Dynamic Default Values for Entity Attributes
Working on my of our internal systems this afternoon I hit one of those requirements that you push to the end of the to-do list and never quite get around to doing until they turn around and bite you.
In my case it was a matter of default values. This particular system needs to know about which Quarter something happens in, and I'd sort of temporarily hard coded FY09Q1 into the system until I go around to doing it properly. You guessed it I then totally forgot to do that and deployed the system live with the hardcoded value.
Sure enough everything was fine for a month or so until FY09Q1 was no longer the right answer anymore and weird stuff started to happen.
Ok we've all done it, fix it and move on.
So let's consider the use case...
Here the quarters are actually being read from a table which maps the start and end of each of Oracle's quarters for the next 10 years or so. Therefore the requirement here is really to set up the default value for the Quarter attribute to equal the Quarter name from the correct row in the Quarters table that matches today's date. Essentially a dynamic default value being read from a table based on some other calculation or factor, rather than being just a simple hardcoded value which we usually use in the default.
The question is can you do this in a declarative way? The answer is yes, thanks to the power of Groovy!
- The first step is to create a new View Object that gives me access to the row I needed that contains the correct Quarter for today's date. Simple enough to do with a Where startdate < = :TODAY and endate >= :TODAY, where :TODAY is a bind variable which has a default value which is set to the Groovy expression
adf.currentdate
- VO does not need to be exposed through the AM as I only use it internally within the model. To get to it using Groovy I needed to open up the entity that contains the Quarter attribute that I wanted to default and create a View Accessor to this VO. This allows me to programmatically walk from the entity to the rows returned by the VO and if I needed to, pass values to the bind variable used by that VO. In this case, the default value of today's date will be just fine though. I called the accessor
DefaultQuarterAccessor -
Finally onto the default value itself. Change the value type to Expression and set the expression to
DefaultQuarterAccessor.first().Quarter, where first() indicates that I want the first (and only) row in the Accessor rowset and from that I want the Quarter Attribute from within that row.
This same technique can be used to do all sorts of useful dynamic things with validators, error messages and default values without having to code up Impl files in Java. For example I use the same approach to look up friendly values for error messages. So rather than a basic message like "salary is too high for department 40" we can walk the related rows and come up with a much more meaningful messages such as: "Salary in Sales Administration is capped at 20,000" where I'm looking up both the proper name of the department and it's actual maximum value using Groovy expressions.
One note of caution though, it's easy to get confused between Groovy usage notation like this and the expression language you use in your view pages. They are different.
Note: This code refers to the production release of JDeveloper 11.1.1.0 (and patches) AKA "Boxer".