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Back in the shell and still connected to that same mongodb instance we used earlier,

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let's again check out our flight data collection with find

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and as you saw, use find pretty to get a pretty output,

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you see we got three documents in there.

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We got the documents with our own custom ID,

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we got the other document with incomplete information but the automatically generated ID and our original

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document, we already saw insertOne.

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Now let me actually start with clearing this database now before we then insert data again but this

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time with insertMany. Now to clear the database, we have to use a delete command and we can simply use

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our flight data collection because all these commands are always executed on a collection where you

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want to add or delete documents and there, we got deleteOne or deleteMany.

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Now here I want to clear all the data and for this, actually the easiest way would be to simply get rid

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of the entire collection

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but since that would be boring,

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let's go with delete one to simply see how that works.

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Delete one takes a filter to find out which document to delete

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and now we could filter for all kinds of things.

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A filter is defined as a document,

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so with curly braces and then in its simplest form, you now simply define which key and which value

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you want to delete or the document with that key and value you want to delete.

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So for example here, we could say departure airport,

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if that is equal to txl so to Berlin tegle, then I want to delete that document

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and what this would do is it would go ahead, find the first document in our database where the departure

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airport is txl and delete it.

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So if I hit enter here, you see this was accepted, it was successful

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and we see it deleted one document.

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So if we now again use our flight data and find all documents and pretty print them, we only see two documents

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now, the one with our custom ID and the first document we ever entered.

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This means that this middle document was now deleted because it was the first document with the departure

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airport of txl which it found,

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obviously we don't have to use the departure airport,

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we could have looked for an ID too. So I could also use db.flightdata.deleteOne and I could have also

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looked for _ID and then maybe my custom ID, this one here,

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txl-lhr-1,

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if I do this, it would delete this document. But let me also show you how to delete many documents.

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Now the thing is we got nothing in common,

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if I just execute deleteMany like this, it fails though,

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so I need to pass in a criteria.

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Now we got nothing in common for our elements here,

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so let's first of all update them real quick to add a new field to both documents so that they have something

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in common, so that we can delete them in bulk, so that I can also show you this.

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So for this, let's go back to the collection and now use update one, update one updates as the name suggests, one

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element and for update one, we first of all define a filter where we say which document should be updated

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and now let's say we want to start with this first element here.

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So I want to update the document where in my case here,

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maybe distance is 12000,

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this should select this first document.

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The second argument then is how I want to change it

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and now the question is what do you pass there.

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Now let's say we want to add a new field

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and therefore we pass a document here, marker which hold some text, delete. By the way as you see here, you

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can use single quotation marks for the value too

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but you can also use double quotation marks.

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Now this might add a new field marker with the value delete to that first element

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but if I hit enter, I actually get an error and the error is that the update operation document must contain

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an atomic operator or atomic operators.

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Now this is a very valid error

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and there's not really much you can do with it if you're just starting with mongodb.

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The thing is it simply is an incorrect type of data for updating,

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you don't pass in a document describing your change like this because Mongodb doesn't know how to interpret

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this,

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instead what you do pass here is a document with curly braces with a special keyword, $set

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and this is something you can remember, whenever you see something with dollar sign in mongodb, it's

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a reserved operator or word, $set is simply identified by mongodb when used in the update

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one operation to describe the changes you want to make.

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The value of $set then is a document with curly braces and we need to close another pair

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of curly braces for our surrounding curly braces now marked in purple and now what this does is it tells

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mongodb hey for this document you're finding,

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please set this value and if this value did exist, it would change it to delete and if it does not exist

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as in our case, it will add it.

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So if I now hit enter, this succeeded,

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you see it found one document that matched our filter and it modified one document then and now if

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I again access my flight data, find it and pretty print it like this,

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we see we now added the marker here.

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Now let's say I want to add that marker to all my documents,

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now what you can do then is you can use update many, so on flight data, you can use update many

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and now you need to pass something as a filter but you can simply pass empty curly braces

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and we could have done that on delete many too by the way,

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this will select all documents. And now here, I again use set and then describe what to change

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and now let's say I'll set the marker to toDelete here, so not delete but toDelete. If you hit enter,

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you see it matched two documents and it modified two documents

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and now if you again access your flight data, you find it and you pretty print it, you see that both

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now have the marker to delete.

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Please also note that for the first document, this was delete before and it was changed to toDelete

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and now with that, we can use that marker to delete all.

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Now as I mentioned, what we could do here is we could also use delete many and pass an empty pair of

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curly braces too,

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this should delete all elements but of course I also wanted to show you how to use update,

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so here I will now use my new condition and check for all elements where the marker is to delete.

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If we now hit enter, you see this was accepted and two elements were deleted and now if I access flight data

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and I use find and pretty print this, well

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we get back nothing because there is nothing in there anymore.

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So this is how we can insert elements, how we can delete and update them and how we find them

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but it's not all I wanted to show you.

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Let's continue in the next lecture.