种子代码优先流利的api数据库EF代码优先问题

Question

我在播种和创建符合我需要的数据库时遇到了麻烦。 我有2个实体； 用户和部门。 用户需要一个部门，一个部门可以有多个用户，但同时，一个部门需要一个用户作为其DepartmentChief。 我可以弄清楚如何进行设置，每次尝试都会导致各种异常。

---

这是我的情况 ：

public class FravaerContext : DbContext
{
    public FravaerContext() : base("name=DefaultConnection")
    {
        Database.SetInitializer(new DBInitializer());
    }


    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<User>().HasRequired<Department>(u => u.Department).WithMany(d => d.Users).WillCascadeOnDelete(false);
        modelBuilder.Entity<Department>().HasRequired(d => d.DepartmentChief);

        //modelBuilder.Entity<Absence>().HasRequired<User>(a => a.User).WithMany(u => u.Absences).WillCascadeOnDelete(false);
        base.OnModelCreating(modelBuilder);
    }

    public DbSet<Absence> Absences { get; set; }
    public DbSet<Department> Departments { get; set; }
    public DbSet<User> Users { get; set; }

}

public class DBInitializer : DropCreateDatabaseAlways<FravaerContext>
{
    protected override void Seed(FravaerContext context)
    {
        for (int i = 1; i <= 4; i++)
        {
            User chief = new User() {Id = i, Absences = new List<Absence>(), Email = $"chief{i}@chief.dk", FirstName = $"Chief{i}",LastName = $"Chiefsen{i}",Password = "admin",UserName = ""+i,Role = Role.DepartmentChief};

            Department d = new Department() {Id = i, Users = new List<User>() { chief }, DepartmentChief = chief };


            chief.Department = d;

            context.Departments.Add(d);
        }
        base.Seed(context);
    }
}

和我的实体 （AbstractEntity仅向实体类提供ID）：

  public class Department : AbstractEntity
{
    public List<User> Users { get; set; }
    public User DepartmentChief { get; set; }

}

public class User : AbstractEntity
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string UserName { get; set; }
    public string Password { get; set; }
    public string Email { get; set; }
    public List<Absence> Absences { get; set; }
    public Department Department { get; set; }
    public Role Role { get; set; }

}

Answer 1

嗯，您遇到了所有这些错误，因为您已经在实体之间创建了循环所需的依赖关系，现在数据库不知道如何正确插入它们。

然后，您需要稍微放松一下模型，以便所需的属性之一变为可选，然后就可以正确地播种数据库了。

假设您在用户实体中将“部门”设置为可选。

目前，由于您没有在OnModelCreating方法上定义外键，因此让EF生成外键。

因此，首先将外键暴露给用户：

public class User : AbstractEntity
{
    public int? DepartmentId { get; set; }
}

请注意，我已将其设置为可为空，因此我们现在可以将此关系定义为可选：

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<User>()
        .HasOptional(u => u.Department)
        .WithMany(d => d.Users)
        .HasForeignKey(x => x.DepartmentId)
        .WillCascadeOnDelete(false);
}

确保在Department的构造函数上初始化Users属性，以便在尝试将用户添加到Department时不会出现NullReferenceException：

public Department()
{
    Users = new List<User>();    
}

创建一个新的迁移并更新您的数据库。 然后，您可以使用以下初始化程序：

public class DBInitializer : DropCreateDatabaseAlways<FravaerContext>
{
    protected override void Seed(FravaerContext context)
    {
        //create the users first
        //save the users in a list so later we can create the departments
        var users = new List<User>();

        for (var i = 1; i <= 4; i++)
        {
            var user = new User
            {
                Id = i,
                Absences = new List<Absence>(),
                Email = $"chief{i}@chief.dk",
                FirstName = $"Chief{i}",
                LastName = $"Chiefsen{i}",
                Password = "admin",
                UserName = "user" + i,
                Role = Role.DepartmentChief
            };
            users.Add(user);
            context.Users.Add(user);
        }

        //create one department for each user (with that user as the chief)
        var departmentId = 1;

        //save the departments so later we can seed them with users
        var departments = new List<Department>();
        foreach (var user in users)
        {
            var d = new Department
            {
                Id = departmentId++,
                DepartmentChief = user
            };

            departments.Add(d);
            context.Departments.Add(d);
        }

        //seeed the departments with users
        foreach (var department in departments)
        {
            //add 5 users to each department
            for (var i = 1; i < 5; i++)
            {
                var user = new User
                {
                    Absences = new List<Absence>(),
                    Email = $"user{department.Id}{i}@user.dk",
                    FirstName = $"User{department.Id}{i}",
                    LastName = $"Username{department.Id}{i}",
                    Password = "user",
                    UserName = "" + department.Id + i,
                    Role = Role.DepartmentChief
                };
                department.Users.Add(user);
            }
        }
    }
}

我在这里所做的是将播种任务分解为较小的部分，因此我首先创建主要用户（不包含部门），然后创建具有先前创建的用户的部门，最后创建每个部门的用户。

这种方法的优点是您可以完全控制创建的用户，部门和负责人的数量。

希望这可以帮助！