The case of the missing Coveo facet picker [by Jeremy Davis]

Quick one this week. Mostly to try and save my own blushes, because the issue here was completely my fault. For the first time on a particular project I was trying to do some Coveo development work. I had created a page based on the default MVC templates they provide for search, but when I tried to add a Facet in Content Editor, I found myself staring at this:

No picker for the field to facet on – so no way to make the Facet component work...

Over the course of a frustrating couple of hours I tried a load of tests to see if I could spot an issue:

Was Coveo working?

Diagnostics said yes.

Any obvious config errors?

Nope, nothing looks like an error here. But a subtle hint that I failed to spot.

Were there any Javascript errors in the client?

Nope – nothing that seems relevant.

I disappeared off down a few rabbit holes at this point, wondering if the project had done something in its custom configuration that might have been breaking bits of the Sitecore UI. I spent a good hour bouncing between config files and Google, getting nowhere. But that finally lead me to looking at the HTML DOM around the missing control:

And suddenly the clouds lifted: The picker control is there – but it's expressed as a custom UI control: <coveoui:facetpicker/>. The custom control isn't being executed. I'd been assuming that because it looks like an ordinary dropdown list, it was just a standard UI control. But custom controls in the Sitecore UI need to be registered so that Sitecore knows what code to run to make them work. And when I fired up the ShowConfig.aspx page and tried searching for this control's definition I found nothing.

So checking the Coveo configuration include files I found: