On Jun 13, 1:24 am, Steven D'Aprano <[EMAIL PROTECTED]> wrote: > On Tue, 12 Jun 2007 08:53:11 -0700, Frank Millman wrote: > > Ah, if I had ever read that there were two instances involved, I hadn't > noticed. Sorry! >
No problem - I really appreciate your input. I snipped the rest of your post, as I have figured out a way to make my problem go away, without changing my code radically. It was a 'sequence of events' problem. 1. create table a 2. create table b, create pseudo column on table a, link to column on table b 3. create table c, create pseudo column on table b, link to column on table c This works most times, but if, in step 2, I want to link the pseudo column in table a to the pseudo column in table b, it fails, as the pseudo column in table b does not get created until step 3. Now, if I try to link to a column that does not exist, I set a flag and carry on. When I go to the next step, after creating the pseudo column, I check if the flag exists, and if so I go back and create the link that it tried to create in the first place. It is not very pretty, but it has the big advantage that, by the time all the tables are opened, all the links are correctly set up, and I never have the problem of trying to access non-existent attributes. I will try to explain what benefit all this gives me. Assume a Customers table and an Invoices table. Customers has a primary key which is a generated 'next number', and an alternate key which is the Customer Code. Users will never see the primary key, only the alternate key. Invoices has a foreign key CustomerId, which references the primary key of Customers. However, when capturing an invoice, the user wants to enter the code, not the primary key. It is not difficult to program for this, but it is tedious to do it on every data-entry form. Therefore, in my framework, I set up a pseudo column on Invoices called CustomerCode, which the application programmer can use as if it is a real column. Behind the scenes, I automatically use that to check that it is a valid code, and populate the real column with the primary key. This has been working for some time, and really simplifies the 'business logic' side of things by abstracting a common idiom which would otherwise have to be coded explicitly every time. Now I have added a complication. I have decided to implement the idea of using a single table to store details of all parties with whom we have a relationship, such as Customers, Suppliers, Agents, etc, instead of separate tables each with their own Code, Name, and Address details. Therefore I now have the following- a Parties table with a 'next number' primary key and a PartyCode alternate key, a Customers table with a 'next number' primary key and a PartyId foreign key reference to the Parties table, and an Invoices table with a CustomerId foreign key reference to the Customers table. Now when capturing an invoice, the user enters a Code, then the program must check that the code exists on Parties, retrieve the primary key, then check that it exists on Customers, retrieve that primary key, then store the result on Invoices, with appropriate error messages if any step fails. I have successfully abstracted all of that, so all that complication is removed from the application. Hope that makes sense. Thanks very much for all your attempts to help me, Steven. You have succeeded in getting me to think properly about my problem and come up with a much cleaner solution. I really appreciate it. Frank -- http://mail.python.org/mailman/listinfo/python-list
