问题
Am new to mongodb and am liking how easy it is not to worry about schema stuff, I have a question suppose you want an Id property in mongo and mongo uses ObjectId to denote property Id's, so far i see you can have or decorate an Id as follows,
public ObjectId Id {get; set;}
//or
[BsonId]
public string Id {get; set;}
//or
[BsonId]
[BsonRepresentation(BsonType.ObjectId)]
public string Id {get; set;}
Can anyone explain to me why most people choose the last type, and whats going on and how the flexibility helps. Thanks?
回答1:
1) If you have a column named Id, id or _id, in your strongly typed TDocument class (the item type in a collection), then a column named "_id" will be generated in Mongo. It will also create an index for that column. You get a duplicate key error exception if trying to insert an item with a key that already exists.
public ObjectId Id { get; set; } will use the type generator for ObjectId and it will look like _id: ObjectId("57ade20771e59f422cc652d9").
public Guid _id { get; set; } will use the Guid generator to produce smth like "_id" : BinData(3,"s2Td7qdghkywlfMSWMPzaA==").
public int Id { get; set; }, public string id { get; set; }, public byte[] _id { get; set; } will also be index columns using the defaults for each type if not specified.
2) [BsonId] gives you the flexibility of naming that index any way you want. [BsonId] public Guid SmthElseOtherThanId { get; set; } and [BsonId] public string StringId { get; set; } will be indexes; public Guid SmthElseOtherThanId { get; set; } and public string StringId { get; set; } won't. mongodb will still use _id internally.
Same logic, public ObjectId SmthElseOtherThanId {get; set;} with no [BsonId] decoration won't be an index column.
3) [BsonRepresentation] lets you juggle with the Mongo type vs the internal .Net type, if there's a conversion between them.
Having [BsonId] [BsonRepresentation(BsonType.ObjectId)] public ObjectId Id { get; set; } is identical to public ObjectId Id { get; set; }.
Having [BsonId] [BsonRepresentation(BsonType.ObjectId)] public string Id { get; set; } is different however. Mongo will auto generate object ids by himself, however you will be able to use strings in .net, filter queries etc., because there is a conversion between object id and string.
Having [BsonId] [BsonRepresentation(BsonType.ObjectId)] public byte[] Id { get; set; } or [BsonId] [BsonRepresentation(BsonType.ObjectId)] public int Id { get; set; } will fail with a ObjectId not a valid representation for a ByteArraySerializer / Int32Serializer message.
But [BsonId] [BsonRepresentation(BsonType.String)] public int StringId { get; set; } will be just fine.
来源：https://stackoverflow.com/questions/38871416/difference-between-decorating-a-property-in-c-sharp-with-bsonrepresentationbson