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: 20220214175650_media_cleanup.go
    path of file: ./repos/gotosocial/internal/db/bundb/migrations
    
    the code of the file until where you have to start completion: // GoToSocial
// Copyright (C) GoToSocial Authors admin@gotosocial.org
// SPDX-License-Identifier: AGPL-3.0-or-later
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

package migrations

import (
	"context"
	"database/sql"
	"time"

	previousgtsmodel "github.com/superseriousbusiness/gotosocial/internal/db/bundb/migrations/20211113114307_init"
	newgtsmodel "github.com/superseriousbusiness/gotosocial/internal/db/bundb/migrations/20220214175650_media_cleanup"
	"github.com/uptrace/bun"
)

func init() {
	const batchSize = 100
	up := func(ctx context.Context, db *bun.DB) error {
		// we need to migrate media attachments into a new table
		// see section 6 here: https://www.sqlite.org/lang_altertable.html

		return db.RunInTx(ctx, nil, func(ctx context.Context, tx bun.Tx) error {
			// create the new media attachments table
			if _, err := tx.
				NewCreateTable().
				ModelTableExpr("new_media_attachments").
				Model(&newgtsmodel.MediaAttachment{}).
				IfNotExists().
				Exec(ctx); err != nil {
				return err
			}

			offset := time.Now()
			// migrate existing media attachments into new table
		migrateLoop:
			for {
				oldAttachments := []*previousgtsmodel.MediaAttachment{}
				err := tx.
					NewSelect().
					Model(&oldAttachments).
					// subtract a millisecond from the offset just to make sure we're not getting double entries (this happens sometimes)
					Where("media_attachment.created_at < ?", offset.Add(-1*time.Millisecond)).
					Order("media_attachment.created_at DESC").
					Limit(batchSize).
					Scan(ctx)
				if err != nil && err != sql.ErrNoRows {
					// there's been a real error
					return err
				}

				if err == sql.ErrNoRows || len(oldAttachments) == 0 {
					// we're finished migrating
					break migrateLoop
				}

				// update the offset to the createdAt time of the oldest media attachment in the slice
				offset = oldAttachments[len(oldAttachments)-1].CreatedAt

				// for every old attachment, we need to make a new attachment out of it by taking the same values
				newAttachments := []*newgtsmodel.MediaAttachment{}
				for _, old := range oldAttachments {
					new := &newgtsmodel.MediaAttachment{
						ID:        old.ID,
						CreatedAt: old.CreatedAt,
						UpdatedAt: old.UpdatedAt,
						StatusID:  old.StatusID,
						URL:       old.URL,
						RemoteURL: old.RemoteURL,
						Type:      newgtsmodel.FileType(old.Type),
						FileMeta: newgtsmodel.FileMeta{
							Original: newgtsmodel.Original{
								Width:  old.FileMeta.Original.Width,
								Height: old.FileMeta.Original.Height,
								Size:   old.FileMeta.Original.Size,
								Aspect: old.FileMeta.Original.Aspect,
							},
							Small: newgtsmodel.Small{
								Width:  old.FileMeta.Small.Width,
								Height: old.FileMeta.Small.Height,
								Size:   old.FileMeta.Small.Size,
								Aspect: old.FileMeta.Small.Aspect,
							},
							Focus: newgtsmodel.Focus{
								X: old.FileMeta.Focus.X,
								Y: old.FileMeta.Focus.Y,
							},
						},
						AccountID:         old.AccountID,
						Description:       old.Description,
						ScheduledStatusID: old.ScheduledStatusID,
						Blurhash:          old.Blurhash,
						Processing:        newgtsmodel.ProcessingStatus(old.Processing),
						File: newgtsmodel.File{
							Path:        old.File.Path,
							ContentType: old.File.ContentType,
							FileSize:    old.File.FileSize,
							UpdatedAt:   old.File.UpdatedAt,
						},
						Thumbnail: newgtsmodel.Thumbnail{
							Path:        old.Thumbnail.Path,
							ContentType: old.Thu
    
func init() {
	const batchSize = 100
	up := func(ctx context.Context, db *bun.DB) error {
		// we need to migrate media attachments into a new table
		// see section 6 here: https://www.sqlite.org/lang_altertable.html

		return db.RunInTx(ctx, nil, func(ctx context.Context, tx bun.Tx) error {
			// create the new media attachments table
			if _, err := tx.
				NewCreateTable().
				ModelTableExpr("new_media_attachments").
				Model(&newgtsmodel.MediaAttachment{}).
				IfNotExists().
				Exec(ctx); err != nil {
				return err
			}

			offset := time.Now()
			// migrate existing media attachments into new table
		migrateLoop:
			for {
				oldAttachments := []*previousgtsmodel.MediaAttachment{}
				err := tx.
					NewSelect().
					Model(&oldAttachments).
					// subtract a millisecond from the offset just to make sure we're not getting double entries (this happens sometimes)
					Where("media_attachment.created_at < ?", offset.Add(-1*time.Millisecond)).
					Order("media_attachment.created_at DESC").
					Limit(batchSize).
					Scan(ctx)
				if err != nil && err != sql.ErrNoRows {
					// there's been a real error
					return err
				}

				if err == sql.ErrNoRows || len(oldAttachments) == 0 {
					// we're finished migrating
					break migrateLoop
				}

				// update the offset to the createdAt time of the oldest media attachment in the slice
				offset = oldAttachments[len(oldAttachments)-1].CreatedAt

				// for every old attachment, we need to make a new attachment out of it by taking the same values
				newAttachments := []*newgtsmodel.MediaAttachment{}
				for _, old := range oldAttachments {
					new := &newgtsmodel.MediaAttachment{
						ID:        old.ID,
						CreatedAt: old.CreatedAt,
						UpdatedAt: old.UpdatedAt,
						StatusID:  old.StatusID,
						URL:       old.URL,
						RemoteURL: old.RemoteURL,
						Type:      newgtsmodel.FileType(old.Type),
						FileMeta: newgtsmodel.FileMeta{
							Original: newgtsmodel.Original{
								Width:  old.FileMeta.Original.Width,
								Height: old.FileMeta.Original.Height,
								Size:   old.FileMeta.Original.Size,
								Aspect: old.FileMeta.Original.Aspect,
							},
							Small: newgtsmodel.Small{
								Width:  old.FileMeta.Small.Width,
								Height: old.FileMeta.Small.Height,
								Size:   old.FileMeta.Small.Size,
								Aspect: old.FileMeta.Small.Aspect,
							},
							Focus: newgtsmodel.Focus{
								X: old.FileMeta.Focus.X,
								Y: old.FileMeta.Focus.Y,
							},
						},
						AccountID:         old.AccountID,
						Description:       old.Description,
						ScheduledStatusID: old.ScheduledStatusID,
						Blurhash:          old.Blurhash,
						Processing:        newgtsmodel.ProcessingStatus(old.Processing),
						File: newgtsmodel.File{
							Path:        old.File.Path,
							ContentType: old.File.ContentType,
							FileSize:    old.File.FileSize,
							UpdatedAt:   old.File.UpdatedAt,
						},
						Thumbnail: newgtsmodel.Thumbnail{
							Path:        old.Thumbnail.Path,
							ContentType: old.Thumbnail.ContentType,
							FileSize:    old.Thumbnail.FileSize,
							UpdatedAt:   old.Thumbnail.UpdatedAt,
							URL:         old.Thumbnail.URL,
							RemoteURL:   old.Thumbnail.RemoteURL,
						},
						Avatar: old.Avatar,
						Header: old.Header,
						Cached: true,
					}
					newAttachments = append(newAttachments, new)
				}

				// insert this batch of new attachments, and then continue the loop
				if _, err := tx.
					NewInsert().
					Model(&newAttachments).
					ModelTableExpr("new_media_attachments").
					Exec(ctx); err != nil {
					return err
				}
			}

			// we have all the data we need from the old table, so we can safely drop it now
			if _, err := tx.NewDropTable().Model(&previousgtsmodel.MediaAttachment{}).Exec(ctx); err != nil {
				return err
			}

			// rename the new table to the same name as the old table was
			if _, err := tx.ExecContext(ctx, "ALTER TABLE new_media_attachments RENAME TO media_attachments;"); err != nil {
				return err
			}

			// add an index to the new table
			if _, err := tx.
				NewCreateIndex().
				Model(&newgtsmodel.MediaAttachment{}).
				Index("media_attachments_id_idx").
				Column("id").
				Exec(ctx); err != nil {
				return err
			}

			return nil
		})
	}

	down := func(ctx context.Context, db *bun.DB) error {
		return db.RunInTx(ctx, nil, func(ctx context.Context, tx bun.Tx) error {
			return nil
		})
	}

	if err := Migrations.Register(up, down); err != nil {
		panic(err)
	}
}