DML currently not allowed

This almost belongs in my sarcastically titled, “Meaningful Error Messages ..” series, but as it has traceable causes, I thought I’d write something up on its most frequent causes (I meant to write this up a few months ago, but it slipped my mind (find it in your heart to forgive an ol’ developer) and now I think it’s time the world knew).

‘DML currently not allowed’, what could that mean? Well it means that DML isn’t allowed. And that it’s not allowed right now, but with the slight promise that given time, it will be allowed. Often in the past, hoping against hope, I kept on pressing that button/link/onclick-area, but the cloud never changed its mind. With a sigh I constructed the Google search query and began trundling through the results.

Strangely enough I’ve found myself in similar situations a few times, but with a different source and solution each time. So now let’s try, you and I, to write-up a nice, consolidated list of the whys-and-hows of ‘DML currently not allowed’.

Situations that may cause ‘DML currently not allowed’

1. Venturing a DML operation within a component controller.
2. Set in motion a DML operation within a class constructor.
3. Undertaking a DML operation within a getter or setter.

A short time in the past, I wanted to write my own debugger within Apex, something similar to log4j (I have big love for that logger).

Alas alack,

it could not be,

mainly because,

of points 2 and 3.

– Wes

Don’t judge me because I wax lyrical.  So about those situations..

Fixing previously mentioned situations

1. This one is surprisingly easy to fix. There is a VisualForce component attribute that can be set such that DML is allowed within its controller i.e. <apex:component allowDml="true" ../>
2. This one is a bit trickier. Often you need some sort of DML to occur on a page-load, so you try to schtuck it into the controller constructor.. but that ain’t gonna work. I would suggest in most cases that you use the ‘action’ attribute on the tag i.e. <apex:page action="{!doStuff}" .. />. This method will run after the constructor and work in the same way as all other action methods.
3. In my opinion, this is the trickiest one of all, and it’s a doozy… especially for developers who’ve worked in other OOP languages. The problem here is that only action methods can be used to perform DML. What about DML with getters and setters? No can do. With VisualForce driven by Apex your mindset has to shift, and you need to think of methods as belonging to two broad categories, action methods, and data methods. You should only be performing DML in action methods (those guys that do stuff and return PageReferences or Void), and use data methods to pass object and variable values around.

In the dark recesses of my mind there’s a whisper of a memory of similar errors happening if you get your trigger -> callout -> DML operation in the wrong order. But I’m (mostly) certain that this results in a ‘You have uncommitted work pending. Please commit or rollback before calling out’ error, so that resolution doesn’t belong here.

That’s all I have on this topic, maybe you have another point I’ve missed. If so, I’d be rather happy if you shared it.