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In the previous lessons, we looked at how we might implement CRUD using MongoDB.
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Now in this lesson, I want to show you how you might establish relationships in your database with MongoDB.
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There's two main ways of doing this
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and the first way is in most cases the preferred method.
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But what you choose will depend on the architecture of your data.
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So it depends on how your data relates to each other and how it's structured.
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Coming back to our shop database, let's say that we wanted to have another product so we might say something
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like db.products.insert
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and in this case the product that we want to insert happens to be a rubber.
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So we might say that this one has an id of 3
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and it's got a name of Rubber.
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It's got a price of 1.30 and a stock of 43.
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But this Rubber has also received reviews on our website.
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And so we can represent these reviews using an array.
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And inside the array we might have- we might have a few embedded documents.
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Every document is represented by a set of curly braces.
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And if we had a document that will represent the data for a review then we could embed that into another
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document.
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So our reviews document might have say an authorName
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so in this case it was somebody called Sally, and it might have a rating
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so they said that this was 5 stars.
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And then it might have a review
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right?
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Best rubber ever.
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OK so this would be a single document and because we're embedding it inside this particular product
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then we can associate the relationship in this way.
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And it's very very natural because it means that if we had another review come in for this rubber product
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then we can simply append it into that array and we simply have some different data for it.
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And now we've established a relationship between a single product document and a single review document.
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Now this style of embedding documents in each other is really well-suited to this kind of one to many
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kind of relationship because one product might have many reviews or one user might have created many
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comments. And this is a very natural way of expressing that relationship using MongoDB.
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I'm going to copy and paste this into my Mongo shell.
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Now the reason why I'm doing it inside Atom is just because it is much easier to format it for you to
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see what's going on and inside the shell
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it's much harder to hit enter without executing your statement.
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So let's hit paste and hit enter.
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And now I've inserted one document into my database and if I look through all of my products I now have
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a product with id 1 and a product with id 3. As a challenge
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I want you to practice embedding documents inside other documents.
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I want you to recreate that pencil document that we deleted earlier on and to add in some reviews for
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that document as well.
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The challenge is to follow the format that we created the last document and you're going to add two
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reviews to this pensil product which looks something like this
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but you're going to make up two reviews for the pencil and that each going to have a author name, a rating
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and also a review.
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Pause the video now and give that a go.
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OK.
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So I recommend writing this out inside a blank document in Atom just because it makes it so much easier
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to write multi-line code.
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So we're going to use db.products to tap into our products collection and then we're going to say insert
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One and the product that we're going to insert is going to have a id of 2, name of pencil, price of 80
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cents, stock of 12
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and finally it's going to have that review field which is going to be an array of imbedded documents.
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Each embedded document will be enclosed in a set of curly braces
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and we're just going to make up some reviews right?
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So now we can check that we don't have any errors in our code and we can copy the entire thing over
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to our Mongo shell hit paste and hit enter.
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And now we've inserted our second document. And now you can see we've got a pen, our pencil and our rubber
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in our products collection.
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Now another format you might see out there in the wild is you might say have two product documents, one
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which is the pen and the other which is the pencil and they each have a unique identifying id which
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is simply 1 and 2.
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And then you could create another collection say a collection of orders this time.
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And for each document in that collection you might have a order number but you might also have a field
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that's products ordered and this can simply be an array that references the id of the products in the
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products collection.
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And so at a later date you can pull up the product ordered and you can find which products they are
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based off those IDs.