Defaulting parent entity of custom activity - dynamics-crm-2011

Question

This should be a simple thing, but as I am quickly finding out with CRM 2011, simple things almost never are...
Anyway, I have a custom activity type, a "Trip". A Trip represents a single visit by a service technician to a client's site. Trips are always created in connection with a Case (Incident) and never with any other entity type, so the entity exists as a custom activity that does not "Display In Activity Menus". That allows me to drop a subgrid of Trips into the primary Case view, making it the only place these things can be created.
The problem is that when I try to create a new Trip from the Case form (by clicking the subgrid and choosing "Add New Trip" from the ribbon), the "Regarding" field isn't populated with the Case that I was on when I clicked "Add". Any of the "built-in" activity types will default their corresponding field with no problem, so it would seem to me that I can do the same here. The field's supposed to be read-only once this defaulting works, but just so I can set up the reference when creating new Trips, I've made it editable, and of course when you pop up the lookup, you can assign this Trip to anything (which is why I want the field read-only in the first place).
I need the form to default the parent Case when the Trip form loads. It has to be there when it's first created, because if it isn't there on the first save, then some plugin code that sets other default values (namely data relating to other Trips that may or may not exist for the same Case) won't work properly.
I read about relationship field mapping, where you can default the values of various fields based on fields of the parent entity, but the relationship between the Incident and my Trip is not listed as mappable from either side and I can't figure out how to make it mappable. I also know it's possible to set default values using JavaScript, but if there's a less "custom" way to set this particular behavior up I would love to know how.

Answer 1

As an epitaph based on Mike's comment, it does indeed seem that Microsoft doesn't want this done. But, it's still possible to do it. The basic steps are:
Download the Ribbon Workbench solution and import it into CRM.
Create a new solution containing the entity or entities you want to be able to customize.
Create a JavaScript file containing functions that will open the form you want and pass the information you need as a parameter. There are two ways to specify the parameters; you can define custom parameters on the form, or you can use Microsoft's undocumented parameters for the Regarding field:
function OpenNewTripFromCase()
{
var entityId = Xrm.Page.data.entity.getId();
var entityTypeCode= Xrm.Page.context.getQueryStringParameters().etc;
var entityDisplayName = Xrm.Page.getAttribute("title").getValue();
var params = {};
params["pId"] = entityId;
params["pType"] = entityTypeCode;
params["pName"] = entityDisplayName;
Xrm.Utility.openEntityForm("cst_trip", null, params);
}
Import this JavaScript file as a Web Resource, and add it to the Solution so it will be available to Ribbon Workbench.
Open up Ribbon Workbench and open the solution you created with your entity and resources. Click the Solution Elements tab, and find the "Commands" item. Right-click and select "Add New". Now expand the list, find and click on your new command. In the Properties pane, change the name of the command to be more descriptive (it's recommended you only change the third "Command#" term), then click the magnifying glass button to the right of the Actions field.
In the window that pops up, click "Add", then choose "JavaScript Function Action". A new item will be added to the action list. Click it, then in the properties pane to the right, find the library containing your function, then type in the function name. I didn't get any sort of IntelliSense here, so be careful typing in the function name. Add parameters if you need to (you shouldn't need to with the code above), then click OK
Now click the custom activity entity in the Entities list of Ribbon Workbench, then in the dropdown to the upper right of the ribbon layout, select the SubGrid ribbon. Create a button in this Ribbon, give it the icon and text you want, and in the Properties pane, set the Command to the command you just created.
Publish the customized Solution, and you should, if you did everything correctly, have a new button that will do the same thing as the "Add New" button when you've selected a sub-grid, but will additionally populate the Regarding field.