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We already had a look at a couple of pipeline stages and operators, now I want to dive into some stages

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in operators that help you with working with arrays and therefore, I prepared some new data which you

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can either import with mongo import or you just go into your mongo shell, create a new collection on

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the fly,

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I'll name it friends and I'll insert many documents and then add an opening parentheses,

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no closing parentheses,

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copy in the values and then close it and hit enter

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and this should now also add the three elements.

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So in the friends collection, I now have these three documents, three persons with name, age and hobbies

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and then some exam scores.

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And I just need some arrays here because you can do quite a lot of things with arrays in the aggregation

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framework.

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Now what can we do with these arrays?

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Well one thing you often want to do is that you want to kind of merge or combine arrays in a grouping

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stage,

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so let's start with that.

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So for that, I prepared a brand new aggregation pipeline and we'll start with grouping our results.

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I will group by age, let's say and by the way, you can also group if you just group by one field, like this

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but I'm a fan of the document syntax because there you can assign your own names and make it clearer

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by what you grouped.

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So I will group by age and of course I do have an age on my documents,

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I only got three, so I will only have, well not that many groups but still let's do that. Let's group by age

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and let's then say we want to combine the hobby arrays, so that we know which hobbies exist for this age group

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and for that age group. And we can do that, we can add an array, we can name it hobbies or you give it

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a brand new name, like all hobbies, whatever you want

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and now there are two operators mainly that help you with combining array values because you have to

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keep in mind that in the group stage, multiple documents are merged into one,

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so we all see the way of handling multiple array values in one array. And we can do that here with the

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push operator, the push operator allows you to push a new element into the all hobbies array for every

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incoming document.

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Now every incoming document here will have a hobbies array,

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this one, this hobbies field and you could push any field value into the array, not just existing arrays,

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so what I will do is I will now push $hobbies.

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Now let's give this a try,

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let's take this aggregation pipeline and run it,

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however not on persons but on friends,

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so let's rerun it, like this.

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Now what you see is that we have two groups, age 30 and age 29 and all hobbies is an array of arrays because

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we push hobbies into our array and hobbies happens to be an array itself, here

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hobbies is an array and we just push that value into our all hobbies array here.

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That is what I meant, you can push any value of course, not just existing arrays but what if you wanted

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to push the values of existing arrays in there but not as one array but you want to pull these values

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out of this array and then add them to all hobbies.

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Well that is also something you can do and I will show you how it works in the next lecture.