.NET Core利用BsonDocumentProjectionDefinition和Lookup进行 join 关联查询(推荐)_实用技巧

/// <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; }
    }

 var client = new MongoClient("xxx");
           var database = client.GetDatabase("xxx");

BsonDocumentProjectionDefinition<BsonDocument> projectionDefinition = new BsonDocumentProjectionDefinition<BsonDocument>(
                        new BsonDocument("UserId", "$UserId")
                       .Add("UserName", "$UserName")
                       .Add("CreateTime", "$CreateTime")
                       .Add("RoleId", "$RoleId")
                       .Add("RoleName", new BsonDocument("$arrayElemAt", new BsonArray().Add("$Role.RoleName").Add(0)))
                    );

Guid roleId = Guid.Empty;
            List<UserDto> list = database.GetCollection<BsonDocument>(typeof(User).Name)
                .Aggregate()
                //过滤条件
                .Match(Builders<BsonDocument>.Filter.Eq("IsDelete", false))
                .Match(Builders<BsonDocument>.Filter.Eq("RoleId", roleId))
                //连接Role
                .Lookup(typeof(Role).Name, "RoleId", "RoleId", typeof(UserDto).Name)
                //查询需要显示的列
                .Project(projectionDefinition)
                .As<UserDto>().ToList();