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So in the last lecture, we had a look at or, let's now have a look at and.

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For that again, let's add a find query here and let's pass a document where we describe our filter

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and now let's use $and,

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just as or, this takes an array of documents.

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And now let's say we want to find all documents where we have a rating

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and we found that on rating.average remember, a rating that is greater than 9, so $gt

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9, so high quality item, high quality series or movie

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and and that's the second element

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in this array

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and just as with or, you can have as many as you want,

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you're not limited to two and let's say we're looking for documents that have a genre of drama.

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Again I'm querying that index with this syntax to find all items that must have drama but may have additional

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genres, so we find high quality dramas here with and. If I hit enter, we got some results and we see the rating

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is quite good here and we get drama in the genre and we got another result with a very high rating

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and drama in the genre,

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so this is working. Now

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actually, there is a shorter way of writing this.

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We can totally get rid of this,

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so this is the old syntax and I'll just count that, we find three results to leave it here on the page

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whilst I write the alternative.

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The alternative is that we don't use and but we just pass a document, one document here and in there, we

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just start writing our conditions,

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so rating.average should be and then that's exactly the same condition I have above,

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so greater than nine and then just a comma inside of our filter document to add another field to that

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filter document, genres drama, if I count that, you'll see I get back three items as well.

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So these two ways of getting the data are exactly equal because by default, mongodb ends all key

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fields

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you add to that filter document and is the default concatenation mechanism of mongodb. That of course

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leaves one question, why is there a $and operator if this does exactly the same?

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There is a $and operator because you might have multiple conditions on the same field,

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let's say on genres, if I want to find the count of items and I'm just using count to not put all the

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data on the screen immediately, on genres,

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where the genre is drama but where the genre also is horror,

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this does work here but this will actually not work in all drivers because in javascript, having repeat

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names in the same object is not permitted.

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It is for cases like this that you then can use and, so even though it works here, does not have to work

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in all drivers,

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so then you can simply wrap these queries into separate documents which you passed to that and array

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and now you will have an approach that will work fine.

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Now you will also notice that the number of results returned is different

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and the reason for that is simple,

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let me repeat that approach where I mentioned it will not work everywhere and let's look into that data.

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Now

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in these 23 results, we got drama and horror, drama, crime and horror, drama, horror

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again, drama, fantasy and horror

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but then if you scroll up,

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also an entry with just horror,

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now why is that? Because and that is what I meant, even though we don't get an error here,

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it essentially, the second genres of horror will essentially just replace that key because in json

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documents, you can't have the same key more than once

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and if you do specify it more than once, the second one will just override the first one.

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So actually the query like this which gives us back 23 results is equivalent to this query where

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we get back 23 results,

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so if we want to look for both genres, we have to use the and operator and this is why there is an and

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operator even though queries are

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anded by default. If you look for values on the same field, you need $and.