Overview
This is an article in a series of Sitecore how-to articles. These articles are meant to be quick guides to accomplish various tasks within Sitecore. The how-to articles have proven to be very helpful internally at One North Interactive https://www.onenorth.com. These articles assume working Sitecore knowledge. I just wanted to share the articles with the community. Hopefully you find them helpful.
How-To
How to limit the number of items selected in a treelist field or create a pseudo-single-select field that restricts what templates can be selected.
Currently Sitecore does not support restrictions like IncludeTemplatesForSelection and IncludeTemplatesForDisplay on any single select fields, so you can instead use a treelist that uses validation to restrict the number of items selected. You may also have other reasons to limit a picker to X amount of items
Regular expression validation
Replace the number 1 with any number to allow more than 1 item. (Note the pipe character is escaped)
^({[^}]+}\|?){0,1}$
Step-by-step guide
-
Create a TreeList field that works for selecting multiple of the kind of items you want to pick.
DataSource=/sitecore/content/Sites/Main/Home/Insights/Media Entities&IncludeTemplatesForSelection=MediaEntity&IncludeTemplatesForDisplay=MediaEntity,MediaEntityFolder - Add the regular expression in the validation field on the field item
- Try to violate your rule and make sure your warning pops up