[{"_id":0,"group":"Electronics",/** other fields omitted for brevity*/"articles":[{"name":"Gameboy",/** Even more fields omitted for brevity*/"area":"Video Games"},{"name":"Playstation","area":"Video Games"},{"name":"Refrigerator","area":"White Goods"}]},{"_id":1,"group":"Food","articles":[{"name":"Apple","area":"Fruit"},{"name":"Pear","area":"Fruit"}]}]

db.collection.aggregate([{$addFields:{articleAreas:{$setUnion:"$articles.area"}}}])