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Now another typical use case would be the opposite, that you have an application where you want to

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find out whether the user is in a certain area,

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so we don't want to find all places in an area but we want to store a couple of different areas potentially

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in the database,

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let's say the neighborhoods of a city and then your user sends some coordinates because he located

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himself and you want to find out in which neighborhood the user is.

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So essentially the same query as before, just the other way around.

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Now to keep things simple, I will work with the same area but I will now store it in the database and

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I then want to find out if a user let's say is here, if this user is inside this area and we'll also

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check this with a point outside of the area then.

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So let's first of all store our nice polygon which we used here and therefore, I will just copy this geo

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json object, let's store this in the database.

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For that I'll add a new collection, areas,

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of course you can name this however you want

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where I will insert one area and that area will have a name of let's say Golden Gate Park and then I'll

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add a field,

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I could name it

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polygon but I'll just name it area,

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that name is totally up to you,

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the value will be my geo json polygon object with the coordinates I stored before.

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So this added this object and we can have a quick look into our areas to confirm that we have all the

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coordinates for the corners here,

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first and last coordinate pair have to match. Ok now, so we got that place in here,

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now with that place added, let's find out if this user is inside of that area.

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Now how can we do that?

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First of all we need to create an index again,

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so let's run db areas create index and then the index should be created on the area key which I added

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and should be a 2D sphere index of course.

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So with the index added, we can write our query and there we want to find out basically if the user is

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inside of this area,

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we can do this by checking for geoIntersects.

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Now what does geoIntersects do? geoWithin allowed us to find out which places are inside of an area,

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geoIntersects simply returns all areas that have a common point or a common area.

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Now why don't I use geoWithin because I can't find areas that are within the point where the user is,

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the user is just a point

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and the area will never be within that point.

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The other way around is what I want to see and I get this by simply checking

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does this point intersect with the area?

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If the answer is yes,

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so if I do get an area returned, I know that the user is well in that area which is returned because

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that is what I'm trying to find out, in which area is the user.

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So geoIntersects is what I need to use there and as before, we can simply pass a geometry here with

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$geometry which is now again a geo json object,

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this is now just a point, the coordinates of the user and the coordinates here are a simple array of

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the longitude and the latitude.

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So let's grab the longitude of that user here,

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like this and let's grab the latitude,

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so this first value here. If I hit enter, I get an error because of course geoIntersects is generally

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all right

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but the field for which I'm looking is the area field and then this geoIntersects operator gets used on

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the area field,

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so my mistake. Just as before,

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this doesn't change,

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we need to let mongodb know hey in which field is the data I want to compare and that of course is the

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area field in our database because the area field is where do I store the area I'm looking for

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and then I use geoIntersects.

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So with this fixed, now we do get a place here and that is looking good.

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I'm getting the only place I have in that database but I'm getting the place with which my point intersects

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and if my point would be in multiple areas, I would of course get all these areas which intersect with

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my point.

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You can also not just intersect points with areas, you could also intersect areas with areas and see

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if two areas touch. Let's say we want to find out which neighborhood contains the Golden Gate Park, the Golden

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Gate Park could be a polygon and the neighborhood around it could be one too and you could find out

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if they intersect and if yes, which neighborhood intersects with the Golden Gate Park. Here we just looked

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at a point

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and now to complete this, let's also look at a point which is not inside the Golden Gate Park.

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So I'll add a new point outside of it, here and definitely feel free to pause the video at this point and do this

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on your own,

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I will do it together with you.

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Here are the coordinates

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and in the end, we just have to repeat the last query, this time add the new coordinates,

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so the new coordinates of this new point for the point we want to see if it intersects with the Golden

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Gate Park and hit enter and we don't get back a result because in our areas collection, we have no areas

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that would contain this point.

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This is the answer this gives us.