.NET？Core利用BsonDocumentProjectionDefinition和Lookup进行？join？关联查询(推荐)（.net core 调用内部方法）一看就会

/// <summary>
/// 用户实体（Collection）
/// </summary>
public class User
{
public Guid UserId { get; set; }

public string UserName { get; set; }

public string Password { get; set; }

public bool IsDelete { get; set; }

public DateTime CreateTime { get; set; }

public Guid RoleId { get; set; }
}
/// <summary>
/// 角色实体（Collection）
/// </summary>
public class Role
{
public Guid RoleId { get; set; }

public string RoleName { get; set; }

public DateTime CreateTime { get; set; }
}
/// <summary>
/// 构建用户Dto（不在Mongo创建Collection）
/// </summary>
public class UserDto
{
public Guid UserId { get; set; }

public string UserName { get; set; }

public DateTime CreateTime { get; set; }

public Guid RoleId { get; set; }

public string RoleName { get; set; }
}