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.
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