I ran into the following error today while searching MongoDb for an object:
The issue was that there were extra fields on the returned document that were not specified on the mapping target object. A bit of Googling revealed that there is a provided attribute that allows you to ignore all extra fields.
Simply add [BsonIgnoreExtraElements] to any relevant model definitions.
Thanks to this stackoverflow post for the lead: https://stackoverflow.com/a/23448987/522859