I have the following structure:

public abstract class DbEntity
{
[BsonId]
public ObjectId Id

{ get; set; }

[BsonDateTimeOptions(Kind = DateTimeKind.Utc)]
public DateTime CreatedAt { get; set; }

[BsonDateTimeOptions(Kind = DateTimeKind.Utc)]
public DateTime ModifiedAt

{ get; set; }

[BsonDefaultValue(EntityState.Active)]
public EntityState State { get; set; }

[BsonIgnoreIfNull]
[BsonDateTimeOptions(Kind = DateTimeKind.Utc)]
public DateTime? DeletedOn

{ get; set; }
}


public class Post : DbEntity
{
public ObjectId UserId { get; set; }

public string SourceUrl

{ get; set; }

public Guid? PictureId { get; set; }

public PostType Type

{ get; set; }

public string Caption { get; set; }

public GeoJsonPoint<GeoJson2DGeographicCoordinates> Location

{ get; set; }

public string LocationName { get; set; }

public List<ObjectId> TaggedFriends

{ get; set; }

public List<string> Tags { get; set; }

public List<string> GearTags

{ get; set; }

public Activity Activity { get; set; }

public int LikesAmount

{ get; set; }

public int CommentsAmount { get; set; }

public List<SharingTarget> Shared

{ get; set; }

public List<string> SearchTokens
{
get
{ var result = new List<string>(); result.AddRange(Caption.GetTokens()); result.AddRange(Tags); result.AddRange(GearTags); result.AddRange(LocationName.GetTokens()); return result; }
set { }
}
}

private class AggregatedPost : Followings
{
public AggregatedPost(ObjectId ownerId) : base(ownerId)
{
}

public List<Post> Posts { get; set; }

}

private class UnwindPost : Followings
{
public UnwindPost(ObjectId ownerId) : base(ownerId)
{
}

public Post Posts

{ get; set; }

}

I run the query where as a projection I use an Expression `<Func<UnwindPost,Post>>`

var query = await
followingsCollection.Aggregate()
.Match(Builders<Followings>.Filter.Where(v => v.Id == userId))
.Unwind<Followings>(new ExpressionFieldDefinition<Followings>(LocalFollowingFieldName))
.Lookup(postCollection, new ExpressionFieldDefinition<Followings>(LocalFollowingFieldName), new ExpressionFieldDefinition<Post>(ForeignPostFieldName),
new ExpressionFieldDefinition<AggregatedPost>(AggregatedPostAliasFieldName))
.Unwind<UnwindPost>(new ExpressionFieldDefinition<AggregatedPost>(UnwindPostFieldName))
.Limit(pageInfo.Take)
.Skip(pageInfo.Skip)
.Project(v => new Post

{ Id = v.Posts.Id, UserId = v.Posts.UserId, CreatedAt = v.Posts.CreatedAt, SourceUrl = v.Posts.SourceUrl, Type = v.Posts.Type, Caption = v.Posts.Caption, Activity = v.Posts.Activity, LocationName = v.Posts.LocationName, LikesAmount = v.Posts.LikesAmount, CommentsAmount = v.Posts.CommentsAmount, PictureId = v.Posts.PictureId }

).ToListAsync();

As a result I have an exception that says
Element 'Id' does not match any field or property of class.
If I run this query on mongodb, it returns the next record:

` Id: ObjectId("57d91c5393846c3c14792522")
UserId: ObjectId("57d91c5293846c3c14792512")
CreatedAt: 09/14/2016 12:45:55 PM (+0300)
SourceUrl: "test url"
Type: 0
Caption: "Test1"
Activity: NULL
LocationName: "TestLocationName"
LikesAmount: 0
CommentsAmount: 0
PictureId: NULL`

As you may see, there is no field '_id' here, however id is returned as 'Id'. All my records are stored in mongodb with '_id' field.

How can I project the record with '_id' field in mongo syntax that will be mapped correctly to my Post class (preferably using projection that has Expression as an argument)?