C# 用户只能喜欢一个帖子
我有一个图书类,作者是ApplicationUser。在ApplicationUser类中,我有一个用户编写的书籍列表(一对多) Books类还有一个ApplicationUsers列表,用于记录喜欢的数量。但是,单个应用程序用户只能像一本书。当用户喜欢另一本书时,该书将从以前的任何书中删除C# 用户只能喜欢一个帖子,c#,asp.net,entity-framework,C#,Asp.net,Entity Framework,我有一个图书类,作者是ApplicationUser。在ApplicationUser类中,我有一个用户编写的书籍列表(一对多) Books类还有一个ApplicationUsers列表,用于记录喜欢的数量。但是,单个应用程序用户只能像一本书。当用户喜欢另一本书时,该书将从以前的任何书中删除 public class ApplicationUser : IdentityUser { public string Name { get; set; } public virtual
public class ApplicationUser : IdentityUser
{
public string Name { get; set; }
public virtual ICollection<Book> Books { get; set; }
public virtual ICollection<Book> Likes { get; set; } //NEW
}
public class Book
{
public string Id { get; set; }
public virtual ApplicationUser ApplicationUser { get; set; } //Author
.......
public virtual ICollection<ApplicationUser> Likes { get; set; }
}
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
base.OnModelCreating(modelBuilder);
modelBuilder.Entity<Book>()
.HasRequired<ApplicationUser>(s => s.ApplicationUser)
.WithMany(s => s.Books)
.WillCascadeOnDelete(true);
modelBuilder.Entity<Book>() //NEW
.HasMany<ApplicationUser>(s => s.Likes)
.WithMany(s => s.Likes);
}
更新:所以我按照@Alex Kudryashev的建议将Likes集合添加到ApplicationUser类中。但是我现在在迁移和更新数据库时遇到了这个错误
在表“BookApplicationUsers”上引入外键约束“FK_dbo.BookApplicationUsers_dbo.AspNetUsers_ApplicationUsers_Id”可能会导致循环或多个级联路径。指定“在删除时不执行操作”或“在更新时不执行操作”,或修改其他外键约束。
无法创建约束或索引。请参阅前面的错误
问题的原因在于您不清楚
应用程序用户
和书籍
之间的关系
每个ApplicationUser
都编写过零本或多本书籍
,每个书籍
都受到零本或多本ApplicationUser
的喜爱。因此,应用程序用户
和书籍
之间的关系是多对多的
在EF代码中,首先编写如下:
class ApplicationUser
{
public int Id {get; set;}
// every ApplicationUser has Authored zero or more books (many-to-many)
public virtual ICollection<Book> AuthoredBooks {get; set;}
... // other properties
}
class Book
{
public int Id {get; set;}
// every Book has been Liked by zero or more ApplicationUsers (many-to-many)
public virtual ICollection<ApplicationUsers> Likers {get; set;}
... // other properties
}
class MyDbContext : DbContext
{
public DbSet<ApplicationUser> ApplicationUsers {get; set;}
public DbSet<Book> Books {get; set;}
}
与多对多一样,尽管创建了连接表,但在dbContext
中没有提到它。无论何时使用ICollection,entity framework都会知道GroupJoin需要连接表
获取所有编写了非常古老书籍的年轻应用程序用户:
var result = myDbContext.ApplicationUsers
.Where(user => user.BirtDate > new DateTime(2010, 1, 1)
.Select(user => new
{
Id = user.Id,
Name = user.Name,
AuthoredOldBooks = user.Books
.Where(book => book.PublicationDate < new DateTime(1900, 1, 1)
.ToList(),
}
您使用的是什么数据库技术?能否显示添加类似实体框架的完整代码?请先显示代码。添加了喜欢的代码。谢谢@johnw为什么喜欢的东西是由书而不是用户储存的?您当前的关系是“用户喜欢书籍”,而不是“用户喜欢书籍”。@RenanB因为我想在用户通过调用book.likes.count请求书籍时获得喜欢的数量。
likes
集合应该属于这两个类,以解决多对多
关系。
modelBuilder.Entity<ApplicationUser>()
// every applicationUser has zero or more AuthoredBooks
.HasMany(applicationUser => applicationUser.AuthoredBooks)
// where every Book has zero or more Likers:
.WithMany(book => book.Likers)
// we put the many-to-many in the following junction table:
.Map(junctionTable=>
{
junctionTable.MapLeftKey("ApplicationUserId");
junctionTable.MapRightKey("BookId");
junctionTable.ToTable("ApplicationUsersBooks");
});
var result = myDbContext.ApplicationUsers
.Where(user => user.BirtDate > new DateTime(2010, 1, 1)
.Select(user => new
{
Id = user.Id,
Name = user.Name,
AuthoredOldBooks = user.Books
.Where(book => book.PublicationDate < new DateTime(1900, 1, 1)
.ToList(),
}
var results = myDbContext.Books
.Where(book => book.Genre == BookGenre.Software
&& book.Likers.Where(liker => liker.Gender == Gender.Female).Any());