How to query a sub document collection using MongoDB and C# driver

10gen-csharp-driver, mongodb, mongodb-.net-driver, mongodb-query

Solution

Try ElemMatch:

var thread = threadHelper.Collection.Find(
    Query.And(
        Query.ElemMatch("Recipients", Query.EQ("UserId", user.Id)), 
        Query.ElemMatch("Recipients", Query.EQ("UserId", sendToUser.Id))
    )
).SingleOrDefault();

Problem

I have the following structure: ``` public class ThreadDocument { public ThreadDocument() { Messages = new List<Message>(); Recipients = new List<Recipient>(); } [JsonIgnore] public ObjectId Id { get; set; } public IList<Recipient> Recipients { get; set; } public IList<Message> Messages { get; set; } public DateTime LastMessageSent { get; set; } public string LastSentByUserName { get; set; } public string LastSentAvatarUrl { get; set; } public string Snippet { get; set; } public int MessageCount { get; set; } } public class Recipient { public string UserId { get; set; } public int Status { get; set; } } public class Message { public string FromUserId { get; set; } public string FromUsername { get; set; } public string FromAvatarUrl { get; set; } public DateTime Sent { get; set; } public string Text { get; set; } } ``` when I save, it produces something like this: ``` { "_id" : ObjectId("4fa5eab4bfeddf23fcd01e4a"), "Recipients" : [{ "UserId" : "4fa5d4d8bfeddf23fc72e590", "Status" : 1 }, { "UserId" : "4fa5d4f9bfeddf23fc72e592", "Status" : 0 }], "Messages" : [{ "FromUserId" : "4fa5d4d8bfeddf23fc72e590", "FromUsername" : "a", "FromAvatarUrl" : null, "Sent" : ISODate("2012-05-06T03:06:28.396Z"), "Text" : "b" }], "LastMessageSent" : ISODate("2012-05-06T03:06:28.395Z"), "LastSentByUserName" : "a", "LastSentAvatarUrl" : null, "Snippet" : "b", "MessageCount" : 1 } ``` What I would like to do is if there is already a thread created, to use the same one and tack on messages to that one based upon if the user is sending to the same user or vice versa. I thought something like this would work, but it is returning null (no values): ``` var thread = threadHelper.Collection.Find( Query.And(Query.EQ("Recipients.UserId", user.Id), Query.EQ("Recipients.UserId", sendToUser.Id)) ).SingleOrDefault(); ``` I'm thinking like a contains all? or has all? Not really sure how to make the query.

Original source