EF简单例子 Entity Framework

1. 管理NuGet程序包，为项目添加Entity Framework
2. 建立模型model对应表的实体
3. 新建一个DBContext的类，继承DbContext
4. DBContext类加上表的属性，有多少个表就写多少个属性
5. 属性的类型用 IDbSet<> 表示
6. 实体写好Id，类型是Guid类型
7. 使用的时候先new一个DBContext类的句柄
8. 使用句柄可以做增删改查等操作

public class DBTestContext : DbContext
    {
        public IDbSet<User> Users { get; set; }
        public IDbSet<Company> Companies { get; set; } 
    }

public class User
    {
        public Guid Id { get; set; }
        public string Name { get; set; }

        public Guid CompanyId { get; set; }
        public Company Company { get; set; }

    }

var ef = new DBTestContext();

            //var c = new Company()
            //{
            //    Id = Guid.NewGuid(),
            //    Name = "nx"
            //};
            //var u = new User()
            //{
            //    Id = Guid.NewGuid(),
            //    Name = "我呵呵哒",
            //    CompanyId = c.Id,
            //};
            //ef.Users.Add(u);
            //ef.Companies.Add(c);
             
            var df = from u in ef.Users
                where u.Company.Name == "nx"
                select u;
            var fddf = df.ToList();
             
            var dfe = from u in ef.Users
                join c in ef.Companies on u.CompanyId equals c.Id
                where c.Name == "nx"
                select u;
            var fesi = dfe.ToList();

            ef.SaveChanges();