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So I'm still in that same database

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I was in the end of the last video and let's now use that aggregate framework.

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We use it in a very simple way, instead of running find on persons or find one,

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we now run aggregate and we already saw this one time in this course already with look up in the schemas

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and relations module. The aggregate method takes an array and it takes an array because we define

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a series of steps that should be run on our data.

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The first step will receive the entire data right from the collection you could say

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and the next step can then do something with the data returned by the first step and so on.

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Now one important piece of information, aggregate does not go ahead, fetch all the data from the database

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and then give it to you and then do something on it,

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of course the first step runs on the database and can take advantage of indexes,

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so if you filter in the first step or you sort there, you can take advantage of your indexes,

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so you don't have to fetch all the documents just because you're using aggregate. Aggregate as find executes

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on the mongodb server and therefore can take advantages of things like indexes

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and it will.

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So what could be a first step that we run? Now to make this a bit easier to read, I'll type this in multiple

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lines by not closing that expression immediately

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and the first step is then a document, every step is a document

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I should say. Now the first step here could be a match phase and match essentially is just a filtering step,

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so you define some criteria on which you want to filter your data in that persons collection.

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Now you can filter here, in the same way you can filter in find,

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so all the things you learn there on how you can query documents, how you can match for greater than

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values and so on applies here too.

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So what we could do here is we could for example and if we have a look at our data structure again, we

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could for example look at females only.

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So to do that, let's simply add a condition here where we say gender should be equal to female.

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Now we can already finish our pipeline at this point if we want to,

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we can close the square brackets around our stages and close the parentheses of aggregate and I get

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back a cursor here,

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you can tell by the fact that I see type it here too.

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So just like the find method, the aggregate method returns a cursor,

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so you have to use it as you use the cursor returned by the find method in your application.

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Besides that, we can also pretty print that

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and now you see, well some results at least of the results we fetched and we should only see females

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in there.

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This was of course only one step, the match step and the match step is not that exciting because there,

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we as I said query it exactly the same way we can query in the find method or we filter in the same

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way I should say. More interesting is the group stage so

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let's take a look at that in the next lecture.