1
00:00:01,110 --> 00:00:02,910
‫So, this is part two

2
00:00:02,910 --> 00:00:05,230
‫of calculating the review statistics.

3
00:00:05,230 --> 00:00:08,873
‫This time, for when a review is updated or deleted.

4
00:00:10,780 --> 00:00:13,310
‫And this part is actually a bit harder

5
00:00:13,310 --> 00:00:15,450
‫because, keep in mind that a review

6
00:00:15,450 --> 00:00:17,730
‫is updated or deleted using

7
00:00:19,829 --> 00:00:21,246
‫findByIdAndUpdate

8
00:00:25,490 --> 00:00:28,277
‫or also findByIdAndDelete, right?

9
00:00:33,540 --> 00:00:37,020
‫So for these, we actually do not have document middleware,

10
00:00:37,020 --> 00:00:39,830
‫but only query middleware, okay.

11
00:00:39,830 --> 00:00:42,630
‫And so in the query, we actually don't have direct access

12
00:00:42,630 --> 00:00:46,140
‫to the document in order to then do something similar

13
00:00:46,140 --> 00:00:48,300
‫to this, okay.

14
00:00:48,300 --> 00:00:51,380
‫Because, remember, we need access to the current review,

15
00:00:51,380 --> 00:00:54,130
‫so that from there, we can extract the tour ID,

16
00:00:54,130 --> 00:00:58,030
‫and then calculate the statistics from there, right,

17
00:00:58,030 --> 00:01:00,130
‫but again, for these hooks here,

18
00:01:00,130 --> 00:01:02,830
‫we only have query middleware, okay.

19
00:01:02,830 --> 00:01:04,890
‫But let me now show you a nice trick

20
00:01:04,890 --> 00:01:07,610
‫to actually go around this limitation.

21
00:01:07,610 --> 00:01:10,040
‫So, we're going to implement a pre-middleware

22
00:01:10,040 --> 00:01:14,073
‫for these hooks, for these events basically.

23
00:01:15,860 --> 00:01:19,730
‫So pre, and then again I'm going to use

24
00:01:19,730 --> 00:01:22,870
‫a regular expression for a string starting

25
00:01:22,870 --> 00:01:25,457
‫with findOneAnd and that's it.

26
00:01:28,641 --> 00:01:30,410
‫And so this one is then going to work

27
00:01:30,410 --> 00:01:33,970
‫for findOneAndUpdate, and findOneAndDelete

28
00:01:34,830 --> 00:01:37,150
‫because, remember that behind the scenes,

29
00:01:37,150 --> 00:01:40,480
‫findByIdAndUpdate is only just a shorthand

30
00:01:40,480 --> 00:01:44,750
‫for findOneAndUpdate with the current ID, right.

31
00:01:44,750 --> 00:01:48,530
‫So here, we actually need to use the findOneAndDelete

32
00:01:48,530 --> 00:01:52,833
‫and findOneAndUpdate middleware hooks, all right.

33
00:01:55,500 --> 00:01:59,970
‫So, function, and it gets the next keyword

34
00:01:59,970 --> 00:02:02,420
‫because it's pre-middleware.

35
00:02:02,420 --> 00:02:05,310
‫So, remember that the goal is to get access

36
00:02:05,310 --> 00:02:07,990
‫to the current review document, okay,

37
00:02:07,990 --> 00:02:11,280
‫but here the, this keyword is the current query.

38
00:02:11,280 --> 00:02:13,650
‫Now, how are we going to go around this?

39
00:02:13,650 --> 00:02:16,150
‫Well, we can basically execute a query,

40
00:02:16,150 --> 00:02:18,200
‫and then that will give us the document

41
00:02:18,200 --> 00:02:20,040
‫that's currently being processed.

42
00:02:20,040 --> 00:02:22,650
‫So in order to do that, we can use findOne.

43
00:02:25,590 --> 00:02:26,590
‫And that's it.

44
00:02:26,590 --> 00:02:29,740
‫So then all we need to do is await this query

45
00:02:29,740 --> 00:02:31,090
‫and then save it somewhere.

46
00:02:34,140 --> 00:02:38,850
‫So let's call it r, which is gonna stand for review, okay.

47
00:02:38,850 --> 00:02:40,763
‫Then save it as async.

48
00:02:42,650 --> 00:02:43,773
‫And, that's it.

49
00:02:45,000 --> 00:02:47,830
‫And just to make sure that this works here,

50
00:02:47,830 --> 00:02:52,340
‫let's for now just log this to the console, okay.

51
00:02:52,340 --> 00:02:55,080
‫So without doing any calculations,

52
00:02:55,080 --> 00:02:56,760
‫all we're really interested in

53
00:02:56,760 --> 00:02:59,960
‫is to see if this nice trick here works.

54
00:02:59,960 --> 00:03:02,410
‫So basically, the trick of going around

55
00:03:02,410 --> 00:03:04,070
‫that in a query midddleware,

56
00:03:04,070 --> 00:03:05,853
‫we only have access to the query.

57
00:03:06,890 --> 00:03:09,850
‫So again, we need to get access to the document,

58
00:03:09,850 --> 00:03:13,210
‫and so we basically execute this query

59
00:03:13,210 --> 00:03:14,463
‫by using findOne.

60
00:03:15,620 --> 00:03:20,603
‫All right, so let's update a review,

61
00:03:22,570 --> 00:03:26,313
‫and so let's actually update the last one that we did,

62
00:03:31,310 --> 00:03:33,050
‫all right, and the rating,

63
00:03:33,050 --> 00:03:35,010
‫now let's actually set it to four.

64
00:03:35,010 --> 00:03:37,080
‫So it was five before

65
00:03:38,130 --> 00:03:42,023
‫and so now we are changing it to four.

66
00:03:43,040 --> 00:03:44,083
‫So it sent that,

67
00:03:46,900 --> 00:03:50,620
‫and here is the review.

68
00:03:50,620 --> 00:03:53,140
‫Now of course, the rating is still set to five

69
00:03:53,140 --> 00:03:56,130
‫at this point, because this findOne here

70
00:03:56,130 --> 00:03:58,850
‫really gets the document from the database,

71
00:03:58,850 --> 00:04:01,660
‫and so at this point of time, in pre,

72
00:04:01,660 --> 00:04:04,900
‫it still didn't persist any changes to the database,

73
00:04:04,900 --> 00:04:06,540
‫and so it was five before,

74
00:04:06,540 --> 00:04:08,990
‫and so now it's still gonna be five.

75
00:04:08,990 --> 00:04:10,460
‫But that doesn't really matter here

76
00:04:10,460 --> 00:04:13,750
‫because all we are interested in is this ID.

77
00:04:13,750 --> 00:04:16,580
‫Actually, this tour ID, right,

78
00:04:16,580 --> 00:04:18,350
‫because that is what we're gonna need

79
00:04:18,350 --> 00:04:21,220
‫in order to calculate the average ratings.

80
00:04:21,220 --> 00:04:24,363
‫Okay, and so now, let's actually use that function.

81
00:04:25,240 --> 00:04:27,850
‫Okay, now, let's think about this

82
00:04:27,850 --> 00:04:32,190
‫because if we were to use this calcAverageRatings function

83
00:04:32,190 --> 00:04:33,850
‫at this point in time,

84
00:04:33,850 --> 00:04:36,100
‫then we would calculate the statistics

85
00:04:36,100 --> 00:04:39,490
‫using the non-updated data, okay.

86
00:04:39,490 --> 00:04:42,390
‫And so that's the exact same reason why up here,

87
00:04:42,390 --> 00:04:45,400
‫we also needed to use post and not pre,

88
00:04:45,400 --> 00:04:48,650
‫okay, because only after the document is already saved

89
00:04:48,650 --> 00:04:50,620
‫to the database it makes sense

90
00:04:50,620 --> 00:04:52,850
‫to then calculate the ratings.

91
00:04:52,850 --> 00:04:55,570
‫And so here, it's the exact same thing,

92
00:04:55,570 --> 00:04:58,650
‫with the big difference that we cannot simply change

93
00:04:58,650 --> 00:05:00,533
‫this pre to post.

94
00:05:01,390 --> 00:05:04,500
‫So we cannot do that because at this point in time

95
00:05:04,500 --> 00:05:07,260
‫we no longer have access to the query

96
00:05:07,260 --> 00:05:10,410
‫because the query has already executed, right,

97
00:05:10,410 --> 00:05:11,500
‫and so without the query,

98
00:05:11,500 --> 00:05:13,670
‫we cannot save the review document,

99
00:05:13,670 --> 00:05:16,410
‫and we can then not run this function.

100
00:05:16,410 --> 00:05:20,360
‫So, this is really confusing, I understand,

101
00:05:20,360 --> 00:05:23,710
‫but I really decided to create this lecture in this way

102
00:05:23,710 --> 00:05:25,790
‫because, well, it's really the only solution

103
00:05:25,790 --> 00:05:28,730
‫around this problem, and it's a really great exercise

104
00:05:28,730 --> 00:05:32,240
‫for you to basically understand this Hello Experience.

105
00:05:32,240 --> 00:05:35,833
‫So, the solution for this is to now use post,

106
00:05:37,520 --> 00:05:40,690
‫so, reviewSchema, then basically the same,

107
00:05:40,690 --> 00:05:44,460
‫so let's just, actually let's just copy all of this,

108
00:05:44,460 --> 00:05:47,640
‫make our lives a bit easier,

109
00:05:47,640 --> 00:05:50,033
‫get rid of this, and get rid of the next.

110
00:05:51,850 --> 00:05:54,320
‫Here, we also need to call next, okay,

111
00:05:54,320 --> 00:05:58,170
‫but then we do it post, okay.

112
00:05:58,170 --> 00:05:59,730
‫And so now this point in time,

113
00:05:59,730 --> 00:06:01,840
‫so after the query has already finished,

114
00:06:01,840 --> 00:06:04,700
‫and so therefore the review has been updated,

115
00:06:04,700 --> 00:06:06,290
‫this is a perfect point in time

116
00:06:06,290 --> 00:06:08,283
‫where we can then call this function.

117
00:06:09,510 --> 00:06:12,720
‫So, calculate average ratings.

118
00:06:12,720 --> 00:06:15,930
‫But where do we now get the tour ID from?

119
00:06:15,930 --> 00:06:17,860
‫Well, we're gonna have to use a trick

120
00:06:17,860 --> 00:06:21,260
‫which is basically to pass data from the pre-middleware

121
00:06:21,260 --> 00:06:23,000
‫to the post middleware.

122
00:06:23,000 --> 00:06:26,690
‫And so instead of saving this document to a simple variable,

123
00:06:26,690 --> 00:06:30,403
‫we're gonna save it to this.r.

124
00:06:31,750 --> 00:06:35,773
‫So basically, we create a property on this variable.

125
00:06:36,700 --> 00:06:40,390
‫Okay, and so now here, we still have access to that.

126
00:06:40,390 --> 00:06:45,350
‫And so now, we can say this.r, which remember is the review,

127
00:06:45,350 --> 00:06:47,603
‫and then .tour.

128
00:06:49,220 --> 00:06:53,300
‫So, this is again quite confusing,

129
00:06:53,300 --> 00:06:56,850
‫but again also quite fun once you understand

130
00:06:56,850 --> 00:06:58,873
‫how this really works, okay.

131
00:06:59,800 --> 00:07:03,200
‫Now, again, we need something like this here

132
00:07:03,200 --> 00:07:06,320
‫in order to actually call this function here

133
00:07:06,320 --> 00:07:08,460
‫because remember that this in fact

134
00:07:08,460 --> 00:07:12,570
‫is a static method, and so we need to call it on the model.

135
00:07:12,570 --> 00:07:15,500
‫Now where is this model in this case?

136
00:07:15,500 --> 00:07:20,500
‫Well, it's at this.r,

137
00:07:20,848 --> 00:07:22,550
‫which is in this case, equivalent

138
00:07:22,550 --> 00:07:25,113
‫to this this here in this middleware,

139
00:07:26,427 --> 00:07:29,217
‫.constructor.calcAverageRatings.

140
00:07:31,860 --> 00:07:35,403
‫Woo, that looks quite overwhelming, doesn't it?

141
00:07:36,600 --> 00:07:38,100
‫Of course, we have to await it,

142
00:07:38,100 --> 00:07:42,600
‫so that's why we declared this as async, okay.

143
00:07:42,600 --> 00:07:46,210
‫So, again, we basically used this way here

144
00:07:46,210 --> 00:07:48,530
‫of passing the data from the pre-middleware

145
00:07:48,530 --> 00:07:50,930
‫to the post middleware, and so then here

146
00:07:50,930 --> 00:07:55,060
‫we retrieved the review document from this variable.

147
00:07:55,060 --> 00:07:59,970
‫Okay, and again, we did have to do it in this way

148
00:07:59,970 --> 00:08:01,317
‫because at this point in time here,

149
00:08:01,317 --> 00:08:03,550
‫the query was already executed,

150
00:08:03,550 --> 00:08:06,783
‫and so we could not do this here.

151
00:08:11,950 --> 00:08:16,753
‫And let's actually write that down; does NOT work here,

152
00:08:19,690 --> 00:08:21,253
‫query has already executed.

153
00:08:22,610 --> 00:08:24,130
‫Great, and with this,

154
00:08:24,130 --> 00:08:27,103
‫we should actually now be ready to test this.

155
00:08:28,100 --> 00:08:32,850
‫So let's go ahead and update this one here once more,

156
00:08:32,850 --> 00:08:37,493
‫and set it to a rating of one, all right.

157
00:08:39,160 --> 00:08:41,060
‫Let's now let's take a look here,

158
00:08:41,060 --> 00:08:44,020
‫until you see that the number of ratings is still five,

159
00:08:44,020 --> 00:08:46,620
‫but the average is now only three,

160
00:08:46,620 --> 00:08:49,220
‫and now the question is if this really updated

161
00:08:49,220 --> 00:08:50,593
‫also on the tour,

162
00:08:53,740 --> 00:08:58,150
‫and, yep, indeed, it did.

163
00:08:58,150 --> 00:09:02,040
‫So, five ratings with an average of three.

164
00:09:02,040 --> 00:09:06,220
‫Great, let's now update another one.

165
00:09:06,220 --> 00:09:09,543
‫So, in order to that, let's get all our reviews,

166
00:09:11,570 --> 00:09:14,280
‫and actually we do not want all of them,

167
00:09:14,280 --> 00:09:16,201
‫but only the last one.

168
00:09:16,201 --> 00:09:18,730
‫And here I still I have this filter,

169
00:09:18,730 --> 00:09:20,253
‫so I should get rid of that,

170
00:09:21,650 --> 00:09:26,650
‫so we get all 65 results, so all of these reviews,

171
00:09:26,980 --> 00:09:29,280
‫but we only want the last ones,

172
00:09:29,280 --> 00:09:31,073
‫so the ones that we just created.

173
00:09:32,820 --> 00:09:34,160
‫So let's update...

174
00:09:37,570 --> 00:09:42,090
‫I'm not sure, because actually they're not in order here.

175
00:09:42,090 --> 00:09:47,090
‫Let's, okay, so here we have the most recent ones.

176
00:09:47,490 --> 00:09:50,963
‫So let me now update this one here, to let's say, four,

177
00:09:56,540 --> 00:09:59,410
‫just as a final test, and so that should

178
00:09:59,410 --> 00:10:01,173
‫bring the average up a little bit.

179
00:10:02,490 --> 00:10:04,090
‫And indeed it did.

180
00:10:04,090 --> 00:10:07,400
‫So, 3.6 now, and of course, the number is still five

181
00:10:07,400 --> 00:10:10,400
‫because you didn't add a new review.

182
00:10:10,400 --> 00:10:12,700
‫So it works great on update,

183
00:10:12,700 --> 00:10:15,463
‫let's now actually also test it with deleting.

184
00:10:17,470 --> 00:10:21,710
‫So let's go ahead and delete the review that we just updated

185
00:10:23,350 --> 00:10:26,860
‫and so now we should be down to only four,

186
00:10:26,860 --> 00:10:28,550
‫and indeed, here we are.

187
00:10:28,550 --> 00:10:30,100
‫So now, only four.

188
00:10:30,100 --> 00:10:32,053
‫And let's actually delete all of them.

189
00:10:35,410 --> 00:10:36,723
‫So, this one is next.

190
00:10:45,038 --> 00:10:46,180
‫Ah, here we are.

191
00:10:46,180 --> 00:10:47,303
‫Then also this one.

192
00:10:53,560 --> 00:10:55,760
‫So, now we should be down to three only

193
00:10:56,970 --> 00:11:00,350
‫or, actually, we only just have two left

194
00:11:00,350 --> 00:11:01,893
‫with an average of four.

195
00:11:05,600 --> 00:11:08,300
‫So let's see which one we have still left.

196
00:11:08,300 --> 00:11:12,330
‫So it's this one with three and this one with five.

197
00:11:12,330 --> 00:11:15,203
‫And so that's why we have this average of four.

198
00:11:19,210 --> 00:11:21,320
‫So let's get rid of this one

199
00:11:21,320 --> 00:11:25,020
‫and so now our average should be five, right?

200
00:11:25,020 --> 00:11:26,340
‫Or actually three

201
00:11:26,340 --> 00:11:29,340
‫because the only one that's left has three.

202
00:11:29,340 --> 00:11:33,350
‫And now the final test, deleting the last one,

203
00:11:33,350 --> 00:11:35,993
‫just to see what's gonna happen once we do that.

204
00:11:41,070 --> 00:11:42,530
‫And, we get an error here.

205
00:11:42,530 --> 00:11:46,400
‫So, cannot read property nRating of undefined.

206
00:11:46,400 --> 00:11:50,683
‫So that's calcAverageRatings at line number 69.

207
00:11:53,280 --> 00:11:54,903
‫So, don't know what this is.

208
00:11:55,760 --> 00:12:00,440
‫So, line 69, and so, it's this problem here.

209
00:12:00,440 --> 00:12:03,530
‫So we're trying to read nRating of undefined.

210
00:12:03,530 --> 00:12:06,420
‫So stats zero is basically undefined.

211
00:12:06,420 --> 00:12:09,400
‫And that's because if there are no document

212
00:12:09,400 --> 00:12:11,780
‫matching this query here basically,

213
00:12:11,780 --> 00:12:13,947
‫well then we simply get back an empty array.

214
00:12:13,947 --> 00:12:17,100
‫And so that's actually exactly what we have down here.

215
00:12:17,100 --> 00:12:19,350
‫So this is the stats array

216
00:12:19,350 --> 00:12:22,630
‫and so right now it doesn't have any results, okay,

217
00:12:22,630 --> 00:12:25,500
‫so we should only execute this piece of code here

218
00:12:25,500 --> 00:12:28,653
‫whenever we actually do have something in the stats array.

219
00:12:29,810 --> 00:12:30,903
‫So let's do that.

220
00:12:31,760 --> 00:12:36,760
‫So if stats.length is greater than zero,

221
00:12:40,090 --> 00:12:42,623
‫well, then do this.

222
00:12:45,450 --> 00:12:48,100
‫And if not, well, basically that means

223
00:12:48,100 --> 00:12:50,260
‫that all our reviews are gone,

224
00:12:50,260 --> 00:12:54,280
‫well then we basically want to go back to the default.

225
00:12:54,280 --> 00:12:58,780
‫So we're gonna set it then to the quantity of zero

226
00:12:58,780 --> 00:13:02,030
‫and the average of 4.5, which remember,

227
00:13:02,030 --> 00:13:04,943
‫is the default when there are no reviews at all.

228
00:13:08,540 --> 00:13:10,823
‫So let's quickly create a new review,

229
00:13:12,200 --> 00:13:13,150
‫and it can be this,

230
00:13:15,880 --> 00:13:19,400
‫all right, so we're back to having one rating,

231
00:13:19,400 --> 00:13:20,780
‫and now delete it right away,

232
00:13:20,780 --> 00:13:24,963
‫just to test that piece of code we just wrote,

233
00:13:28,080 --> 00:13:30,310
‫just to see it here as well,

234
00:13:30,310 --> 00:13:32,573
‫so one rating, average five.

235
00:13:33,850 --> 00:13:36,113
‫And now when we delete it,

236
00:13:38,910 --> 00:13:41,790
‫we no longer get an error.

237
00:13:41,790 --> 00:13:45,630
‫And on our tour, we are back to zero

238
00:13:45,630 --> 00:13:48,450
‫and a default of 4.5.

239
00:13:48,450 --> 00:13:51,560
‫All right, all right, all right, perfect.

240
00:13:51,560 --> 00:13:54,920
‫So let's again take just a quick second here

241
00:13:54,920 --> 00:13:57,090
‫to recap what we just did.

242
00:13:57,090 --> 00:13:59,970
‫So in order to be able to run this function here

243
00:13:59,970 --> 00:14:02,320
‫also on update and on delete,

244
00:14:02,320 --> 00:14:04,590
‫we actually need to use the query middleware

245
00:14:04,590 --> 00:14:07,110
‫that Mongoose gives us for these situations.

246
00:14:07,110 --> 00:14:11,230
‫Okay, so, we do not have a handy document middleware,

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‫which works, for these functions,

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‫but instead we need to use the query middleware,

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‫and in that one, we do not directly have access

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‫to the current document.

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‫And so we need to go around that

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‫by using this findOne here, and so basically retrieving

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‫the current document from the database.

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‫We then store it on the current query variable,

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‫and so that's this, and by doing that,

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‫we then get access to it in the post middleware.

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‫And it's then only in the post middleware

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‫where we actually calculate the statistics for reviews.

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‫And remember that we do it this way

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‫because if we did it right in this middleware function,

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‫then the underlying data would not have been updated

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‫at that point and so the calculated statistics

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‫would not really be up to date.

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‫And so that's why we used

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‫this two-step process here basically.

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‫Now, let's get rid of this console.log here,

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‫since we're finished now, and also of these statistics.

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‫So, I hope that this was fun to you

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‫and not all too overwhelming.

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‫I designed this exercise specifically

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‫so that we could solve a real world business problem

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‫by using all the tools that Mongoose gives us.

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‫And so, with that, I hope that you learned a bit better

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‫how to work with all these different middlewares

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‫in different situations whenever needed

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‫in a real world situation.

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‫So, great job for finishing this quite challenging lecture

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‫and for still being with me at this point of the course.

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‫It's really good for me to see,

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‫so I'm really happy about that.

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‫But anyway, there's still some more great content coming up

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‫in this section actually,

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‫and so let's now move on together, right to the next video.