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Now that we got the basic field operators set, let's see how we can work with arrays because there are

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some nice operators that help us with updating existing arrays in documents, like the hobbies array in my

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users,

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so I switch back to that users collection

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we worked with before.

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Let's say we want to update all documents,

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so Chris, Anna, Max, whoever matches, all documents where the person has a hobby of sports with a frequency

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greater or equal to three, this also practices something we used in the read section.

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So how could this look like?

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Well we want to first of all find all the persons so that we get a feeling for the correct filter we should

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write,

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so I want to find a certain amount of persons and I want to find persons based on the hobbies array

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and then on the hobbies array, I got two conditions. Two conditions on the same field means that I need

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and and then here, the first condition is that for hobbies.title,

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I want to have sports and the second condition is that for hobbies.frequency, I want to have a value greater

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or equal to three.

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Now if I now hit enter, we get Chris, sports frequency five, makes sense, we get Anna, sports frequency

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two.

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Well we've been there, right, we've been in that situation

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in the read module, if you haven't been there, you should go there now.

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Now how did this happen?

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This simply checks for documents where you have a hobby with a title of sports and where you have a hobby

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with a frequency greater or equal to three

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and that is the case for Anna,

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she has a hobby with the title of sports and she has a hobby with a frequency of three or greater or equal

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three.

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Now of course, we're looking for one and the same document but we are not expressing this in this query.

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The correct query would be to use elemMatch,

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so there we would normally say hobbies should have a value which is a document where we use elemMatch

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and elemMatch is now looking at the elements and hobbies which happened to be embedded documents,

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so now we can say the title key should be sports and for that same embedded document, the frequency should

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be greater or equal to three.

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And if I now hit enter, we find Chris because we have sports with the frequency greater or equal to

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three and we find Max where the same is true,

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Anna is not part of the results set anymore. So this is the right query to find the documents I want

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to update.

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Now let's say this document is the document I want to update

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but then, I want to update that document in the array,

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so essentially I know which overarching document is what I want to change but then in the document,

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I want to change something in exactly that element which I found in the array.

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Now we can do that, I'll quickly count here so that this stays on the same page, we can do that with update

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many because we'll have potentially and also in reality more than one result, I can now copy that whole

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query here because that is the query I need down there

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and now how do I want to update this? By default if I now use $set and then define something here,

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these are of course changes that will be applied to the overall document,

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so to the overall person, not to the document in the array which I found. If I want to update that array

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document, I can use a different syntax, I can use set,

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that is correct but then in here, I don't want to update hobbies as a whole,

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I don't want to assign a brand new value to hobbies,

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I only want to update the element in hobbies which matched my condition here. And to do that, we can use

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a different syntax,

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I can enclose it in quotation marks and say hobbies.$ and this will automatically refer

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to the element matched in our filter,

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so in our query and then here, I can define the new value.

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So here I could now say that should still have a title of sports, that should still have a frequency of,

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well of

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what? I could do that but the frequency of course differs, it's five for Chris and three for Max and I don't

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want to override it, essentially I don't want to override the entire document here,

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instead what I want to do is I want to add let's say a new field and we can do that by moving back to

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hobbies.$

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and dot something else, we could add a new field here, we could say dot high frequency and

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set this to true as a value.

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So what this will do is it will dig into hobbies, find the element which we found in our query and then

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access the high frequency field which if it doesn't exist simply gets added. If I now hit enter,

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you see it matched two documents, two persons and it modified two persons

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and if we now have a look at all persons again, we see that Chris still has sports and five but also

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high frequency true,

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Anna was not edited because she had a low frequency here and therefore was not selected in our query,

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Manuel doesn't have sports at all

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but Max also has sports and now also got the high frequency field.

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So this dollar sign placeholder

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here is a great helper

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when you have a query where you do select a specific element in an array and you then want to update

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exactly that element in your set operation.

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And just to make it really clear, inside of set, you could of course have updated totally other fields

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as well,

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you could update this high frequency field and then also update isSporty or the name or add an age

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to the overall document.

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$set to works as before, it's just this special placeholder which you can use in the path to

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a field to quickly get access to the matched array element.