c# - 如何使用 LINQ 限制来自包含类的数据？

Question

我有两个由 ASP.NET Identity 使用的类。一个是用户列表的支持类，另一个具有角色。在数据库中有三个表。 AspNetUsers、AspNetRoles 和 AspNetUserRoles。由于它们在 Entity Framework 中的映射方式，这三个映射到两个类:

public partial class AspNetUser
{
    public AspNetUser()
    {
        this.AspNetUserClaims = new List<AspNetUserClaim>();
        this.AspNetUserLogins = new List<AspNetUserLogin>();
        this.AspNetRoles = new List<AspNetRole>();
    }
    public int Id { get; set; }
    public virtual ICollection<AspNetRole> AspNetRoles { get; set; }
    public virtual ICollection<AspNetUserClaim> AspNetUserClaims { get; set; }
    public virtual ICollection<AspNetUserLogin> AspNetUserLogins { get; set; }
}

和

public partial class AspNetRole
{
    public AspNetRole()
    {
        this.AspNetUsers = new List<AspNetUser>();
    }

    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<AspNetUser> AspNetUsers { get; set; }
}

谁能告诉我如何创建一个 LINQ 语句来检索具有特定角色类型的所有用户，或者如果我提供的 roleId 为零，那么我想查看所有用户而不限制 roleId。这是我尝试过的:

        var user = await db.AspNetUsers
            .Where(u => u.AspNetRoles.SelectMany(r => r.Id == roleId || roleId == 0))
            .Include(u => u.AspNetRoles);

我也试过:

        var user = await db.AspNetUsers
            .Where(u => u.AspNetRoles.Select(r => r.Id == roleId || roleId == 0))
            .Include(u => u.AspNetRoles);

Answer 1

Where 的参数是

predicate
Type: System.Linq.Expressions.Expression<Func<TSource, Boolean>>
A function to test each element for a condition.

所以你需要传递一个返回bool的函数

.Where(u => roleId == 0 //if roleId is 0 return all
         || u.AspNetRoles.Any(r => r.Id == roleId) // check that user have any roles with id equals to roleId
      )