4 Apr
2011
4 Apr
'11
4:01 p.m.
On 04/02/2011 02:22 PM, Caleb James DeLisle wrote:
After searching through documentation on JPQL
(JPA's query language) I was unable to find any
example of the "doc.object(XWiki.XWikiUsers)" construct. This means XWQL is
it's own standard and
there is no authoritative reference on it. What makes an implementation compliant? I have
found that
most HQL queries can be executed as XWQL queries with little or no modification so if
compliance is
defined as being "just like the reference implementation" then nearly all HQL
must be implemented in
order to be compliant.
The goal of XWQL was to not be bound to a certain query language, but to
be able to map it to as many QLs as possible, be they SQL-related, like
HQL or JPQL, or other types of queries, like QBE, XPath, SPARQL. So, it
wasn't meant from the start to be compatible with any standard.
Now, I'm not sure if the right thing to do is to move to a standard
query language, or to stick with our own.
- Is there any tool that allows mapping a JPQL or JDOQL query into other
query languages?
- Is there a way to parse a query into a tree/AST?
- Other than the fact that it's a non-standard language (and all the
consequences of this, like no support from tools and libraries), are
there any downsides to having our own query language?
The benefit of XWQL was that it allowed to write domain specific
queries, which are shorter and easier to understand (at least in theory).
Looking at the specifications I have rewritten the
example query in compliant JPQL and JDOQL.
I wrote these so that they would work if all objects were custom mapped which is similar
to the
appearance XWQL gives.
XWQL:
(SELECT doc.fullName FROM XWikiDocument as doc) where doc.author =
'XWiki.LudovicDubost' and
doc.object(XWiki.XWikiUsers).email like '%xwiki.com'
JPQL:
SELECT doc.fullName FROM XWikiDocument as doc, IN(doc.xObjects) obj WHERE obj.className
=
'XWiki.XWikiUsers' and obj.email LIKE '%xwiki.com'
JDOQL:
SELECT this.fullName FROM XWikiDocument WHERE this.xObjects.containsValue(obj)&&
obj.className ==
"XWiki.XWikiUsers"&& obj.email.startsWith("xwiki.com")
I understood that XWQL was simply a translation scheme which made it appear that we were
using JPQL
with the schema we wanted when really we were using HQL with the schema we had. Given
that it is not
compliant JPQL that is not the case.
I think when we update the schema, we should cut our losses with this thing and move to
something
which has a reference document and is more widely used.
WDYT?
Caleb
The JPQL specification (originally called EJBQL):
ejb-3_0-fr-spec-ejbcore.pdf chapter 9.
http://jcp.org/aboutJava/communityprocess/final/jsr220/
The JDOQL specification:
jdo-3_0-mrel3-spec.pdf chapter 26.
http://db.apache.org/jdo/specifications.html
Easy to read, example rich descriptions:
http://www.datanucleus.org/products/accessplatform_3_0/jpa/jpql.html
http://www.datanucleus.org/products/accessplatform_3_0/jdo/jdoql.html
--
Sergiu Dumitriu
http://purl.org/net/sergiu/