Hi Pinaki, On Feb 26, 2007, at 10:01 AM, Pinaki Poddar wrote:
Respected Sir,Sorry to disagree. If you fetch Department instances, I think you always want the _employees collection to contain all the related Employees. You never want a filtered collection unless you provide a filter via some Java behavior.I agree that department.getEmployees() must return all the employees.However, i was considering the state of a Department instance in memory that has just been selected by a query. Which of the native (String/intetc.) field values are populated and which of its related objects are instantiated is controlled by the currently active fetch plan (if not explicitly then at least implicitly by 'default' fetch group).
Right.
If the application accesses any unrealized field of this 'partiallyfilled' department, then that field is populated irrespective of whetherit is in active fetch group or not via a new, separate database access call.
Right.
In a sense, the fetch plan determines a 'partial' view of the 'complete' department -- both in terms of its native state and its relations -- as and when it appears for the first time in memory. In fact, if the department instance leaves the managed environment viadetachment at that point -- the detached department in a remote processwill only have this partially filled 'view'. And, of course, theapplication can explictly load any unrealized state of the partial viewvia any getter method.
Right.
I was trying to draw an equivalence/parallel of this normal fetch behavior to the use case where a owner object has a high,multi-cardinalty relationship but only wants a subset of the elements ofthat relationship. In such a scheme, the traversal on a fetch path relation (which is now a binary decision) can be further adorned to support a predicate i.e. Query query = // a JPQL query that selects a Department based on some critria query.getFetchPlan().addField("employees", "yearsOfSevice>10"); Department dept = query.getSingleResult(); // dept will be constructed in memory with a collection of Employees with yearsOfService>10
The standard way to get the employees with the filter you show is to explicitly return the employee instances. The query language does not accommodate returning the collection as a result value. One of the reasons to exclude this is to disambiguate the result collection as being filtered or not. Disallowing return values of collection types neatly avoids the issue.
dept.getEmployees(); // now the dept will load a collection of all Employees
Right.And you can also envision filtering the collection directly via non- standard techniques. Given the collection, that might not be fully instantiated, you can pass that collection to a proprietary method that does the filtering in the database.
Of course, the suggested alternative approach where the applicationexplictly constructs this partial/filtered multi-cardinality relatinshipby choosing the elements via query is feasible and available right now with current features.
Right. Craig
Pinaki Poddar BEA Systems 415.402.7317 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Friday, February 23, 2007 8:32 PM To: open-jpa-dev@incubator.apache.org Subject: Re: possible to write a JPA Query to that filters both an Entity and its relationship entities? On Feb 23, 2007, at 7:49 PM, Pinaki Poddar wrote:One way to realize a owner object with a partially filled multi-cardinality relationship is to expand fetchplan concept. Forexample, if we consider a Department of 100 Employees of which only 20are oldtimers, then a query can select the particular Department from the datastore but to realize that Department as a Java object in memory with only 20 senior Employees in _employees collection will need to tune the fetch plan acconrdingly.In JDO, fetch plan is specified to certain details to extract a subset of native states and subset of relationships. JPA does not yet specifyfetch plan, but OpenJPA does via @FetchGroup extension. However currently, no filtering criterion is applied to filter the content while a particular relationship path is traversed. It is a all-or-nothing affair. Either Department is fetched with all its 100 Employees or the Employees relation is not traversed at all. The only control available now is to specify which native states and relationship will be considered for fetching and in case of recursive relationship how many times a particular relation path will be traversed. Essentially query and fecth paths are working in conjunction -- query selects the root object and fetch plan decides which subset of the closure of this root object is realized in memory. It will surely be a useful feature to consider expanding fetch plan with filtering criteria.Sorry to disagree. If you fetch Department instances, I think you always want the _employees collection to contain all the related Employees. Younever want a filtered collection unless you provide a filter via some Java behavior.In the query case, you want to get references to the filtered instancesbut there's no connection to the _employees field of the department. This is a good thing. CraigPinaki Poddar BEA Systems 415.402.7317 -----Original Message----- From: Patrick Linskey Sent: Friday, February 23, 2007 4:37 PM To: open-jpa-dev@incubator.apache.org Subject: RE: possible to write a JPA Query to that filters both an Entity and its relationship entities? I would expect that your query would return the unfiltered employees collection. But I have not run any tests. In other words, I would expect that the oldtimers collection would be the full set of employees for each dept that had any oldtimers >= 15. That said, I don't think that your interpretation is necessarily wrong. Just not my expectation. -Patrick -- Patrick Linskey BEA Systems, Inc._____________________________________________________________________ __ Notice: This email message, together with any attachments, may contain information of BEA Systems, Inc., its subsidiaries and affiliated entities, that may be confidential, proprietary, copyrighted and/or legally privileged, and is intended solely for the use of the individual or entity named in this message. If you are not the intended recipient, and have received this message in error, please immediately return this by email and then delete it.-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Friday, February 23, 2007 4:14 PM To: open-jpa-dev@incubator.apache.org Subject: Re: possible to write a JPA Query to that filters both an Entity and its relationship entities? Hi Patrick, From the JPQL select dept, oldtimers from Department dept LEFT JOIN dept.employeeCollection oldtimers WHERE dept.deptno >= 100 AND oldtimers.yearsOfService >= 15 I expect to get SQL that looks something like select dept.id, dept.name, oldtimers.id, oldtimers.firstname, oldtimers.lastname, oldtimers.salary, oldtimers.ssn from Department dept, Employeeoldtimers WHERE dept.id = oldtimers.deptid AND dept.deptno >= 100 ANDoldtimers.yearsOfService >= 15 I then construct instances from the returned dept and oldtimers columns and give back to the user the results, with one result row foreach SQL ResultSet row. So the filtering from the JPQL translated intothe WHERE clause in the SQL should exclude oldtimers that don't qualify. If you don't use the oldtimers.yearsOfService >= 15 in the SQL, then you will get all of the employees in the department returned in the result. But why do you not use the oldtimers.yearsOfService >= 15 in the SQL? Craig On Feb 23, 2007, at 2:12 PM, Patrick Linskey wrote:In fact, you have to do a bit of work to get SQL to return you non-filtered instances. So I don't get it. Does OpenJPA not construct the obvious SQL that filters oldtimers?I don't think that you and I are really in sync here. Maybesome moreconcrete examples are in order? -Patrick -- Patrick Linskey BEA Systems, Inc.______________________________________________________________ _________ Notice: This email message, together with any attachments, may contain information of BEA Systems, Inc., its subsidiaries and affiliated entities, that may be confidential, proprietary, copyrighted and/or legally privileged, and is intended solely for the use of the individual or entity named in this message. If you are not the intended recipient, and have received this message in error, please immediately return this by email and then delete it.-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Friday, February 23, 2007 2:09 PM To: open-jpa-dev@incubator.apache.org Subject: Re: possible to write a JPA Query to that filters both an Entity and its relationship entities? So, I can tell you that having a result column that is filtered in thequery is not what a user would expect. In JDO, the oldtimers columnwould be filtered.In fact, you have to do a bit of work to get SQL to return you non-filtered instances. So I don't get it. Does OpenJPA not construct the obvious SQL that filters oldtimers? I've re-read the JPA specification, and it appears to be silent on the issue of filtering. Is this a portability issue? Craig On Feb 23, 2007, at 1:57 PM, Tom Mutdosch wrote:Hi Patrick, Thanks for the query suggestion. I guess what I was initiallythinking of doing was incorrect in that JPA doesn't really give mea "view" of what I want. That is, I can never get a Department object containing a list of filtered Employees. A JPA object returned from a query is always going to be an exactrepresentationof the database. So your Department object is always going to contain all of the Employees in its relationship. So like you mentioned, I can still get all the information using one query, and then just process those results as I want them. I imagine that this would entail some sort of wrapper beanthat wouldhouse the Department and the filtered list of Employees. Or what if I added a regular method to my Department entity called getFilterEmployees() which would return a List that I populated with the filtered results from my query? Does that seem like a reasonable thing to do -- if I didn't want to deal with a wrapper object but still have all of my desired data captured by a single Entity? Thanks Tom Patrick Linskey wrote:It is, but it doesn't buy you much in this situation --the oldTimerscollection in your example won't be filtered to just theones thatare old. It'll be all the employees in the dept. -PatrickCraig Russell Architect, Sun Java Enterprise Systemhttp://java.sun.com/products/jdo408 276-5638 mailto:[EMAIL PROTECTED] P.S. A good JDO? O, Gasp!Craig Russell Architect, Sun Java Enterprise System http://java.sun.com/products/ jdo 408 276-5638 mailto:[EMAIL PROTECTED] P.S. A good JDO? O, Gasp!Craig Russell Architect, Sun Java Enterprise System http://java.sun.com/products/jdo 408 276-5638 mailto:[EMAIL PROTECTED] P.S. A good JDO? O, Gasp!______________________________________________________________________ _ Notice: This email message, together with any attachments, may contain information of BEA Systems, Inc., its subsidiaries and affiliated entities, that may be confidential, proprietary, copyrighted and/or legally privileged, and is intended solely for the use of the individual or entity named in this message. If you are not the intended recipient, and have received this message in error, please immediately return thisby email and then delete it.
Craig Russell Architect, Sun Java Enterprise System http://java.sun.com/products/jdo 408 276-5638 mailto:[EMAIL PROTECTED] P.S. A good JDO? O, Gasp!
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