Re: Select Query on Table with no Primary Key

From: Craig L Russell (Craig.Russel..un.COM)
Date: Sat Jun 09 2007 - 16:27:57 EDT

  • Next message: Andrus Adamchik: "Re: Select Query on Table with no Primary Key"

    Hi Andrus,

    On Jun 7, 2007, at 11:38 PM, Andrus Adamchik wrote:

    >
    > On Jun 8, 2007, at 7:01 AM, Craig L Russell wrote:
    >
    >> Just FYI, when JDO reads data from tables without PK, it
    >> internally creates a unique id, similar to a generated PK, for the
    >> objects that it reads and these ids are discarded when no longer
    >> needed. The fact that the mapping is for tables without PK is
    >> known by the code that creates the temporary ids.
    >>
    >> Craig
    >
    > Hi Craig,
    >
    > I can probably implement this in Cayenne in about 30 minutes, as
    > Cayenne has a notion of "temporary id" (normally used for new
    > uncommitted objects). The problem of course is uniquing. So say if
    > a DB row is fetched from a table via a query, resulting in object
    > A, and then later the same row is navigated to via a relationship
    > from another object, resulting in a second object B, distinct from
    > A. This breaks the fundamental assumption about object identity. So
    > we chose not to go this way.

    If you can navigate to a single A via a relationship, to me that
    implies that you have some unique constraint by which a single A is
    associated with another row. And if you have a unique constraint,
    then you can use that instead of a primary key.

    Craig
    >
    > Andrus
    >

    Craig Russell
    Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
    408 276-5638 mailto:Craig.Russel..un.com
    P.S. A good JDO? O, Gasp!





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