MongoDB for Developers with Python Transcripts
Chapter: MongoDB's shell and native query syntax
Lecture: Exact subdocument matches
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0:01
So here's an interesting question— what if I want to find all the books where user 720 has rated that book exactly a nine.
0:10
You would think that this would do it, right, we're using both values in this prototypical object or this document here
0:16
and it says that the book is going to have to have a rating of nine and user id 720 has rated it.
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However, when we run this, you'll see we get mixed results. The bottom one looks perfect, we got a book with the user id 720
0:30
an a value of nine in the ratings, great; but this other one, what's up with this, the red one? Well, user 601 rated this as a nine,
0:39
and user 720 actually hated the book, they gave it a one. However, taken as a whole, does the book have a rating by user id 720— yes,
0:47
does it have a rating of nine— yes, so it matches this and clause. So, oftentimes if you're looking for this exact subdocument match
0:55
and that thing you're looking in is an array so ratings is an array of documents, if ratings was one subdocument,
1:01
this would work fine, but if it's an array and you want to say I need to make sure that the thing in that array is
1:08
that subdocument itself matches value and user id as I've specified here you need a different query operator, and that is dollar element match;
1:16
so you can run this and it'll look down inside and say I want to find all the things in ratings, where both, the user id is 720 and the value is nine.
1:26
So this is a slightly more complex version that you have to run and you have to use because you run into that problem we had before
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where somebody voted a 9, user 720 voted, but it was not user 720 who voted nine. So a little bit different than if you were working in
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say a sequel traditional tabular language because you don't ever have this kind of duplication within the one result,
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so it would be a lot simpler, but this is something that you kind of got to get your head around a little bit,
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you luckily don't use it very often, and if you are using the higher level of things like MongoEngine, you won't run into it,
1:57
but down here at the shell or in PyMongo, you have to be really careful if this is actually the question you're trying to ask and answer.