Set filter in merged query rule

I don't think you can directly put the query filter options on the result. better you can put the filters in your query rules mentioned here. Or you can put your logical filters using whereContains or where functions. Try it out. 

Fascinating work.  I'm sure others will tell you, that you should do the filtering before the merge, and that would likely be the best option.

However, if you do indeed need to proceed in this direction, I think you could wrap the "expression:" of that forEach with a logic function like a!match and then reject(fn!null on your data.