MongoDB for Developers with Python Transcripts
Chapter: Mapping classes to MongoDB with the ODM MongoEngine
Lecture: A parting glitch
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Let's look at one final thing, I think it is not beautiful but knowing about it and expecting it is really, really important,
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not in the beginning, but as you evolve your application, you'll end up with some funky complications.
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I actually chose the cars that I wanted to update very, very carefully, let me run this again, if I list the cars, notice here in particular state
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far away from this Ferrari F40 from 2005, there's only one of those, right, notice the id it's d15 and ends in 7e, if I try to list the cars again
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there's that one but oh, it ends in ae, list the cars again— now it's ending in a1, what is up with that?
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Just to be clear, the other ones are not changing, like 0f that's always the value, for the first one 0f,
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there's not a problem with Mongo or anything like that, what is going on here is this car was inserted into the database
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when we just had a little bit of our class to find here, remember in the beginning, we didn't have this default concept
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when I first introduced it, and somewhere along the way after we had inserted a few cars, then we added this, let's look at Robomongo.
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If you flip through here, you'll see almost all the cars have a vi number, vehicle id number, vehicle id number
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except for that F40 from 2005, right at the top— none. Because when it was inserted, there was no definition for a vi number
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what the heck was that thing anyway, how was it supposed to know that was not here yet but would eventually exist; so we can do a couple of things,
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the reason you keep seeing this changing numbers is that there's a default value and basically it gets created every time
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it comes back from the serialization layer, but it doesn't get set from the database because there is nothing to set it to,
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so every time it goes back, it reruns that lambda and gets a new value and we're not saving it.
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So basically what we need to do is we need to upgrade our documents, now sometimes like I said, this doesn't matter,
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but this one where we kind of counted on a default value to be there, and then it wasn't, well that's unfortunate.
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So here's something, there's a couple of things we could do, I could simply come up here and write the script in Javascript
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and apply that, that's one option, another option is let's go here and let's write make sure
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we just below configure Mongo I'll say update_doc_versions, or something like this define that function here, and what we can do is something like this
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it's not exactly in a work, but it'll give you the idea what we're after; so we'll say for car in car.objects so basically
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let's look through everything in this collection and let's save it back, I'm going to run this, and let's list the cars and see if we solved it,
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I really wish it wasn't at the top but there is, 19, 02 what's going on, well, it turns out that unless we somehow forcibly change this object
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it's like hey, this object is not dirty and we don't need to push it in there. And say mark as changed, vi number and let's try it again—
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here we go, so we told it that it changed sadly even though it generates a default value
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it doesn't look back into the database which I guess would be super expensive, it just says hey someone changed this, right
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it didn't really trigger that, it came from the databases none and then we set it,
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so it doesn't know to push that through, so you have to do this little bit of a trick here
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to say mark has changed, and PyCharm says you're not supposed to do that, let's just tell it hey, don't make me look bad,
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we have to do this you understand it, right? Very well, okay, so now we've got this save back to the database,
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we only want to run this the one time, right, we don't want to run at all the time, this is just like a one time upgrade of our documents
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and if you have a 100 thousand records, probably fine, if you have a billion records this is not how you want to do it,
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you want to do some kind of in place updater or something better, write the script, so let's run this again, and now we should see our car here,
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this is the 2005 F40, you know what, it is time for new tires, let's service this puppy.
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Now we come down here and say I don't know how much new tires are in Ferrari, but let's say they're 2000, new tires,
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the customer is pretty happy they had the low profile ones they were looking for but they could have been like a certain kind, who knows, whatever.
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Perfect, that worked, now if we list our cars again, you can see that this one that was basically, we couldn't get to
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because its vin number kept changing is now fixed and that's because when we reran that we said hey, force of the default of the vin number in there,
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notice that none of these other ones changed, it did write them back to the database, but it wrote them back
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in exactly the way they were before, so nothing changed there, I'll just run it one more time, this second one is,
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I'll just copy this and we can go pull it back in a second, so if we put this back one more time, and then we try to service this car
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one dollar test service they were pretty thrilled with that here you can see the test service, okay. So the ids are not changing when we do this,
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it's just if they're not there they are created. Alright, so if that seemed kind of annoying, I'm sure it was annoying,
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but let's think how this would be in a relational database, what would have happened if this was a say SQLAlchemy or something to that effect,
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or if this was some other thing, we would have a lot more trouble evolving from one to the next, right, so we wouldn't have the problem of
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hey here's a car that doesn't have a vin number, because if we didn't actually go and manually changed the database
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every time we added something here, when we added this we would have had to go back to the database
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and do like a migration or data transformation SQL query to actually change and add this column, same thing for this,
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but none of that was required, it was just this one case where we went back in time that we had to do a little bit of work here.
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So, sometimes you still have these scripts you've got to run, sometimes you still have these changes, you got to do and consider the version history,
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but it's much much less often than with relational databases where every little change requires a script
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or it's just oops things are out of sync, bam we can no longer work, but I did want to point that out to you that look,
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you're going to have to be really careful, some of the time. as these things evolve, how are you going to deal with the fact
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that in the database there is this thing that has no vehicle id number. If we're using PyMongo, it would have just come up as none
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or key error or something like that, it would have been a little more obvious but that's just one of the trade-offs you get with these ODMs.