[hibernate-dev] [OGM] MongoDB dialect and treatment of _id

[Emmanuel Bernard]

I would like to discuss the problem of _id in MongoDB and how to map that in Hibernate OGM.

MongoDB is a bit psycho-rigid in how it uniquely identifies a document. A special property named _id is used for that and must be unique across a collection. It is also strongly recommended to let MongoDB generate this id (a UUID essentially).

In the MongoDb dialect we have not settled on how to use _id. and I would like to clarify that. Today we use `dbObject.put(ID_FIELDNAME, key.getColumnValues()[0])` but that is only correct if the id property is mapped to a single column. (ie `key.getColumnNames().length == 1`)

## Use _id as a OracleDB rowid

We could decide to use _id as a purely internal identifier for a document and basically never ever rely on it. All queries and lookup with use the identifier columns and their value to find a document.
That has the benefit of not having to deal with _id but I don't know if that's an OK practice in MongoDB or if it's not recommended at all as it would lead to costly lookups. Anybody familiar with MongoDB can shime in?

## Map _id when we have a identifier mapped on one column

In this case, I will only discuss the case where an id is mapped to a single column.
We could decide to map the id column value to both the id column and to _id. That creates some duplication but OGM would be happy and MongoDB's queries could be efficient.
Alternatively, we could decide to completely ignore the id column name and use _id for this. The TupleShapshot would then be responsible for binding the id column name to the value stored in _id. My concern with the alternative is that someone reading the data from the mongodb store will not find the JPA id column but rather see _id. On the other hand it seems to be the norm in the MongoDB land.

### Identifiers mapped on several columns

In this case, we have three approaches that can be combined:

1. treat _id as rowid (see avove)
2. map id values as a complex object and put that in _id  eg { "_id": { "firstname": "Emmanuel", "lastname": "Bernard"} }

Note that we can then decide to bind the id columns as top level attributes of the document as well.

Do you guys have any thoughts on the best approach?