code: def generate_migration_body(tables)
        current_tables, new_tables = table_names(tables).partition do |table_name| 
          @db_table_names.include?(table_name)
        end
  
        add_line "change do"
        create_new_tables(new_tables, tables)
        alter_tables(current_tables, tables)
        add_line "end"
      end
docs: Generates the 'change' block of the migration.