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?
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