[Zope3-dev] Florent's O-R blog entry

[Santi Camps]

En/na zope3-dev-request at zope.org ha escrit:

>Martijn Faassen wrote:
>  
>
>>Missing powerful query concepts
>>-------------------------------
>>
>>Certain powerful query concepts like joins, available in a relational
>>setting, are missing. I've already run into a scenario where I wanted to
>> someting like this: given a bunch of version objects with field 'id',
>>where multiple objects can have the same 'id' to indicate they're
>>versions of the same object, I want all objects where field
>>'workflow_state' is 'PUBLISHED' unless there is another object with the
>>same id that have workflow_state 'NEW', in which case I want that one'.
>>
>>I think joins would be a way to solve it, though I haven't figured out
>>the details, nor how to implement them efficiently on top of the
>>catalog. This kind of thing is where a relational database makes life a
>>lot simpler.
>>    
>>
>
>I used to have the same complaints in Zope 2, but so far I've been happy
>with Dieter's AdvancedQuery product.  See
>http://www.dieter.handshake.de/pyprojects/zope/AdvancedQuery.html
>It might be worth a look while thinking about what to implement for zope 3.
>  
>
A very interesing discussion.  I'm just and advanced Zope 2 user, and a 
Zope 3 beginner, but those questions are over my head since I start to 
develop using ZODB, so I will put here my opinions.

The use of ZODB and ZCatalogs has a lot of advantatges, that's obvious, 
but also has some limitations.   We also use AdvancedQuery to solve some 
of them, but there are another not solved ones:

1) Join's.    There isn't an standard way to make joins, and also to 
make sorts and filters over joined data.   I know that with enought 
python knowledge this can be done manually, but that's hard (at least 
much more hard than using SQL).   Moreover, programmers writing business 
application reports are the ones with less python experience and, 
believe me, some joins can be writted in a very unefficient way :-)

2) Relational Integrity.   I like to make a data definition and rely on 
it, so if I define that an invoice has a restricted foreign key with a 
customer, the customer will not be deleted while the invoice exists.   
Despide this behaviours could (and should) be implemented in the 
application logic, sometimes there are bugs in the application and data 
inconsistencies are generated.   Working with RDBM's you could rely on 
your data definition, so data inconsistencies never will be done.

3) Huge amounts of data.   When working with applications writing a lot 
of data or making queries (in special join's) over huge amount of data, 
the performance of a ZODB database is poor in front of some RDBM's (I'm 
talking about Zope 2.7, I haven't this experience with Z3 yet).

Despide of this, I think the advantatges of use ZODB are more than the 
limitations, so we continue using it.   I think a good approach could be 
a transparent O/R mapping plus a way to use SQL over the RDBM's and get 
objects back.   That way, a pure ZODB application can work with 
FileStorage or with a RDBM's storage, and applications requiring it will 
be able to take advantage of SQL.

I just want to express an opinion from a user that simply uses ZODB and 
ZCatalogs, and that has been using RDBM's for a long time before.

Regards

Santi Camps