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