Re: Select Query on Table with no Primary Key

From: Dave Merrin (dmerri..pasystems.co.uk)
Date: Thu Jun 07 2007 - 09:36:22 EDT

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

    Hi Andrus,

    I was playing about with putting in my own EntityResolver. For now I'm
    going to take your approach and see how I get on.

    Cheers,

    Dave

    Andrus Adamchik wrote:
    > To give an example - if you have an EMPLOYEE table, you can make an
    > assumption that it is highly unlikely that there are two people with
    > the same name, born on the same date, and working in the same
    > department. On that assumption you can mark these 4 columns as PK in
    > the Modeler: FIRST_NAME, LAST_NAME, DATE_OF_BIRTH, DEPARTMENT_ID.
    >
    > Works well with views or tables (updateable or read-only) on any DB.
    >
    > Andrus
    >
    >
    > On Jun 7, 2007, at 1:29 PM, Andrus Adamchik wrote:
    >
    >> Tore is right - for Cayenne to handle an object (whether read-only on
    >> read/write), it needs to know which column or columns uniquely
    >> identify each row. Now... you can fake a PK in your model, even if
    >> there's none in the db - just select a really unique combination of
    >> columns, and mark those columns as the PK in the modeler. I've mapped
    >> tables with such "imaginary PK" a lot.
    >>
    >> If it is not possible (i.e. duplicate rows are expected to be
    >> fetched), you will have to use DataRows.
    >>
    >> Andrus
    >>
    >>
    >> On Jun 7, 2007, at 1:04 PM, Tore Halset wrote:
    >>
    >>> On Jun 6, 2007, at 16:12 , Dave Merrin wrote:
    >>>
    >>>> I'm trying to run a SelectQuery on a table with no primary key.
    >>>> Unfortunately it's not working. Can anybody help? I have no control
    >>>> over the database so I can't add in primary keys.
    >>>
    >>> As you know the PK are essential not only to update a row, but to
    >>> make sure a single row maps to a single DataObject in your context.
    >>>
    >>> Some database engines do have a unique invisible column. If your
    >>> database does this, then perhaps you could map that column as your
    >>> primary key? What database engine are you using?
    >>>
    >>> Regards,
    >>> - Tore.
    >>>
    >>
    >>
    >
    >



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