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We already covered a lot about how to retrieve data, how to store data, update and delete it

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and we're nearing the end of this module

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but before we conclude the module, there are two other important things you gotta know

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and the first one is projection.

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Now what is projection?

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Imagine you have this kind of data in your database, some information about a person,

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in this case me with name, age, job and that ID.

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Now imagine in your application,

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you don't need all this data,

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maybe you just need the name and age for a given page on your website.

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Of course you could fetch all the data and filter it in your application, in any programming language

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you can of course manipulate the data you're getting back but you would still transfer it over to the

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wire,

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so you would kind of impact your bandwidth, you would fetch unnecessary data and you want to prevent this,

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so it would be better to filter this out on the mongodb server

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and this is exactly what projection can help you with. Projection is super easy to use,

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let me show it to you.

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We've got our passengers and on the passengers, let's do a quick find again, here are our passengers and

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now let's say I'm only interested in the names of the passengers.

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Now you can do projection by using find,

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now you need to pass a first argument because I actually want to pass a second argument and that first

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argument now is an empty document because I want to find all passengers and I don't want to pass a filter

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and the second argument now allows us to project. And here, the projection is set up by passing another

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document,

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so another pair of curly braces

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and here you simply specify which key value pairs you want to get.

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For that, you simply name the field you want to get,

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so let's say in our passengers, we have a name and we want to get that name,

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so we type name here and then as a value, simply a one,

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this means include it in the data you're returning to me. If we now output that, we get all our fields

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with just a name and well still the ID,

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so the age is missing and the age is of course not missing in a database but only in the data we retrieved

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from it,

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so in the data we're showing here.

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So this is projection doing its job

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but what about the id?

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The id is a special field in your data, by default it's always included,

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so the default in general is everything is included

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but even if you use projection as we do it with the second argument, the id is always included too.

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If you use projection like this normally, only what you add here, so name in this case is included but

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the id is also always included, you have to explicitly exclude it if you don't want to add it.

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Now you exclude something by simply specifying its name and then using 0 instead of 1,

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we could do the same for age but this is not required because the default for everything but the id

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is that it's not included if you don't add it in your projection configuration but for the ID, we have to

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be very clear about it.

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And now with that if I hit enter, we get only the objects with the names.

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So this is projection doing its job

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and the important thing to understand here is that this filtering or this data transformation is happening

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on the mongodb server,

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so this happens before the data is shipped to you

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and this is of course what you want because then you don't get unnecessary data,

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you don't impact your bandwidth and therefore you can only work with what you need in this case.