前段时间由于项目需要用到MongoDB，但是MongoDB不建议Collection join 查询，网上很多例子查询都是基于linq 进行关联查询。但是在stackoverflow找到一个例子，程序员的朋友们请善于利用google搜索。主要介绍一个查询角色的所有用户的例子。MongoDB创建Collection 和准备数据，请自行处理。
/// <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();
