[Karl Anderson] Casey Duncan <cduncan@kaivo.com> writes:
I am not arguing necessarily for SQL as a query language for the ZODB. Although it is an accepted standard, but not a perfect one by any means especially for OODBs. Its appeal lies mainly in the high level of community familiarity and the plethora of SQL software to borrow from.
Does anyone have an opinion on the possible usefulness of XPath, XQuery, and other XML standards for this? Someone suggested (on the zope-xml wiki) that it would be nice to be able to drop in a cataloger that supported a presumably standard and presumably well-known XML query syntax, and which would work throughout the database because Zope objects would support DOM. This is all speculation, and I personally don't know much right now about XML database interfaces and how finished or well-regarded they are. [Albert] An excellent introduction to this topic is: "Putting XML in context with hierarchical, relational, and object-oriented models" by David Mertz. ftp://www6.software.ibm.com/software/developer/library/x-matters8.pdf Author is a python developer with lots of interesting XML stuff. See also his xml_matters 1 and 2 for xml_object and xml_pickle with much nicer "pythonic" syntax instead of using DOM directly. Article is also *essential* background for the distinction between "Object Mapping" and "Object Relational Mapping" which needs to be understood by anyone participating in this discussion. An example of a python ODBMS with some partial support for OQL is 4ODS from 4 Suite, which uses a very natural "pythonic" syntax for objects stored in and queried from PostgreSQL: Following is from 4Suite-docs-0.11/4Suite-0.11/html/4ODS-userguide.html available via: http://4suite.org/download.html#4Suite_Documentation vvvvvvvv How to use the system (a very basic walk through) First create a ODL file that represents what you want to store in test.odl module simple { class person { attribute string name; attribute double weight; relationship Person spouse inverse Person ::spouse_of; relationship Person spouse_of inverse Person ::spouse; relationship list<Person> children inverse Person ::child_of; relationship Person child_of inverse Person ::children; }; class employee (extends person) { attribute string id; }; }; Now create a new database and initialize #OdlParse -ifp test test.odl Now write some python code to do stuff with these people #!/usr/bin/python #Every thing that is persisten must be done inside a transaction and open database from Ft.Ods import Database db = Database.Database() db.open('test') tx = db.new() tx.begin() #Create a new instance of some objects import person import employee dad = employee.new() mom = person.new() son1 = person.new() son2 = person.new() daughter = person.new() #Set some attributes dad.name = "Pops" mom.name = "Ma" son1.name = "Joey" son2.name = "Bobby" daughter.name = "Betty" dad.weight = 240.50 #We can set attributes not defined in the ODL but they will not persist mom.address = "1234 Error Way" #Set some relationships #First set a one to one relationship dad.spouse = mom #Or we could have done it via the ODMG spec #dad.form_spouse(mom) #Add some children to the dad (our data model does not let mom have children. We'd need a family struct (left up to the reader) dad.add_children(son1) #We can create relationships both ways son2.form_child_of(dad) #Shortcut for adding dad.children = daughter #Now root the family to some top level object. db.bind(dad,"The Fam") #Make it so tx.commit() #Out side of a transaction we can still access the objects. #However, any changes we make will not persist. #NOTE, because 4ODS caches relationships, any relationships that were not traversed during the #transaction, cannot be traversed now because an object cannot be loaded from the db outside #of a transaction. print dad.name #Start a new tx to fetch tx = db.new() tx.begin() newDad = db.lookup("The Fam") print newDad.name print newDad.children[0].name print newDad.spouse #Discard this transaction tx.abort() Ft/Ods/test_suite and Ft/Ods/demo are good places to look for more examples ^^^^^^ See also: http://www.xml.com/pub/a/2000/10/11/rdf/ Some other relevant references are: Extraction of DBMS catalogs to XML using python. http://hyperschema.sourceforge.net./ PostgreSQL as XML repository http://zvon.org/index.php?nav_id=61 http://hopla.sourceforge.net/doc/README Note that none of this has much to do with the original topic of Object-*Relational* Mapping. *Essential* background for understanding what an object-relational persistence layer looks like is: http://www.ambysoft.com/persistenceLayer.html It isn't very long and there *absolutely* isn't any point discussing how to design such an OR persistence layer without first reading and fully understanding it. (I say that after having carefully studied all the messages in this discussion - though I also said so before ;-) The rest of that web site has lots of useful references. Two examples of java open source code implementing it are: http://osage.sourceforge.net/userguide.html http://jakarta.apache.org/turbine/ Third is open source java bindings for XML, LDAP and RDBMS with OQL and possible use of UML repository, also likely to be helpful for above. http://castor.exolab.org/ What Zopista's don't know about RDBMS would and does fill many libraries. Here's one online book, just to emphasize that the design of an RDBMS is a highly specialized technical area (not recommended for introductory reading): http://research.microsoft.com/pubs/ccontrol/ I strongly recommend careful study of *why* people (sometimes) need to use a complex RDBMS *despite* the fact that ZODB is so much more transparent and easier to use, before embarking on attempts to implement RDBMS features within ZODB (with or without the use of an actual RDBMS). BTW, as I predicted, both the "great" article on "Why aren't you using an ODBMS" referenced in Zope Weekly News and it's counterpart on slashdot have now been deluged with replies explaining why not. http://slashdot.org/features/01/05/03/1434242.shtml http://www.kuro5hin.org/?op=displaystory;sid=2001/5/3/32853/11281 http://www.zope.org/Documentation/ZWN/ZWN-2001-05-04 The key issue is the difference between an embedded persistence mechanism driven by application programs and an enforced schema independent of the applications (from any platform) that may be using it. Maintaining data integrity with concurrent changes by many users from many and rapidly evolving applications is a *hard* problem, which doesn't get easier by ignoring it. ZODB provides Atomic and Durable transactions. An RDBMS provides ACID transactions - Atomic, Consistent, Isolated and Durable. When you need either Consistent enforcement of a schema or Isolation (so that changes made by one transaction are not based on data that is concurrently being modified by another transaction), there is simply *no way* you can get that from ZODB, regardless of "storage". The easiest way to do an OR mapping for python is to start with one that has already been half built through stored procedures in the database, since Zopistas already know how to do the other half far better but know very little about the DBMS side. Since suitable free (as in speech) SQL is available from ACS 4, use that: http://developer.arsdigita.com/doc/kernel-doc.html It is currently being ported by OpenACS to support both PostgreSQL and Oracle (probably SAP DB, now also free, next): http://www.openacs.org This has a "query extractor" (in python) that collects all the SQL used by the entire comprehensive ACS 4 Tcl web application framework into a python and XML accessible form where it can be conveniently be used for porting to other DBMS and also for porting to other languages such as python. This makes sense for the same reasons that building ZODB with full support for only half of ACID made sense to meet immediate requirements and lay a foundation for others that are less urgent. Once a *specific* OR mapping has been implemented, *then* go on to do a more generic one.