MongoDB / C＃投影误解

Question

I have the following:

var P = Builders<T>.Projection.Include(_ => _.AccountId);
var R = Mongo.Find(E).Project(P).FirstOrDefault();

Here's what I don't understand:

With that projection, I need T to include AccountId but also _id. If I do not include _id, I get the following error:

{"Element '_id' does not match any field or property of class T."}

If I try to exclude _id, by doing the following:

var P = Builders<T>.Projection.Include(_ => _.AccountId).Exclude(_ => _._id);
var R = Mongo.Find(E).Project(P).FirstOrDefault();

I get a different error:

{"Element 'CreatedOn' does not match any field or property of class T."}

CreatedOn is another field from the record in the DB.

So, I'm afraid I don't understand how the projection really works: I need to extract one field (AccountId) and nothing else (no _id), etc (and ideally map it to a string and not a class that contains just a string, but that's a secondary problem)

What am I missing?

Am I to understand that Include means take a blank slate, add all fields named in Include statements (and _id for some reason), while Exclude would mean 'take everything' and exclude specific fields? In that case, these two commands would be exclusive.

Also, I can use:

Expression(_ => _.AccountId)

in my projection, and I get the result I want, but this doesn't clarify Include / Exclude

Answer 1

The error indicates that the document returned by the database contains a field, CreatedOn , which does not exist in the class of type T . You would need to add that field to the class T like so (assuming CreatedOn is a DateTime )

public DateTime CreatedOn { get; set; }