change insert_one() to insert_many() in mongo_db save function.

[bwook00]

close #438

(Plus, if there is duplicate "_id" key in mongo db, it occurs error and stop everything)
-> This feature is already well supported by mongodb itself (�BulkwriteError)

[vkehfdl1]

(Plus, if there is duplicate "_id" key in mongo db, it occurs error and stop everything)

You need to add test code for this kind of situation.

[vkehfdl1]

You have to give option to user about upsert.
If upsert is True, duplicate id does not occur error and just overwrite existing id.
If upsert is False, duplicate id occurs error.
Sometimes upsert option is super convenient, so we might support upsert feature. Find way to implement this with pymongo.

[bwook00]

If I use pymongo's update_many() with upsert=True, it would update all existing passages into a "single passage".

So we take the upsert parameter directly, and if true, we put the existing ids in a list and only update the duplicates (and even then, we use bulk_wirte to minimize the iteration as much as possible).

Instead of updating all of them, we just proceed with insert_many() for non-existing ids.

(I've been googling hard for a function to update all at once in pymongo, but I haven't found one...)

[vkehfdl1]

finally..