private static async Task FillAuthorsAsync(BookDataContext context)
{
// Note that we are jus reading primary keys of books, not entire
// book records. Tip: Always read only those columns that you REALLY need.
var bookIDs = await context.Books.Select(b => b.BookID).ToArrayAsync();
var authors = JsonSerializer.Deserialize <IEnumerable <Author> >(
await File.ReadAllTextAsync("Data/Authors.json"));
using var transaction = context.Database.BeginTransaction();
var rand = new Random();
foreach (var author in authors)
{
//Authors.jsonから読み込んだAuthorデータから
//新しくAuthorインスタンスを作る
//直接authorを使ってはいけない?
//直接authorを使っても良いけれど,
//もしauthorが変更された場合,それがDBに登録されることになる.
//それを避けるためか?
var dbAuthor = new Author
{
AuthorName = author.AuthorName,
Nationality = author.Nationality
};
// Randomly assign each author one book.
// Note that we can use the dbAuthor, although we have not yet written
// it to the database. Also note that we are using the book ID as a
// foreign key.
// 最後に,AuthorをDBに追加するときは
// Bookとの多対多の関係を登録する必要がある
// ここはちょっとくせがあるな.
// AuthorオブジェクトとBookIDを使う.
// AuthorとBookじゃないんだな.
var dbBookAuthor = new BookAuthor
{
Author = dbAuthor,
BookID = bookIDs[rand.Next(bookIDs.Length)]
};
//ここまででまだcontext.Authors.Addが出てきていないことに注目!
// Note that we do NOT need to add dbAuthor. It is referenced by
// dbBookAuthor, that is enough.
// BookAuthorsにAddするだけよい.
// これだけでAuthorsテーブルにもAuthorが登録される.
// これはEntityFrameworkがやってくれる.
context.BookAuthors.Add(dbBookAuthor);
}
await context.SaveChangesAsync();
await transaction.CommitAsync();
}
private static async Task FillAuthorsAsync(BookDataContext context)
{
// Note that we are jus reading primary keys of books, not entire
// book records. Tip: Always read only those columns that you REALLY need.
var bookIDs = await context.Books.Select(b => b.BookID).ToArrayAsync();
var authors = JsonSerializer.Deserialize <IEnumerable <Author> >(
await File.ReadAllTextAsync("Data/Authors.json"));
using var transaction = context.Database.BeginTransaction();
var rand = new Random();
foreach (var author in authors)
{
var dbAuthor = new Author
{
AuthorName = author.AuthorName,
Nationality = author.Nationality
};
// Randomly assign each author one book.
// Note that we can use the dbAuthor, although we have not yet written
// it to the database. Also note that we are using the book ID as a
// foreign key.
var dbBookAuthor = new BookAuthor
{
Author = dbAuthor,
BookID = bookIDs[rand.Next(bookIDs.Length)]
};
// Note that we do NOT need to add dbAuthor. It is referenced by
// dbBookAuthor, that is enough.
context.BookAuthors.Add(dbBookAuthor);
}
await context.SaveChangesAsync();
await transaction.CommitAsync();
}
