[SQLObject] rename notNull -> required, SQL check constraints

[Bud P. B. <bu...@si...>]

Another thought on the relationship of alternateID and notNull:

Doesn't the former imply the latter?  If so, maybe the interface can
be simplified.

Also, considering that SQLObject kind of hides the SQL and attempts a
pythonic touch and feel, I don't like the term "notNull" since Null
really comes from SQL world.  NotNone would be more pythonic (but
ugly), but what about "required"?

And a change of topic:
Just for completeness, it would be nice to also add Col keyword
argument check, for example check="price > 0",  and a special
attribute _checks, for example _checks=[check("marriageDate >
birthDate")], to access Check constraints of SQL.  (See
http://www.postgresql.org/docs/view.php?version=7.3&idoc=1&file=ddl-constraints.html#AEN1793).  

--b

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