Can you Overwrite this.datasource in a ColdFusion App?

I'm a little confused, having read through the whole thread. Are you saying you want to be able to change this within different copies of your application (f/e dev, stage & prod) or are you saying you want one part of your application to have a different value of another part of the same application?

If it's the former, you can easily do this by having some external set of information that the application reads from wherever, then uses to set the values you want within the Application and/or this scopes. For example, we used to use INI files for this. You'd place an INI file in your dev location with dev values, you'd place another within your prod location with prod values, etc. Then, on startup, your application would read from its own INI file and set all of these values as needed. You wouldn't include the INI file in the application's code repo, you'd have another repo for your infrastructure-as-code stuff and the INI file would go in there. This approach works pretty well.

If it's the latter, well, you're saying something that's kind of weird: "I want this minimal time-saving thing that works automatically to not work in this specific case". If that's the case, maybe this.datasource isn't the best tool for your job. Just set the datasource attribute for all your queries.

Dave Watts, Eidolon LLC

 

We all know the benefits of being able to write `this.datasource` in your Application.cfc. However, does anyone know if there's a way to override or change this value elseware in the app?

By @Homestar9

Any answer to the question will contradict the very benefits you mention. Application.cfc contains functionality common to every page and is included at the beginning of a page. As such it is the bootstrapper of the entire application.

That is its power. That power would be lost if any page could one-sidedly change Application.cfc's behaviour in the way you propose.

An analogy to illustrate. Suppose the State Department uses Fort Knox as the default storage location for all US embassies around the world. An urgent situation arises at the embassy in Kabul. The only viable solution is immediate, temporary storage in Islamabad. 

Imagine this change having the effect of automatically switching the storage location of every US embassy to Islamabad. We don't expect their ColdFusion application to do that, do we?

 

My only theory as of now (untested) would be to create a method in the Application.cfc which would re-assign the datasource dynamically like this:

function setDatasource() {
  // read configuration file...
  this.datasource = 'foo';
}

You could then call this method somewhere in your app using the application scope like this:
`application.setDatasource()`

That is good CFML, which will run. But it won't solve your problem. The reason follows.

The this scope pertains to an instance of a CFC. Before ColdFusion runs any page, it includes an instance of Application.cfc. The this scope pertains to that particular instance, during that particular page request. A new page request generates and uses a new Application.cfc instance. Therefore, the value set in the this scope will not persist from one page request to the next.  

One way out would be to make the application.setDatasource()* call wherever and whenever needed. But that might amount to just as much work as adding a datasource attribute to any query that needs one. 

If the use-case you describe - having the freedom to dynamically change the datasource for the entire application - is important to your application, then you will have to make some code changes. To make the change just once, you  could implement it as follows:

1. Delete the this.datasource line from Application.cfc. Replace it with application.datasourceUsed="defaultDSN"; in onRequestStart.
2. Define the datasource attribute for every tag or function that needs it. Give the attribute the value #application.datasourceUsed#.
3. From here onwards, any change in the value of application.datasourceUsed anywhere, at any time, will instantly be in effect throughout the application.

-----------------------------------------------

*Appendix: Alternative to your setDatasource idea

<!--- On the CFM page where required --->
applicationInstance = createObject("component","path.to.Application");
applicationInstance.datasource="foo";

Well, when you refer to "the benefits", they are primarily two: if you do a cfquery/cfstoredproc (or their script equivalents), those use the value in this.datasource if you don't specify one on the cfquery/cfstoredproc...but you can of cours ejust specify one there, and it will use that.

Or (second) if you are using orm, it uses the value in this.datasource...unless you override it in this.ormsettings, to be some other dsn. 

So can you clarify which of those two (or some other benefit) you are getting, and whether somehow either of the offered alternatives don't work? It just wasn't clear from your question.