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Home » Modeling » EMF » [CDO] Database mapping: explicitly specify nullable columns(The default null/not-null is not consistent among every database engine)

[CDO] Database mapping: explicitly specify nullable columns [message #1855834]

Mon, 07 November 2022 04:20

Eclipse User

I found this issue for nullable EDate attributes in a mysql database.
In mysql, the first(!) column of type "timestamp" will get the default value "current time", if the column is not explicitly declared as nullable. This can lead to very strange effects in CDO, because first the value seems to be set correctly (to null), but once the value is read again from the database, the value changes to some timestamp.

Potentially, the fix is quite simple. In DBAdapter.java, line 965, instead of using an empty string for nullable fields, we should return "NULL" instead.

  protected String createFieldDefinition(IDBField field)
  {
    return getTypeName(field) + (field.isNotNull() ? " NOT NULL" : ""); //$NON-NLS-1$ //$NON-NLS-2$
  }

Re: [CDO] Database mapping: explicitly specify nullable columns [message #1855849 is a reply to message #1855834]

Mon, 07 November 2022 12:03

Eclipse User

I'd prefer to keep the impact of applied changes as small as possible. So, why not change just MYSQLAdapter and limit the new behavior to just the datatypes that need it?!

Re: [CDO] Database mapping: explicitly specify nullable columns [message #1855866 is a reply to message #1855849]

Tue, 08 November 2022 04:41

Eclipse User

I am no SQL expert. I only searched the web and found e.g. this post: https://www.red-gate.com/hub/product-learning/sql-prompt/why-you-should-always-specify-whether-a-column-accepts-nulls

But I also get your point that this global change does have some risks. For our use case, the restriction to the mysql engine is sufficient, of course.

Re: [CDO] Database mapping: explicitly specify nullable columns [message #1855875 is a reply to message #1855866]

Tue, 08 November 2022 12:03

Eclipse User

I've read the that article and I agree with it.

More importantly, the behavior compatibility issue is not as severe as I thought.
Your proposed change only impacts the DDL generation, not the internal schema model itself.
So, with your proposed change we'd not create any schema deltas for existing DBs.
Good ;-)

I've submitted and resolved https://bugs.eclipse.org/581014 .
Please CC yourself in case we must discuss regressions caused by the change....

Re: [CDO] Database mapping: explicitly specify nullable columns [message #1857643 is a reply to message #1855875]

Fri, 17 February 2023 13:42

Eclipse User

I think the derby database does not allow that. https://db.apache.org/derby/docs/10.16/ref/rrefsqlj16095.html#rrefsqlj16095
So you have to set -Dorg.eclipse.net4j.spi.db.DBAdapter.NO_NULLABLE_DEFAULT=true

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