On Mon, Feb 1, 2016 at 4:48 PM, Adrian Klaver <adrian.kla...@aklaver.com> wrote:
> > >> As an example of where this leads see: >> >> >> http://www.postgresql.org/message-id/7224.1452275...@sss.pgh.pa.us >> >> Thanks for the heads up. The good news is all machine access to >> the >> data will be via functions and views so I can inline the >> constraint in >> the right places. In other news, this sucks! I have no idea what >> it >> >> >> I could see moving your constraint into a per row trigger. >> >> >> You'd need to basically replicate the current FK constraint setup but >> with custom queries...you need the insert/update trigger on the main >> table and then a insert/update/delete trigger on the referenced table to >> ensure that actions just rejected if the relevant detail on the main >> table isn't changed. Then decide whether you need something like "ON >> UPDATE/DELETE CASCADE" instead of the default 'reject' behavior. >> >> I take it you would need to ensure that these triggers are disabled >> during dump/restore but am not certain on that point. >> > > Well this brings up another part to Danes post(that contained the function > definition): > > "Unfortunately the "type" definition can't be expressed as a primary key > so I can't use foreign keys to enforce consistency." > > Not sure what exactly is meant by "type", though I suspect it is this: > "SELECT type FROM discount_codes WHERE code ..." > Type in the context that I'm using it simply means the type of thing the function is checking to see exists or not. The example I gave unfortunately had a column named type that confuses the situation but in actually that example is the only one that actually has a column named type involved in evaluation. > FYI, I know type is non-reserved word, but I would avoid using it as a > column name. I went down that path and got myself confused in a hurry:) > > In any case it should be pointed out that FKs do not necessarily have to > point to PKs: > > http://www.postgresql.org/docs/9.5/interactive/sql-createtable.html > > "The referenced columns must be the columns of a non-deferrable unique or > primary key constraint in the referenced table" > I am aware of the fact that FKs don't have to point to PKs but they do have to point to something backed by a unique indexed and that is what makes them unsuitable for my needs. Here is an example where foreign keys can't play a role, but this time as a description. The system can send out surveys to solicit feedback. The admin can customize the surveys and associate them w/ specific class events. The constraint is this, once a survey has a respondent then the survey can no longer be edited. I'm going to try out David's idea of using triggers to implement the more complex constraints and inline the simpler ones in functions where appropriate. Regards, Dane
