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: op_drop_not_null_test.go
    path of file: ./repos/pgroll/pkg/migrations
    
    the code of the file until where you have to start completion: // SPDX-License-Identifier: Apache-2.0

package migrations_test

import (
	"database/sql"
	"testing"

	"github.com/stretchr/testify/assert"
	"github.com/xataio/pgroll/pkg/migrations"
	"github.com/xataio/pgroll/pkg/testutils"
)

func TestDropNotNull(t *testing.T) {
	t.Parallel()

	ExecuteTests(t, TestCases{
		{
			name: "remove not null with default up sql",
			migrations: []migrations.Migration{
				{
					Name: "01_add_table",
					Operations: migrations.Operations{
						&migrations.OpCreateTable{
							Name: "reviews",
							Columns: []migrations.Column{
								{
									Name: "id",
									Type: "serial",
									Pk:   ptr(true),
								},
								{
									Name:     "username",
									Type:     "text",
									Nullable: ptr(false),
								},
								{
									Name:     "product",
									Type:     "text",
									Nullable: ptr(false),
								},
								{
									Name:     "review",
									Type:     "text",
									Nullable: ptr(false),
								},
							},
						},
					},
				},
				{
					Name: "02_set_nullable",
					Operations: migrations.Operations{
						&migrations.OpAlterColumn{
							Table:    "reviews",
							Column:   "review",
							Nullable: ptr(true),
							Down:     ptr("(SELECT CASE WHEN review IS NULL THEN product || ' is good' ELSE review END)"),
						},
					},
				},
			},
			afterStart: func(t *testing.T, db *sql.DB, schema string) {
				// The new (temporary) `review` column should exist on the underlying table.
				ColumnMustExist(t, db, schema, "reviews", migrations.TemporaryName("review"))

				// Inserting a NULL into the new `review` column should succeed
				MustInsert(t, db, schema, "02_set_nullable", "reviews", map[string]string{
					"usernam
    
func TestDropNotNullValidation(t *testing.T) {
	t.Parallel()

	ExecuteTests(t, TestCases{
		{
			name: "down SQL is mandatory",
			migrations: []migrations.Migration{
				{
					Name: "01_add_table",
					Operations: migrations.Operations{
						&migrations.OpCreateTable{
							Name: "users",
							Columns: []migrations.Column{
								{
									Name: "id",
									Type: "serial",
									Pk:   ptr(true),
								},
								{
									Name:     "name",
									Type:     "text",
									Nullable: ptr(false),
								},
							},
						},
					},
				},
				{
					Name: "02_set_nullable",
					Operations: migrations.Operations{
						&migrations.OpAlterColumn{
							Table:    "users",
							Column:   "name",
							Nullable: ptr(true),
							Up:       ptr("name"),
						},
					},
				},
			},
			wantStartErr: migrations.FieldRequiredError{Name: "down"},
		},
		{
			name: "can't remove not null from a column that is already nullable",
			migrations: []migrations.Migration{
				{
					Name: "01_add_table",
					Operations: migrations.Operations{
						&migrations.OpCreateTable{
							Name: "users",
							Columns: []migrations.Column{
								{
									Name: "id",
									Type: "serial",
									Pk:   ptr(true),
								},
								{
									Name:     "name",
									Type:     "text",
									Nullable: ptr(true),
								},
							},
						},
					},
				},
				{
					Name: "02_set_nullable",
					Operations: migrations.Operations{
						&migrations.OpAlterColumn{
							Table:    "users",
							Column:   "name",
							Nullable: ptr(true),
							Up:       ptr("name"),
							Down:     ptr("(SELECT CASE WHEN name IS NULL THEN 'placeholder' ELSE name END)"),
						},
					},
				},
			},
			wantStartErr: migrations.ColumnIsNullableError{Table: "users", Name: "name"},
		},
	})
}