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‫In this video, we're gonna use

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‫a simple trick in order to prevent users

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‫from writing multiple reviews for the same tours,

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‫so basically preventing duplicate reviews.

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‫So in the last video, we created all these reviews

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‫in order to calculate the averages.

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‫And we created multiple reviews for one tour,

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‫but all from the same user.

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‫But in practice, that doesn't make much sense, right?

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‫So in the real world, each user

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‫should only review each tour once.

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‫So basically, a duplicate review happens

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‫when there is a review with the same user

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‫and the same tour ID.

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‫And that's what we want to avoid from happening.

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‫And the obvious solution here is

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‫to just use a unique index, right?

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‫However, it is not enough to set

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‫both these fields to unique.

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‫And actually that would really be very wrong,

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‫because that would mean that each tour can get only review,

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‫and each user can only write one review.

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‫And obviously that's not what we want.

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‫So what we do need is them both together to be unique,

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‫so the combination of user and tour to be always unique.

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‫So that sounds a bit complicated, but luckily for us,

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‫that's actually very easy to achieve with indexes.

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‫So we already created a compound index on the tour before,

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‫and so now let's do the same here on the reviews.

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‫So again, right here after the schema definition,

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‫and of course we're still in the review model, right?

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‫So review, and that's not correct,

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‫so review schema dot index...

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‫Tour set to one, and once more it's not really important

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‫if it's one or minus one in this case.

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‫And user also set to one, okay.

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‫And so again, that's similar to what we did before,

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‫but here we're going to take it to the next level

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‫and now add an object for options.

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‫And the option here that we're gonna set is unique

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‫(typing)

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‫set to true.

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‫And that's actually it.

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‫This will achieve exactly what we want.

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‫So now each combination of tour and user

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‫has always to be unique.

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‫So let's give it a save here, or two,

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‫and then go test this out actually.

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‫Now when I first tested this,

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‫it actually didn't work right away.

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‫Really, it only just started working the next day,

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‫and I'm not really sure why, but if that happens to you,

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‫then don't worry about that, okay.

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‫And maybe it might even happen now

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‫as I'm recording this video.

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‫So anyway, right now, there should not be any reviews

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‫on the tour that we were working on.

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‫So just to make sure we still have zero ratings

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‫and also the reviews are empty.

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‫And so let's now once more create

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‫a new review on this tour.

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‫Okay.

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‫So this is one review, so one user and one review.

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‫So that of course should work, but if I now send this again

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‫and let's say I set it to four and say okay,

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‫then this should not be allowed to work, okay.

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‫Just keep in mind that it might still work

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‫because of the issue I was telling you before, okay.

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‫So sometimes this kind of index doesn't get set immediately.

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‫Oh, but actually, in my case here it worked right away.

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‫So we get this duplicate key error on this index here,

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‫and so that's exactly the index that we just created before.

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‫So let's take a look at that also here in Compass.

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‫So here it actually is not,

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‫and so I will do it just like I did before

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‫and reconnect basically.

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‫So going to the recents, then connecting here.

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‫Natours...

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‫Then...

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‫Of course the tours,

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‫and our indexes here are now these four.

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‫And so that's the one that we just created,

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‫or actually it's not (laughs), okay.

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‫So for some reason, it's not here,

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‫so that's just what I was saying earlier,

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‫but don't mind because actually it is working in our code.

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‫Okay, so we were not able to create two reviews

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‫coming from the same user, okay.

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‫But if you now log in as someone else,

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‫well, then we will certainly be able to post this review.

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‫So let's see what user we could use now.

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‫So let's get all users here,

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‫which I can't because I'm not an admin (laughs), okay.

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‫So I forgot about that part,

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‫and so I'm simply going to retrieve this from Compass.

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‫Sometimes that's just easier.

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‫So this is a guide, a user (exclaims),

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‫so let's just use this Eduardo here or something.

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‫So let's log in as this person, as this user.

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‫Yep, with success, and now we should be allowed

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‫to create a review, and indeed here it is, okay.

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‫And if we now update this,

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‫then we have our two ratings, great.

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‫Now just testing it again, so if we send it again,

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‫it's not going to work because again,

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‫we have a duplicate error.

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‫Awesome.

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‫There is just one more thing that I wanted to show you,

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‫and so for that, I'm going to log in as yet another person,

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‫okay, so that I can then create another review.

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‫So this time, I'm gonna be this Max Smith.

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‫And so let's log in.

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‫(mouse clicks)

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‫Okay, so now I'm officially Max,

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‫and I will be able to post this review.

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‫So with five,

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‫I'm able to do that,

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‫and now I want to take a look at the average.

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‫Okay, so right now it is this 4.6666,

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‫which of course does not look good.

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‫And we could fix this in the front end.

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‫For example when we request this data

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‫from the API and then display it,

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‫we could on the front end then do this rounding.

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‫But actually, I want to do it right here on the back end

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‫so that the end user already gets this final rounded value,

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‫which in this case would be 4.7, right?

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‫And for doing that, I'm gonna show you a small new feature

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‫in Mongoose that we didn't use yet.

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‫So let's go to our tour model,

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‫and to that average,

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‫so right here.

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‫And so now, we can use a setter function.

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‫(typing)

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‫So set and this function will be run each time

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‫that a new value is set for this field, okay.

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‫And so here, we usually specify a callback function,

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‫which receives the current value.

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‫And in this case, it returns basically this value,

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‫but rounded.

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‫So we use math dot round for the current value,

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‫but the problem with math dot round is that

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‫it rounds values to integers.

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‫So for example, if we had something like this,

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‫it would then round this to five, okay.

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‫And that's not what we want.

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‫We want it to be rounded to 4.7.

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‫And so we're gonna use a trick here,

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‫which is quite common, so multiplying this by 10,

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‫and so with that, we would get 46.666,

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‫and then this rounded would be 47.

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‫And so then, we divide the results by 10 again,

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‫and that then is 4.7.

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‫All right, and that's actually it.

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‫So again, a setter function here which is going

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‫to be run each time that there is a new value

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‫for the ratings average field.

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‫So just to test this, let's actually simply

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‫update this last review that we added,

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‫which had a five, and let's keep it at five.

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‫So all we basically want to do is to re-run

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‫that function which creates this average.

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‫So if I do this now and now get my tour,

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‫we should get 4.7 here as the average.

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‫And yep, here we go.

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‫Great, and that's actually it already for this video.

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‫So we created this nice setter function,

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‫and we created this unique compound index here

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‫in order to ensure that one user

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‫cannot write multiple reviews for the same tour.