If I have this schema...
person = { name : String, favoriteFoods : Array }
... where the favoriteFoods array is popula
favoriteFoods
Incase of lookup_food_array is array.
match_stage["favoriteFoods"] = {'$elemMatch': {'$in': lookup_food_array}}
Incase of lookup_food_array is string.
match_stage["favoriteFoods"] = {'$elemMatch': lookup_food_string}