Difference between decorating a property in C# with BsonRepresentation(BsonType.ObjectId) vs BsonId vs ObjectId

Question

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?

Answer 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")

Likewise:

public Guid _id { get; set; }

will use the Guid generator to produce smth like

"_id" : BinData(3,"s2Td7qdghkywlfMSWMPzaA==")

Also all of the following properties

public int Id { get; set; }
public string id { get; set; }
public byte[] _id { get; set; }

will 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.

---

These both will be indexes:

[BsonId] 
public Guid SmthElseOtherThanId { get; set; } 

[BsonId] 
public string StringId { get; set; }

However;

public Guid SmthElseOtherThanId { get; set; } 
public string StringId { get; set; }

won't be indexes, 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; }

But,

[BsonId] 
[BsonRepresentation(BsonType.ObjectId)] 
public string Id { get; set; }

is different. Mongo will auto generate object ids by itself, 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.