MongoDB for Developers with Python Transcripts
Chapter: MongoDB's shell and native query syntax
Lecture: Concept: Atomic, in-place updates

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0:01 Despite the fact that MongoDB is a NoSQL database
0:03 it does adhere to the acid properties under certain circumstances.
0:07 Primarily that means updates to individual documents are guaranteed to be atomic,
0:12 and along with those, we can get great performance
0:15 as well as safety if we don't pull the document back for the database,
0:19 make changes and push it back hoping no one else has changed it
0:22 during that intervening time there,
0:24 but in fact we can go to the database and go make this change here
0:27 I don't care if it's a 100k document, don't pull anything back
0:30 just make this little change and that happens atomically and safely.
0:34 So the operators that we have to work with are increment, multiply,
0:37 rename a field, set on insert set unset, like basically delete a field,
0:42 min and max so I would like to set the value
0:45 but only if this value is lower than the one I'm passing,
0:48 or the one that's in the document or set it to the max,
0:51 like only set the value to this if this new value is bigger than the existing one.
0:55 You can also use current date to basically grab the server date and save it there as well.
1:00 So these are the in place individual updates and we can see how that works
1:03 so we'll come over here and let's insert just a book
1:06 and this time our book has a view count, right, the view count is zero,
1:09 maybe every time somebody pulls up the book we want to increment that,
1:13 so we can say test.update and give it the object id
1:16 right here is a real simple one so it was fits onto the screen basically
1:20 you can say $inc increment view count by one,
1:23 and we do this a few times, so we've done it three times
1:26 it should go from zero to— well you guessed it, three
1:29 and it all happened atomically in the database,
1:32 without us ever pulling it back or worrying about any sort of concurrency whatsoever.
1:36 So this is great for working with individual fields
1:39 sometimes we need to work with arrays,
1:42 so we saw like for example our ratings object
1:44 maybe we want to work with that atomically.
1:47 So MongoDB has operators for that as well,
1:50 so we have things like add to set, so suppose it's got like a votes list,
1:55 people who have voted on this book,
1:57 not the values just keep it simple, just the users who have voted
2:00 and that contains user id, so you could say add to set user id when they vote
2:04 and that would actually only add them there, if they're not already in that list;
2:08 what's cool about that is
2:11 if they push the little vote button twice, it doesn't count twice,
2:14 just either you add it there and the person has now voted for or they haven't.
2:17 Another good example is tags, like think stack overflow, I want to tag a post
2:21 so you could say add the tag python, add the tag mongo,
2:24 and if it's already there, it's just going to leave it alone
2:26 if it's new, if it's not there it will actually add the tag.
2:29 So these are really cool to add to set for kind of uniqueness on these subarrays.
2:33 We also have pop and pull for pulling things out,
2:35 pull all say I want to remove all the votes by a particular user, things like that.
2:40 Also push, so push is like add the set
2:42 without the unique desk constraint, and that's it,
2:45 I definitely recommend you think about these atomic updates,
2:47 they are not simple, but they are better performing
2:51 and they are definitely safer as well.
2:54 Like I said before, it's great that the odm, the object document mapper
2:59 that we're going to look at, MongoEngine automatically does this behind the scenes,
3:02 we don't ever have to even know how they work,
3:05 but it's important that you know that they exist and why they're good for you
3:08 when you look at the logs, and you look the performance
3:11 and think about things in that way.