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