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

[Nicolas Helleringer]

My DBA background tells me 'do not do thing behind the back of rou db' : I
shall use the second strategy.

But I am not sure what is the exact contract around _id column from the
MongoDb devs.

Niko

2012/4/6 Emmanuel Bernard <emmanuel at hibernate.org>

> 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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