On 5/20/2011 10:15 AM, Seb wrote:
On Fri, 20 May 2011 09:48:45 -0500,
Jack Christensen<jackc@xxxxxxxxxxxxxxxxx> wrote:
Use a loans table with unique partial index to ensure that only one
unreturned loan per item can exist at a time.
[...]
Thanks, this certainly avoids loaning an item before it's returned, but
it doesn't protect against having loans that overlap in time. For
example, an item can have a start_time that is between start_time and
end_time of a previous loan for that same item. My first thought was to
have some CHECK constraint with a query, but this doesn't seem to be
supported by postgresql.
In a similar project I worked on start time for a loan was always the
current time so overlaps weren't an issue. I don't have any firsthand
experience with them, but it sounds like what you want are exclusion
constraints.
http://www.depesz.com/index.php/2010/01/03/waiting-for-8-5-exclusion-constraints/
--
Jack Christensen
jackc@xxxxxxxxxxxxxxxxx
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