Filter values in dropdown list

Dear all,

we have a two-layered sampling design (->plot->tree) with multiple campaigns.

In a follow-up inventory we want to ensure that all trees from the previous campaign have been monitored. Thus we upload preset-data from the previous campaign.

Each campaign looks as follows:
- plot information
- tree information
- further measures

To ensure that all trees from the previous inventory are monitored, I have the following idea: - include a checkbox 'tree_completed' as last item of tree information
- for tree->new inventory display a dropdown-list with all tree-ids (level2) from the previous campaign, filtered for those that are already monitored (i.e. tree_completed=true) - hide the 'further measures' until all trees from the previous inventory have been monitored

Therefore, my questions are: - how do I retrieve sampling point data for the second level key (note that tree_id corresponds to 'level2_code', idm:samplingPointData('level2_code',parent()/plot_id) doesn't seem to work) --> I assume that I could duplicate the level2_code column and name it distinctly in the preset-data? - how do I filter this list for a given list/array of tree_ids for trees that have already been measured in the current inventory (tree_ids occur in the current survey and checkbox 'tree_completed'=true)? - how do I implement this in the current inventory?

Thank you for your help, Chris