public override Task Official_Driver()
{
var pipeline = BookCollection.Aggregate()
.Match(b => b.ID == "5e572df44467000021005692")
.Lookup <Book, Author, BsonDocument>(
AuthorCollection,
b => b.Author.ID,
a => a.ID,
x => x["authors"])
.AppendStage <Author>("{$replaceWith: { $arrayElemAt: ['$authors', 0] }}")
.AppendStage <Author>("{$set: { Age : '34' }}");
return(pipeline.ToListAsync());
}
