如何使用Linq在带有字符串的集合上加入带有字符串[]的MongoDB集合

Question

I am trying to join two collections. I managed to join the two properly where the joining parts are equivalent in type - but that's not the goal. The difficulty occurs since i have a list of strings (id's) in one of the collections, and i want to replace each string in that list with an object of the other collection which is a collection of users defined as:

public string name { get; set; }
public string email { get; set; }
public string _id { get; set; }
public bool IsDeleted { get; set; }

I have two types of meetings where i want to insert the users to the one replacing the strings hence it's a DTO.

public class MeetingBase : ModelBase
    {
        public string createdBy { get; set; }
        public string facilitator { get; set; }
        public string topic { get; set; }
        public string address { get; set; }
        public string city { get; set; }
        public string country { get; set; }
        public int RoomNumber { get; set; }
        public DateTime meetingDate { get; set; }

    }

public class Meeting : MeetingBase
    {
        public List<string> attendees { get; set; }
    }

public class MeetingDTO : MeetingBase
    {
        public List<User> attendees { get; set; }
    }

As said, i want to join the collection og Meeting with the collection of users , resulting in a collection of MeetingDTO 's

So far, my best attempt is this:

public async Task<List<MeetingDTO>> getMeetingDtos(string userid)
        {
            var userCollection = _userDataManager.GetDataContext().MongoCollection;
            var meetingCollection = _meetingDataManager.GetDataContext().MongoCollection;

            var query = from m in meetingCollection.AsQueryable()
                where (m.createdBy == userid)
                from attendee in m.attendees
                join u in userCollection.AsQueryable() on attendee equals u._id into meetingusers
                select new MeetingDTO()
                {
                    _id = m._id,
                    IsDeleted = m.IsDeleted,
                    attendees = meetingusers.ToList(),
                    address = m.address,
                    city = m.city,
                    country = m.country,
                    createdBy = m.createdBy,
                    facilitator = m.facilitator,
                    meetingDate = m.meetingDate,
                    RoomNumber = m.RoomNumber,
                    topic = m.topic

                };

            var result = query.ToList();
            return result;
        }

However, this attempt gives the following run-time exception System.NotSupportedException: '$project or $group does not support {document}.'

Answer 1

you can do it with the .Aggregate() interface of the collection like so:

    var meetingDTOs = meetingCollection.Aggregate()
                        .Match(m => m.createdBy == userid)
                        .Lookup<Meeting, User, MeetingDTO>(
                            userCollection,
                            m => m.attendees,
                            u => u.Id,
                            dto => dto.attendees)
                        .ToList();

test program:

 using MongoDB.Bson; using MongoDB.Bson.Serialization.Attributes; using MongoDB.Driver; using MongoDB.Entities; using MongoDB.Entities.Core; using System.Collections.Generic; using System.Linq; namespace StackOverFlow { public class User : Entity { public string name { get; set; } } public class MeetingBase : Entity { public string createdBy { get; set; } public string topic { get; set; } } public class Meeting : MeetingBase { [BsonRepresentation(BsonType.ObjectId)] public List<string> attendees { get; set; } } public class MeetingDTO : MeetingBase { public List<User> attendees { get; set; } } public static class Program { private static void Main() { new DB("test"); var user1 = new User { name = "user one" }; var user2 = new User { name = "user two" }; new[] { user1, user2 }.Save(); var meeting = new Meeting { attendees = new List<string> { user1.ID, user2.ID }, createdBy = "god", topic = "corona beer" }; meeting.Save(); var meetingDTOs = DB.Fluent<Meeting>() .Match(m => m.createdBy == "god") .Lookup<Meeting, User, MeetingDTO>( DB.Collection<User>(), m => m.attendees, u => u.ID, dto => dto.attendees) .ToList(); } } }