Ok. Sorry to jump to quickly on that one; I am partially wrong.

<dmtl-var expr="_['2000'].id"> will work; so will <dtml-in expr="_['2000'].objectValues()"> (returns a sequence).

However, in the case of something like this: <dtml-var expr="_['5000']._['5100'].id"> try

<dtml-var expr="_['5000']['5100'].id"> cheers, oliver