Re: Fetching record counts.
Re: Fetching record counts.
- Subject: Re: Fetching record counts.
- From: Marek Wawrzyczny <email@hidden>
- Date: Tue, 1 Feb 2005 15:34:02 +1100
Thanks Arturo,
To be absolutely honest I have never used synchronized keyword in this
way before.
I will still create the "count" attribute dynamically, otherwise I
suspect there might be a risk of a deadlock if the attribute was to be
added to two different entities simultanously. So, the given entity
would be locked for the duration of addition/fetching count/removal of
the "count" attribute.
import com.webobjects.foundation.*;
import com.webobjects.eocontrol.*;
import com.webobjects.eoaccess.*;
/**
* This class impements methods that allow us to count the number of
* rows for a user defined fetch specification.
*
* <p>The count is performed directly on the database source without
having to load
* them into memory.</p>
*
* <p>Original code by Eric Noyau. This code is based on Eric Noyau's
Bugtracker application.</p>
*
* @author Eric Noyau
* @author Marek Wawrzyczny
* @version 1.1
*/
public class EOFetchCounting extends Object {
/**
* Returns the number of rows metching the fetch specifivation.
* @param editingContext - editing context to perform the fetch in
* @param fetchSpecification - the fetch specification
*/
public static Number objectCountWithFetchSpecification(
EOEditingContext editingContext,
EOFetchSpecification fetchSpecification) {
boolean hasCountAttribute = false;
EOFetchSpecification rawFetchspecification;
EOEntity entity;
EOQualifier schemaBasedQualifier;
EOAttribute attribute;
NSArray results = null;
if (fetchSpecification != null) {
try {
rawFetchspecification =
(EOFetchSpecification)fetchSpecification.clone();
} catch (Exception e) {
System.out.println("\nException in
objectCountWithFetchSpecification :\n"+e);
return null;
}
entity =
EOModelGroup.defaultGroup().entityNamed(rawFetchspecification.entityName
());
// Synchronize access to the entity object, the "count" attribute
should 'live' only for the
// duration of the fetch.
synchronized (entity) {
schemaBasedQualifier =
entity.schemaBasedQualifier(rawFetchspecification.qualifier());
attribute = objectCountAttribute();
try {
entity.addAttribute(attribute);
} catch (Exception e) {
NSLog.out.appendln("FetchCounting: Exception while adding attrib "
+ e);
}
try {
rawFetchspecification.setQualifier(schemaBasedQualifier);
rawFetchspecification.setRawRowKeyPaths(new
NSArray(attribute.name()));
results =
editingContext.objectsWithFetchSpecification(rawFetchspecification);
} catch (Exception e) {
NSLog.out.appendln("FetchCounting: Exception while performing
fetch " + e);
} finally {
entity.removeAttribute(attribute);
}
}
if ((results != null) && (results.count() == 1)) {
NSDictionary row = (NSDictionary) results.lastObject();
return (Number)row.objectForKey(attribute.name());
}
}
return null;
}
/**
* Returns the number of rows metching the fetch specifivation.
* @param editingContext - editing context to perform the fetch in
* @param entityName - the entity to perform the count on
* @param fetchSpecification - the fetch specification
* @param bindings - the fetch bindings
*/
public static Number objectCountWithFetchSpecificationAndBindings(
EOEditingContext editingContext,
String entityName,
String fetchSpecName,
NSDictionary bindings) {
EOModelGroup modelGroup = EOModelGroup.defaultGroup();
EOFetchSpecification unboundFetchSpec;
EOFetchSpecification boundFetchSpec;
unboundFetchSpec =
modelGroup.fetchSpecificationNamed(fetchSpecName, entityName);
if (unboundFetchSpec == null) {
return null;
}
boundFetchSpec =
unboundFetchSpec.fetchSpecificationWithQualifierBindings(bindings);
return objectCountWithFetchSpecification(editingContext,
boundFetchSpec);
}
/**
* Returns the "count" entity attribute to perform the fetch on.
* @return "count" entity attribute
*/
private static EOAttribute objectCountAttribute() {
EOAttribute objectCountAttribute = new EOAttribute();
objectCountAttribute.setName("p_objectCountAttribute");
objectCountAttribute.setColumnName("p_objectCountAttribute");
objectCountAttribute.setClassName("Number");
objectCountAttribute.setValueType("i");
objectCountAttribute.setReadFormat("count(*)");
return objectCountAttribute;
}
}
On 01/02/2005, at 14:57, Arturo Pérez wrote:
On Jan 31, 2005, at 9:33 PM, Marek Wawrzyczny wrote:
This is actually where I am stuck. I don't understand the EOModel and
EOAttribute mechanism very well. One solution I tried was to add the
count attribute permanently, that crashed the application. So I tried
creating the attributes on the fly and removing them once finished
this worked. However, it doesn't appear I can make this thread-safe
since there is no locking mechanism on the EOModel (I think this is
why Eric's solution used a static var) and I cannot easily test this
concept under duress (when the exceptions actually start occurring).
Can anyone point me in the right direction on this?
EOF may not provide a locking mechanism but perhaps the Java one will
suffice? Have you tried synchronizing on the entity, something like:
public static Number objectCountWithFetchSpecification(
EOEditingContext editingContext,
EOFetchSpecification fetchSpecification) {
if (fetchSpecification != null) {
EOFetchSpecification rawFetchspecification;
try {
rawFetchspecification =
(EOFetchSpecification)fetchSpecification.clone();
} catch (Exception e) {
System.out.println("\nException in
objectCountWithFetchSpecification :\n"+e);
return null;
}
EOEntity entity =
EOModelGroup.defaultGroup().entityNamed(rawFetchspecification.entityNam
e());
synchronized (entity) { // ADDED SYNCHRONIZED
EOQualifier schemaBasedQualifier =
entity.schemaBasedQualifier(rawFetchspecification.qualifier());
EOAttribute attribute =
EOFetchCounting.objectCountAttribute();
NSArray results = null;
entity.addAttribute(attribute);
// Fix to ensure that fetch counting does not trip the
site when
// the fetch fails for some reason
try {
rawFetchspecification.setQualifier(schemaBasedQualifier);
rawFetchspecification.setRawRowKeyPaths(new
NSArray(attribute.name()));
results =
editingContext.objectsWithFetchSpecification(rawFetchspecification);
} finally {
// IMPORTANT!!!
// This needs to run every time, exception or not
entity.removeAttribute(attribute);
}
} // CLOSE SYNCHRONIZED
if ((results != null) && (results.count() == 1)) {
NSDictionary row = (NSDictionary) results.lastObject();
return (Number)row.objectForKey(attribute.name());
}
}
return null;
}
Marek Wawrzyczny
software engineer
-------------------------->
ish group pty ltd
http://www.ish.com.au
7 Darghan St Glebe 2037 Australia
phone +61 2 9660 1400 fax +61 2 9660 7400
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