1
00:00:02,230 --> 00:00:11,320
To play around with cursors, I'll use my movie data database and there, remember we had movies where

2
00:00:11,690 --> 00:00:18,280
if I counted, we have 240 documents. By the way count is already one method we can chain on a cursor because

3
00:00:18,330 --> 00:00:19,610
find gives us a cursor,

4
00:00:19,610 --> 00:00:25,820
so whatever we call here is called on the cursor and count simply has a look at the cursor and determines

5
00:00:25,820 --> 00:00:29,020
how many elements can I get from the cursor,

6
00:00:29,060 --> 00:00:30,590
so that's important.

7
00:00:30,620 --> 00:00:31,880
How does this work technically?

8
00:00:31,940 --> 00:00:38,750
Well on the database side, the query will already have ran and therefore information like this is already

9
00:00:38,750 --> 00:00:41,670
loaded into memory and ready to be delivered to us,

10
00:00:41,690 --> 00:00:46,460
so it's not like that if we now cycle through all the elements, we have to manually read them from a

11
00:00:46,460 --> 00:00:50,730
file, they have been loaded efficiently into memory there already.

12
00:00:50,750 --> 00:00:58,320
So now with this, we can of course execute find with the pretty command and the pretty command is really

13
00:00:58,320 --> 00:00:59,430
just the utility command,

14
00:00:59,430 --> 00:01:04,470
we could also output it like this and get a bunch of documents, to be precise 20.

15
00:01:04,540 --> 00:01:10,350
That's just something the shell does for us and the shell also gives us this shortcut to type it to

16
00:01:10,350 --> 00:01:11,510
get the next 20

17
00:01:11,610 --> 00:01:16,620
and we can do this until this is exhausted, the it command will not be available to you when working with

18
00:01:16,680 --> 00:01:20,470
a driver and there, you will indeed work differently with the cursor.

19
00:01:20,640 --> 00:01:28,290
So on find, then in most drivers, you will have some next method you can call. If I call next here, I get

20
00:01:28,290 --> 00:01:29,970
back exactly one document,

21
00:01:30,060 --> 00:01:33,570
hence you also see no type it to see more at the bottom.

22
00:01:33,670 --> 00:01:38,320
I get back exactly one document because next gives me the next document,

23
00:01:38,370 --> 00:01:41,300
the cool thing is now that I called next,

24
00:01:41,460 --> 00:01:43,780
I got back Glee with the id 8,

25
00:01:43,980 --> 00:01:49,470
if I call it again, I get back the next document in line.

26
00:01:49,610 --> 00:01:55,220
However, this again is glee because this query is now basically executed from scratch.

27
00:01:55,550 --> 00:01:57,720
This is different

28
00:01:57,740 --> 00:02:05,510
if I store my data cursor, however you want to name it, you could name it just cursor in a variable or constant

29
00:02:05,630 --> 00:02:10,550
and since the shell uses javascript, I can use javascript syntax to create such a variable here.

30
00:02:10,790 --> 00:02:16,230
So here I can use db.movies find without any condition to find all

31
00:02:16,610 --> 00:02:22,730
and now I can use my data cursor here with next to get the first element which again is Glee with the

32
00:02:22,730 --> 00:02:23,510
id 8

33
00:02:23,660 --> 00:02:31,160
and if I now use that same cursor again with next, I get a different element with the id 7, Homeland

34
00:02:31,290 --> 00:02:36,110
and If I type this again, I get yet another document ID 5,

35
00:02:36,140 --> 00:02:37,580
True Detective.

36
00:02:37,610 --> 00:02:40,090
So this is the cursor doing its work.

37
00:02:40,190 --> 00:02:47,580
The cursor also has other methods, so that was our data cursor here, if I execute it like this, it gives

38
00:02:47,580 --> 00:02:49,030
me 20 documents again

39
00:02:49,290 --> 00:02:54,210
but if I use that data cursor, I also can specify forEach method.

40
00:02:54,210 --> 00:02:58,600
Now again this will vary on the driver you are using, for javascript,

41
00:02:58,650 --> 00:03:04,860
you can use it like this, forEach takes a function that will be executed for every element that can

42
00:03:04,860 --> 00:03:06,610
be loaded through the cursor

43
00:03:06,960 --> 00:03:09,780
and now in there, we therefore get the document.

44
00:03:09,780 --> 00:03:13,680
Now this is just a special function syntax provided by javascript

45
00:03:13,680 --> 00:03:19,320
with that arrow here, here is the input to the function which is given to us by the forEach method

46
00:03:19,470 --> 00:03:23,070
and here is then our function body where we write what we want to do

47
00:03:23,310 --> 00:03:28,420
and then we got print json which is a method provided by the shell to output the document.

48
00:03:28,500 --> 00:03:33,820
And what this would do now is it would cycle through all the documents that are in the cursor

49
00:03:34,020 --> 00:03:37,440
and that by the way are only documents we didn't look at yet,

50
00:03:37,470 --> 00:03:43,860
so the two we fetched via next and the 20 we fetched via just typing the cursor name will be

51
00:03:43,860 --> 00:03:44,910
excluded

52
00:03:44,910 --> 00:03:49,500
and now we will cycle through the remaining documents which we haven't fetched yet and it will fetch

53
00:03:49,500 --> 00:03:52,590
them from the database and output them here as you see.

54
00:03:52,740 --> 00:03:55,080
And that is why you see no type it

55
00:03:55,090 --> 00:04:02,540
to see more at the bottom because we did now fetch all documents in the database simply with forEach.

56
00:04:02,940 --> 00:04:07,260
So this another way of using the cursor to go through all the documents

57
00:04:07,320 --> 00:04:11,170
and of course inside of the function you execute on every document,

58
00:04:11,310 --> 00:04:17,420
you could also have some condition, an if check to for example see if you want to output that document,

59
00:04:17,500 --> 00:04:21,030
though just be aware, if you use an if check to filter out a document,

60
00:04:21,060 --> 00:04:26,460
it would be more efficient to filter that already as a filter in the find method and not fetch it from

61
00:04:26,460 --> 00:04:30,690
the database because here, you're only filtering on the client side,

62
00:04:30,690 --> 00:04:36,060
that of course is not as efficient as filtering on the mongodb server side but you can run

63
00:04:36,090 --> 00:04:42,410
any code you want on all the documents or as I mentioned, use next to only fetch one at a time.

64
00:04:42,540 --> 00:04:48,070
By the way, if I execute next now, I get an error because the cursor has been exhausted.

65
00:04:48,360 --> 00:04:54,460
You can check for that manually with the hasNext field here,

66
00:04:55,740 --> 00:04:58,330
function excuse me, hasNext function,

67
00:04:58,320 --> 00:04:59,130
this is false

68
00:04:59,130 --> 00:05:01,920
and this indicates there is no data to be fetched.

69
00:05:02,250 --> 00:05:09,400
If I recreate my cursor by again reassigning it and this was a constant, so I'm not allowed to do that,

70
00:05:09,400 --> 00:05:13,090
so let's create a new one, just named cursor.

71
00:05:13,240 --> 00:05:18,200
So now I'm creating a new cursor based on the same query but the shell doesn't know that.

72
00:05:18,670 --> 00:05:23,290
So I got a new cursor, now on that cursor if I call hasNext, I get

73
00:05:23,300 --> 00:05:28,320
true and hence I can safely execute next.

74
00:05:28,600 --> 00:05:31,230
Now with cursors, you can do a couple of other things too,

75
00:05:31,240 --> 00:05:33,350
there are other methods you can execute

76
00:05:33,370 --> 00:05:39,130
and again this is something which you can always of course find in the official docs. There, under

77
00:05:39,160 --> 00:05:44,410
mongodb crud operations, query documents, you want to have a look at iterate a cursor in the mongo shell to

78
00:05:44,410 --> 00:05:48,380
learn more about using that cursor and different ways of iterating through it

79
00:05:48,520 --> 00:05:54,660
and we'll also have a look at how to use a cursor in the environment of our mongodb drivers.

80
00:05:54,700 --> 00:05:59,170
It is important for you to understand what a cursor is and why it exists though and I hope this became

81
00:05:59,170 --> 00:05:59,880
clearer.