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Now what we did in the last lecture where we updated the matched array element can also be done to multiple

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matching elements.

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So if I again find some documents based on an array based query, so where I maybe find hobbies with a frequency

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that is greater than two,

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so now I should find way more documents if I pretty print this, I find a bunch of documents and again always

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remember, the document is the overall person that documented the collection, not the document

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you filter for in your query, that is just the key factor for giving you back to the overall document.

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So we found Maria because she has a hobby with a frequency greater than two,

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same is true for Chris,

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he also has a hobby with a lower frequency but that does not matter,

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not all has to match,

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he has at least one that matches,

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that's the thing. We also found Anna and Manuel and Max,

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so we found all documents. However not all array documents fulfilled our filter,

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we had greater than two,

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so for example cars for Manuel did not fulfill it

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and so did hiking.

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Now let's say we want to change all embedded documents in the hobbies array that did fulfill it.

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So this was our query,

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it gave us back all documents in our collection,

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now let's update many documents

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and here, I will just repeat my query, so hobbies frequency greater than 2

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and now I want to update them, whoops

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I pressed enter accidentally,

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so now I want to update them. Again

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we can use $set inc

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and so on

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but here, I want to set a new field value, so $set is the right option here

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and I want to change all these matched hobbies.

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So before, we had hobbies.$ and then we could override this as a whole value or add a new field

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or edit an existing field with another dot notation.

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However this only matches or edits the first matching hobby for each person

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and if there are multiple matching hobbies per person, it will not update them all.

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I can show this to you by simply adding another field, good frequency and setting this to true.

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If I hit enter, it modified all five documents

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but now if I find all my persons, whoops and I pretty print that, Maria has a good frequency,

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Chris has it but note that

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these two documents were not changed

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and especially for this one, it's strange because the frequency of three is certainly higher than the

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frequency of two which we looked for, only the first element was updated. Now for Anna it made sense that

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the first one was not touched because frequency of two did not meet our criteria,

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this one was updated though, for Manuel the same is the case,

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the second one shouldn't be updated because the frequency of two is not fulfilling our filter criteria,

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for Max however, again the second document just wasn't edited even though it should have been.

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Well it should have not been because dollar sign as we used it simply refers to the first match.

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Now sometimes you want to edit all elements in an array

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and for now, let's ignore a filter,

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so let's say not the ones that match some filter but you simply want to update all elements in an array.

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Now for this, you can of course again update many

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and let's say we now want to find all persons with an age older than 30, so age should be greater than

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30,

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let's quickly pretty print that,

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whoops we

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first of all find them not update them and of course it should be total age, we renamed that,

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so we find Chris and we find Manuel.

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Now let's say for these people, I want to add a new field to my documents here, to all documents in hobbies

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not just to one. To do that, I can again use update many and then repeat my filter criteria,

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so total age should be greater than 30 and then to update them, I can of course use $set,

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I could also use inc and so on,

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let's actually use inc to match it up, so I can use inc here

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and then here, I want to update my hobbies but I don't want to override the entire field,

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I just want to add a new field to every document inside of hobbies.

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Now let's try accessing frequency with hobbies.frequency and increment it by minus one

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because these people should slow down maybe or do more,

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whatever you want but let's decrement it.

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If I hit enter, I get an error,

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cannot create field frequency in an element hobbies. 

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Now the problem here is that hobbies is an array and not a document itself and even though you can find

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elements with this notation, you can't update

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but you can update all array elements with another placeholder provided by mongodb and that

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placeholder is 

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${}, whoops, $[].

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This simply means update all elements and for each element, simply the frequency field because we have

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embedded documents in our case,

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if you hadn't embedded documents, you could of course stop after this part. And now if I hit enter,

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you see two documents were modified and if I find my documents again, you see the frequency here for

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example is zero now and previously it was one,

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so all the frequencies actually decreased by one.

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So this is the

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$[] syntax for updating all array elements.