Очень старый кусок кода (~2012-13 гг.), но вдруг пригодится когда-нибудь, память освежить.

            var match = new BsonDocument 
            { { 
                    "$match", new BsonDocument 
                    {         
                        {
                            "Date", new BsonDocument 
                            { { "$lte", DateTime.Now } }
                        }
                    } 
            } };
            var group = new BsonDocument
            { {
                    "$group", new BsonDocument
                    {
                        { "_id","$Blog._id" },
                        {
                            "PublishDate", new BsonDocument 
                            { { "$max", "$Date" } }
                        }
                    }
            } };

            var project = new BsonDocument
            { {
                    "$project", new BsonDocument
                    {
                        { "Blog._id", "$_id" },
                        { "PublishDate", "$PublishDate" }
                    }
            } };

            var sort = new BsonDocument { { "$sort", new BsonDocument { { "PublishDate", -1 } } } };
            var skip = new BsonDocument { { "$skip", (page - 1) * count } };
            var limit = new BsonDocument { { "$limit", count } };

            var pipeline = new[] { match, group, sort, project, skip, limit };
            var lastPostsInBlogs = this.mongoContext.Posts
                .Aggregate(pipeline)
                .ResultDocuments.Select(x => BsonSerializer.Deserialize(x))
                .ToList();

            return
                lastPostsInBlogs.Join(
                    this.mongoContext.Blogs.AsQueryable()
                        .Where(condition),
                    pb => pb.Blog.Id,
                    b => b.Id,
                    (pb, b) =>
                    new BlogItem()
                    {
                        Id = b.StrId,
                        Alias = b.Alias,
                        Title = b.Title,
                        Description = b.Info,
                        CoverFile = b.CoverFile ?? DomainAppSettings.DefaultPreview,
                        CanPost = b.Owner.AccountId == visitorId || (b.IsCommon && b.Members.Contains(visitorId)),
                        LastUpdated = b.LastUpdated,
                        LastPosts = this.mongoContext.Posts
                            .AsQueryable()
                            .Where(p => p.Blog.Id == b.Id && p.PublishDate  p.PublishDate)
                            .Take(3)
                            .Select(p => new PostLine()
                            {
                                Id = p.StrId,
                                Number = p.Number,
                                BlogAlias = p.Blog.Alias,
                                Title = p.Title,
                                Date = p.Date
                            })
                    }).ToArray();