> 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
Sch�tzenbahn 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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