Unfortunately, the pipeline mentioned in Adam Najmanowicz's answer works for some other types, like Droplink and Multilist, but the pipeline isn't run for Droptree fields.
After looking into this deeper I found that the Source of a Droptree field IS using the wrong context item, as Adam mentioned, but the code comes from the Droptree field itself:-
Sitecore.Shell.Applications.ContentEditor.Tree, Sitecore.Kernel
Utilising the query string code from Adam's answer, we can create a 'fixed' Droptree custom field, that is almost the same as the regular Droptree but will use the correct context item instead.
The code will inherit from the normal Tree control, and only change the way that the Source property is set.
public class QueryableTree : Sitecore.Shell.Applications.ContentEditor.Tree
{
// override the Source property from the base class
public new string Source
{
get
{
return StringUtil.GetString(new string[]
{
base.Source // slightly altered from the original
});
}
set
{
Assert.ArgumentNotNull(value, "value");
if (!value.StartsWith("query:", StringComparison.InvariantCulture))
{
base.Source = value; // slightly altered from the original
return;
}
Item item = Client.ContentDatabase.GetItem(this.ItemID);
// Added code that figures out if we're looking at rendering parameters,
// and if so, figures out what the context item actually is.
string url = WebUtil.GetQueryString();
if (!string.IsNullOrWhiteSpace(url) && url.Contains("hdl"))
{
FieldEditorParameters parameters = FieldEditorOptions.Parse(new UrlString(url)).Parameters;
var currentItemId = parameters["contentitem"];
if (!string.IsNullOrEmpty(currentItemId))
{
Sitecore.Data.ItemUri contentItemUri = new Sitecore.Data.ItemUri(currentItemId);
item = Sitecore.Data.Database.GetItem(contentItemUri);
}
}
if (item == null)
{
return;
}
Item item2 = item.Axes.SelectSingleItem(value.Substring("query:".Length));
if (item2 == null)
{
return;
}
base.Source = item2.ID.ToString(); // slightly altered from the original
}
}
The above code is pretty much the same as the Source property on the base Tree field, except that we figure out the proper context item from the URL if we've detected that we're in the rendering parameters dialog.
To create the custom field, you just need to edit the Web.Config file as described here. Then add the custom field to the core database as described here.
This means that parameters can now have queries for their source, allowing us to limit the available items to the content editor. (Useful for multi-site solutions).
