如何从C#Mongodb强类型驱动程序中的文档加载特定字段

时间:2018-04-09 12:39:57

标签: c# .net mongodb mongodb-query mongodb-.net-driver

我正在使用官方C#MongoDb强类型驱动程序版本2.5.0与MongoDB进行交互。

我有以下课程Book and Page:

public class Book
{
    [BsonId]
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; }

    public string Title { get; set; }

    public string AuthorName { get; set; }

    public int PublishYear { get; set; }

    public List<Page> Pages { get; set; } = new List<Page>();

}


public class Page
{
    [BsonId]
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; }

    public int Number { get; set; }

    public string HTMLContent { get; set; }
}

我的问题是:

1 - 如何在没有Pages字段的情况下查询所有图书?目前这是我的代码:

var repository = _database.GetCollection<Book>("Books");
List<Book> allBooks = await repository.Find(_ => true).ToListAsync();

2 - 如何使用id字段仅获取一本书的pages字段的title,authorName和publishYear字段?目前这是我的代码:

var repository = _database.GetCollection<Book>("Books");
var filter = Builders<Book>.Filter.Eq("Id", bookId);
Book book = await repository.Find(filter).FirstOrDefaultAsync();

3 - 如何仅将一本书的页面字段设为List<Page>?目前这是我的代码:

var repository = _database.GetCollection<Book>("Books");
var filter = Builders<Book>.Filter.Eq("Id", bookId);
Book book = await repository.Find(filter).FirstOrDefaultAsync();
List<Page> pages = book != null ? book.Pages : new List<Page>();

1 个答案:

答案 0 :(得分:4)

您需要Project运营商。要仅排除特定字段,您可以这样做:

var allBooks = await repository.Find(_ => true)
    .Project<Book>(Builders<Book>.Projection.Exclude(c => c.Pages))
    .ToListAsync();

仅包含特定字段:

 var allBooks = await repository.Find(_ => true)
    .Project<Book>(Builders<Book>.Projection.Include(c => c.Pages))
    .ToListAsync();

如果您需要包含\排除多个字段 - 多次调用(Include(x => x.Title).Include(x => x.Title)等)。

仅包含您需要的替代方式:

var allBooks = await repository.Find(_ => true).Project(b => new Book {
    AuthorName = b.AuthorName,
    Id = b.Id,
    PublishYear = b.PublishYear,
    Title = b.Title
    // specify what you need, don't specify what you do not
    // like Pages
}).ToListAsync();