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Subject: re: Detecting failure to retrieve data
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Date: Wed, 7 Oct 2009 22:51:09 +0200
Message-ID: <B297D4737108A0459B66C3A85BB7F90118AA@kn.esteam.de>
In-Reply-To: <4ca747c30910051750r30f0d7d9r2c39408de267d401@mail.gmail.com>
Thread-Topic: re: Detecting failure to retrieve data
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References: <4ca747c30910051750r30f0d7d9r2c39408de267d401@mail.gmail.com>
From: =?iso-8859-1?Q?Rainer_D=F6bele?= <doebele@esteam.de>
To: <empire-db-user@incubator.apache.org>

Hi Andrew,

when you call DBReader.open() the cursor is positioned before the first =
record not on the first record.
The open method assumes that you want to retrieve a list of records =
hence you should always iterate through the reader like this:
	reader.open(cmd, conn);
	while (reader.moveNext())
	{	// Init updateable record
		reader.initRecord(EMP, record);
	}
This will work for 0 to n records.

However, if you know for sure that the result is a single record or =
none, you can instead use the method getRecordData(DBCommandExpr cmd, =
Connection conn) on DBReader. This method opens the reader and moves to =
the first record. If no record was found this function returns false.

About your question whether to subclass DBRecord or use beans:
Basically DBRecord objects are something like dynamic beans in a way =
that the properties don't have to be declared at compile-time and that =
instead of an individual getter / setter for each property you have =
several generic ones that require a field reference.=20
Personally I prefer subclassing a DBRecord and use it to create other =
type safe methods that work on these derived types. This gives you also =
the opportunity to override methods like getValue(), setValue(), =
onFieldChanged(), isFieldReadOnly() etc.
Using beans IMO only makes sense if you either need to work with other =
frameworks that require beans or if you need to serialize your beans.

Regards
Rainer


acooke.org@gmail.com wrote:
> re: Detecting failure to retrieve data
>=20
> Hi,
>=20
> Another question - hopefully simpler.  How do I detect when a query
> has failed to retrieve data?  In particular I have code like:
>=20
>     cmd =3D schema.createCommand()
>     cmd.select(schema.CANONICALS.getColumns())
>     cmd.where(schema.CANONICALS.VALUE.is(value))
>     row =3D new DBRecord(schema.CANONICALS)
>     reader =3D new DBReader()
>     reader.open(cmd, cnxn)
>     ok =3D Schema.CANONICALS.initRecord(row, reader)
>=20
> where ok is true even when there are no data (value does not exist in
> the VALUE column of the CANONICALS table and "row" contains nulls)
> (and similarly if I use the initRecord method on the reader).
>=20
> Should ok be false above?  It appears to be true, but I must confess
> that I am also using Scala (I have modified it to look like Java
> above, I hope) and it is possible I am doing something stupid with
> Scala...
>=20
> Finally, I was originally thinking that I should subclass DBRecord and
> so map rows to objects, but now I am thinking that it would be better
> to use beans.  Is there any reason to prefer one approach rather than
> another, or are they for different cases somehow?
>=20
> Thanks,
> Andrew