[{"categories":["foo","bar","baz"]},{"categories":["foo2","bar","baz"]},{"categories":["foo","bar",["baz"]]},{"categories":["foo","bar","baz"]},{"categories":["foo","bar","baz"]},{"categories":[["foo","bar"],"baz"]},{"categories":["foo9","bar6","baz4"]},{"categories":["foo1","bar3","baz2"]},{"categories":["foo6","bar1","baz1"]}]

db.collection.aggregate([{"$unwind":"$categories"},{"$unwind":"$categories"},{"$group":{_id:null,categories:{$push:"$categories"}}},{"$project":{categories:{$cond:[{$eq:[{$type:"$categories"},"array"]},{"$reduce":{"input":"$categories","initialValue":"","in":{"$concat":["$$value",{"$cond":[{"$eq":["$$value",""]},"",", "]},"$$this"]}}},"$categories"]}}}])