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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> >--
> >***************************************************************
> >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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