey;
      alter table observations
      add constraint observations_user_id_fkey
      foreign key (user_id) references users (id) on delete set null;
    SQL
  end