Thanx for all the responses.
If it is not possible to use the DB Generated values then arent there better
ways to generate a globally unique number(if that is the sole purpose and
not keeping count) which can be used as PKeys? It would save me a database
hit to get the last update value for a loss of a few CPU cycles(depending on
the complexity of the algorithm).
What would be your reactions do this approach? But it would be really
convenient in the future to get some EJB level support for DB generated
values.
Atul.
-----Original Message-----
From: Seibert, Dan [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, July 05, 2000 11:09 AM
To: [EMAIL PROTECTED]
Subject: Re: DB Generated Primary Key
Atul,
I have used a method very similar to the one described by Thomas, and it
works.
I am using CMP Entity beans with ORACLE.
True, I do have Oracle specific SQL statements in my code.
And, I am probably tied to a particular EJB Container,
but at the current level of maturity of CMP EJBs, who isn't?
Check the archives for the thread "Creating Entity Beans with db generated
sequence numbers"
for a similar discussion.
Dan
> -----Original Message-----
> From: Thomas Preston [SMTP:[EMAIL PROTECTED]]
> Sent: Wednesday, July 05, 2000 11:17 AM
> To: [EMAIL PROTECTED]
> Subject: Re: DB Generated Primary Key
>
> I think that you are confused about how the seq gets used in code snippet.
> It is called by the method (select name.nextval ....), a container managed
> variable is set from the return value, and then the seq is never used
> again.
> If you get back 1, I don't care when the container does the insert, the
> pk
> *will be* 1 (the container will just persist the value that I set in
> ejbCreate). The container can insert the row with pk 2 before inserting
> row
> with pk 1. That is ok.
>
>
> >From: Stefan Hanenberg <[EMAIL PROTECTED]>
> >Reply-To: A mailing list for Enterprise JavaBeans development
> ><[EMAIL PROTECTED]>
> >To: [EMAIL PROTECTED]
> >Subject: Re: DB Generated Primary Key
> >Date: Wed, 5 Jul 2000 16:48:45 +0200
> >
> > > Your argument implies that it matters which row gets pk 1 and which
> gets
> >pk
> > > 2 when two ejb entity objects are created at the same time, but to me
> it
> > > doesn't. All I care about is that each of the objects gets a distinct
> >pk
> > > and a known pk. When the getNewFoodId() method returns I will
> *always*
> >get
> > > a unique known pk. So what is the problem? I don't care which one
> gets
> >1
> > > and which one gets 2 as long as I know which gets which (and i do know
> > > because of the return value).
> >
> >But have a look at the following situation:
> >
> >The entity Objects a + b should be created. getNewFoodId() for A returns
> >1 and getNewFoodId() for B returns 2. So within A you set the
> >pk-attribute id to 1 and within B id is 2.
> >After that the container creates the representations of b and a (first b
> >and then a....you cannot be sure what object he creates first!!!). Then
> >the database-entry 1 contains the attributes of B, and conversely. But
> >when the client asks for the attribute id of B he gets 1!
> >So if he creates a primary key with the 1 and executes the
> >findByPrimaryKey() of the client gets the object A!!!!!
> >
> >I think concerning unknown primary keys and database generated pks there
> >is a lack in the ejb-spec. Most of the discussion was about to create a
> >unique identified in your own. If the algorithm creates really unique
> >keys.....that would be perfect.
> > >
> > > I am interested to continue this discussion because the db gen'd pk
> >thread
> > > comes up often with no "right way" to do it closure that I have seen.
> > >
> > > >From: Stefan Hanenberg <[EMAIL PROTECTED]>
> > > >To: A mailing list for Enterprise JavaBeans development
> > > ><[EMAIL PROTECTED]>, [EMAIL PROTECTED]
> > > >Subject: Re: DB Generated Primary Key
> > > >Date: Wed, 05 Jul 2000 16:06:18 +0200
> > > >
> > > >Hi Thomas,
> > > >
> > > >I think there is NO possibility in EJB to use a DB generated primary
> > > >key! The code you are using has one fault:
> > > >
> > > >If two enterprise objects will be created the same time, you cannot
> be
> > > >sure, that you got the "right newFoodID" for every object.
> > > >
> > > >For example: you create object A and Object B the same time. When
> > > >ejbCreate for A is invoked getNewFoodId() returns id 1, When
> ejbCreate
> > > >for B is invoked getNewFoodId() returns id 2. But you cannot be shure
> > > >that A will be really created before B!!!
> > > >
> > > >One solution could be, that getNewFoodId() created a new row in your
> > > >table, and the method ejbCreate does not use an INSERT but an UPDATE
> >for
> > > >accessing the db. But in this case you have to be sure, that every
> > > >ejbContainer, where you bean is installed, allows you to create the
> > > >SQL-statements on you own. That is not required by the EJB-spec.
> > > >
> > > >Within the ejb-spec (9.4.2) you can find:
> > > >"The container may create the representation of the entity in the
> > > >database immediately after ejbCreate(...) returns, or
> > > >it can defer it to a later time (for example to the time after the
> > > >matching ejbPostCreate(...) has been called, or to the end of the
> > > >transaction)."
> > > >
> > > >That means: If your Bean should be deployed in every ejbContainer,
> you
> > > >never know when is accesses the database.
> > > >
> > > >Stefan
> > > >
> > > >Thomas Preston schrieb:
> > > > >
> > > > > In ejbCreate, call a method which gets the next primary key from
> the
> >db.
> > > > > Here is an example of ejbCreate and methhod that gets next seq
> from
> >db
> > > >(this
> > > > > can be a CMP bean):
> > > > >
> > > > > public void ejbCreate( String Product_code) throws
> > > >RemoteException,
> > > > > CreateException
> > > > > {
> > > > > try {
> > > > > //create primary key
> > > > > m_Food_id = getNewFoodId();
> > > > >
> > > > > //init rest of fields
> > > > > //fields passed in to constructor
> > > > > m_Product_code = Product_code;
> > > > >
> > > > > //fields not passed in to constuctor
> > > > > m_Ingredient = null;
> > > > >
> > > > > }
> > > > > catch(Exception exp) {
> > > > > throw new CreateException(
> exp.getMessage()
> >);
> > > > > }
> > > > > }
> > > > >
> > > > > private Integer getNewFoodId() throws Exception
> > > > > {
> > > > > Connection connect = null;
> > > > > Statement statement = null;
> > > > > ResultSet results = null;
> > > > > Integer FoodId = null;
> > > > >
> > > > > try {
> > > > > connect = getConnection();
> > > > > statement = connect.createStatement();
> > > > > statement.executeQuery("select " +
> > > >getSequenceName() + ".nextval from
> > > > > DUAL");
> > > > > results = statement.getResultSet();
> > > > > if ((results != null) && (results.next()))
> > > > > {
> > > > > FoodId = new Integer(
> > > >results.getInt(1) );
> > > > > }
> > > > > else {
> > > > > throw new CreateException
> >("ejbCreate:
> > > >sequence failed to return a
> > > > > value");
> > > > > }
> > > > > }
> > > > > catch (SQLException sqe) {
> > > > > throw new CreateException
> >(sqe.getMessage());
> > > > > }
> > > > > finally {
> > > > > if (statement != null) statement.close();
> > > > > if (results != null) results.close();
> > > > > connect.close();
> > > > > }
> > > > > return FoodId;
> > > > > }
> > > > >
> > > > > >From: "Atul Kulkarni(CTS)" <[EMAIL PROTECTED]>
> > > > > >Reply-To: A mailing list for Enterprise JavaBeans development
> > > > > ><[EMAIL PROTECTED]>
> > > > > >To: [EMAIL PROTECTED]
> > > > > >Subject: Re: DB Generated Primary Key
> > > > > >Date: Wed, 5 Jul 2000 11:53:23 +0530
> > > > > >
> > > > > >As I did not receive any post on this do I consider that there is
> >no
> > > > > >solution to this problem? Any hint will suffice.
> > > > > >
> > > > > >Regards,
> > > > > >Atul.
> > > > > >
> > > > > >-----Original Message-----
> > > > > >From: Atul Kulkarni(CTS) [mailto:[EMAIL PROTECTED]]
> > > > > >Sent: Saturday, July 01, 2000 2:17 AM
> > > > > >To: [EMAIL PROTECTED]
> > > > > >Subject: DB Generated Primary Key
> > > > > >
> > > > > >
> > > > > >Hi all:
> > > > > ><Q1>
> > > > > >I have a case where I create many records of type PayObject where
> >the
> > > > > >primary ky is generated by the DB itself. If in this case I want
> to
> >map
> > > >an
> > > > > >entity bean to this table, how do I do it?
> > > > > >
> > > > > >Also in the database there are some other records being inserted
> in
> >the
> > > > > >same
> > > > > >transaction which hold a reference to PayObject object ( ie have
> a
> > > >column
> > > > > >which holds the primary Key of PayObject ). So how does the
> object
> >know
> > > >its
> > > > > >own primary key on creation?
> > > > > ></Q1>
> > > > > >
> > > > > >I am using WL 4.5.1 with Toplink.
> > > > > >
> > > > > >Thanx in advance.
> > > > > >Atul.
> > > > > >
> > > > >
> > > >
> >
> >=========================================================================
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> > > > >
> > > >
> >
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> >________________________________________________________________________
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> ________________________________________________________________________
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> >
> >--
> >***************************************************************
> >Dipl.-Wirt.Inform. Stefan Hanenberg
> >Mathematics & Computer Science
> >University of Essen
> >Schutzenbahn 70
> >D-45117 Essen
> >Germany
> >
> >email: mailto:[EMAIL PROTECTED]
> >www: http://www.cs.uni-essen.de/shanenbe/
> >phone: ++49-201-183-2168
> >fax: (+49) 201-183 2419
> >***************************************************************
> >
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