当我们在ASP.NET MVC中播种数据库时，为什么我们使用lamba表达式返回元素的id？

Question

In all the examples I see, such as

protected override void Seed(BookService.Models.BookServiceContext context)
{
    context.Authors.AddOrUpdate(x => x.Id,
        new Author() { Id = 1, Name = "Jane Austen" },
        new Author() { Id = 2, Name = "Charles Dickens" },
        new Author() { Id = 3, Name = "Miguel de Cervantes" }
        );

    context.Books.AddOrUpdate(x => x.Id,
        new Book() { Id = 1, Title = "Pride and Prejudice", Year = 1813, AuthorId = 1, 
            Price = 9.99M, Genre = "Comedy of manners" },
        new Book() { Id = 2, Title = "Northanger Abbey", Year = 1817, AuthorId = 1, 
            Price = 12.95M, Genre = "Gothic parody" },
        new Book() { Id = 3, Title = "David Copperfield", Year = 1850, AuthorId = 2, 
            Price = 15, Genre = "Bildungsroman" },
        new Book() { Id = 4, Title = "Don Quixote", Year = 1617, AuthorId = 3, 
            Price = 8.95M, Genre = "Picaresque" }
        );
}

from here , I see that there is a lamba expression as the first parameter of the AddOrUpdate and it's always shown as the Id (primary key?) of the table. Can someone explain what that lamba expression is for? Is it for checking whether the Id is null ? I'm confused.

Answer 1

It's to define which property should be checked to see if the Seed() -method should UPDATE or INSERT . In this case, if the given ID is already in the database, it will not insert it again (but update the existing entry).

But you could also check on Title , Year , ... or other properties to define if the entry should be inserted or updated in your database. Basically it's to prevent duplicates in your database, but you can give meaning to what a duplicate is to you.