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: spanner_update_with_history.go
    path of file: ./repos/golang-samples/spanner/spanner_snippets/spanner
    
    the code of the file until where you have to start completion: // Copyright 2020 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package spanner

import (
	"context"
	"io"

	"cloud.google.com/go/spanner"
)

func updateWithHistory(w io.Writer, db string) error {
	ctx := context.Background()
	client, err := spanner.NewClient(ctx, db)
	if err != nil {
		return err
	}
	defer client.Close()

	_, err = client.ReadWriteTransaction(ctx, func(ctx context.Context, txn *spanner.ReadWriteTransaction) error {
		// Create anonymous function "getContents" to read the current value of the Contents column for a given row.
		getContents := func(key spanner.Key) (string, error) {
			row, err := txn.ReadRow(ctx, "Documents", key, []string{"Contents"})
			if err != nil {
				return "", err
			}
			var content string
			if err := row.Column(0, &content); err != nil {
				return "", err
			}
			return content, nil
		}
		// Create two string arrays corresponding to the columns in each table
    
func updateWithHistory(w io.Writer, db string) error {
	ctx := context.Background()
	client, err := spanner.NewClient(ctx, db)
	if err != nil {
		return err
	}
	defer client.Close()

	_, err = client.ReadWriteTransaction(ctx, func(ctx context.Context, txn *spanner.ReadWriteTransaction) error {
		// Create anonymous function "getContents" to read the current value of the Contents column for a given row.
		getContents := func(key spanner.Key) (string, error) {
			row, err := txn.ReadRow(ctx, "Documents", key, []string{"Contents"})
			if err != nil {
				return "", err
			}
			var content string
			if err := row.Column(0, &content); err != nil {
				return "", err
			}
			return content, nil
		}
		// Create two string arrays corresponding to the columns in each table.
		documentsColumns := []string{"UserId", "DocumentId", "Contents"}
		documentHistoryColumns := []string{"UserId", "DocumentId", "Timestamp", "PreviousContents"}
		// Get row's Contents before updating.
		previousContents, err := getContents(spanner.Key{1, 1})
		if err != nil {
			return err
		}
		// Update row's Contents while saving previous Contents in DocumentHistory table.
		txn.BufferWrite([]*spanner.Mutation{
			spanner.InsertOrUpdate("Documents", documentsColumns,
				[]interface{}{1, 1, "Hello World 1 Updated"}),
			spanner.InsertOrUpdate("DocumentHistory", documentHistoryColumns,
				[]interface{}{1, 1, spanner.CommitTimestamp, previousContents}),
		})
		previousContents, err = getContents(spanner.Key{1, 3})
		if err != nil {
			return err
		}
		txn.BufferWrite([]*spanner.Mutation{
			spanner.InsertOrUpdate("Documents", documentsColumns,
				[]interface{}{1, 3, "Hello World 3 Updated"}),
			spanner.InsertOrUpdate("DocumentHistory", documentHistoryColumns,
				[]interface{}{1, 3, spanner.CommitTimestamp, previousContents}),
		})
		previousContents, err = getContents(spanner.Key{2, 5})
		if err != nil {
			return err
		}
		txn.BufferWrite([]*spanner.Mutation{
			spanner.InsertOrUpdate("Documents", documentsColumns,
				[]interface{}{2, 5, "Hello World 5 Updated"}),
			spanner.InsertOrUpdate("DocumentHistory", documentHistoryColumns,
				[]interface{}{2, 5, spanner.CommitTimestamp, previousContents}),
		})
		return nil
	})
	return err
}