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

/// 
 /// 用户实体（Collection） /// 
 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; } } /// 
 /// 角色实体（Collection） /// 
 public class Role { public Guid RoleId { get; set; } public string RoleName { get; set; } public DateTime CreateTime { get; set; } } /// 
 /// 构建用户Dto（不在Mongo创建Collection） /// 
 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 projectionDefinition = new BsonDocumentProjectionDefinition( 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 list = database.GetCollection(typeof(User).Name) .Aggregate() //过滤条件 .Match(Builders.Filter.Eq("IsDelete", false)) .Match(Builders.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().ToList();