Re: Fetching from related object
Re: Fetching from related object
- Subject: Re: Fetching from related object
- From: Arturo Pérez <email@hidden>
- Date: Fri, 23 Apr 2004 21:28:25 -0400
On Apr 23, 2004, at 6:54 PM, Randall Perry wrote:
EOQualifier qual = EOQualifier.qualifierWithQualifierFormat(...);
EOFetchSpecification custFetchSpec = new EOFetchSpecification(...);
custInfo = ec.objectsWithFetchSpecification(custFetchSpec);
Went to add a second fetch which grabs log entries for the past year by
joining on custNo:
qual = EOQualifier.qualifierWithQualifierFormat(...);
logInfo = ec.objectsWithFetchSpecification(logFetchSpec);
Then I realized that custNo is a hidden key in EOModeler for both
entities,
so this can't be done without making the keys visible.
There is a to-one relationship defined in EOModeler between the Log and
CustView entities.
How would I do the logInfo fetch above without making the keys visible?
Lots of things you should know about in this area. This is an area
that I truly enjoy using in WebObjects/EOF so I'm going to be
long-winded about it. This may also help some of the other newbies.
You have a customer object named CustView. You have a log entry object
named Log. BTW, your second fetch specification above was still
fetching CustView objects, not Log objects. You have a relationship
defined in EOModeller between the two objects. I'm not going to use
relational database terminology because it confuses the issue and
WO/EOF makes you not care. In any case, there's a CustInfo->>Log
relationship defined which is each CustInfo has many Log entries and
the name of that relationship is logEntries. The reverse relationship
from Log to CustInfo is named customer.
So, you fetch a CustInfo as above. Now, how do you get the Log objects
associated with it? The easiest best performing way is to do
NSArray arrayOfLog = (NSArray)custInfo.valueForKey("logEntries");
Notice that there is no fetch specification involved. EOF takes care
of it all for you. The SELECT, the join the whole nine yards. Now
it's obvious why this is the easiest way to get the Log objects. Why
is this the best performing? Because EOF will cache the results of
that relationship traversal and never perform that underlying SELECT
again. As you get more comfortable with EOF you may come to see
problems with that caching but in general it is the way to go. For the
issues feel free to visit
http://wodev.spearway.com/cgi-bin/WebObjects/WODev.
The power in WebObjects/EOF is in getting the model you define with
EOModeler to do all the work. Say I had a Log and needed the CustView
related to it. Then I just do log.valueForKey("customer"); Use the
Model!
----
WO in philadelphia - wanna cheesesteak with that?
Please visit webobjects.meetup.com.
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