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So in the last lesson we looked at how you would create new records or new documents in our database.
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In this lesson I want to show you how we can now read the data that's stored inside our database.
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Again if we type in show collections you can see we have only a single collection called products but
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inside products we can find all of the data. Heading back to our documentation
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you can see that in order to read data from our database we can use a method called find. Following the
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same format
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we say db. the name of the collection .method.ind.
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And inside the find method we pass in a query criteria.
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In this case they're looking to find data within the user's collection where the age is greater than
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18.
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And if we click on this method then we get taken to a detailed page on this find method. And we've got
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the definition, behavior and examples all written out in this very long document.
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You can see the way that you use it is you pass in two parameters.
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One is query and one is projection and they're both optional.
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So you can simply write db.collection.find with empty parentheses and you will find everything.
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So let's try that.
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Let's say db.products.find and then we'll leave the parentheses empty.
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And now if I hit enter I'll bring up all the documents that are inside my product collection and that is
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my pen and my pencil.
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But if you wanted to make a specific query for a piece of data in your collection then you can add a
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query in there. And you can use query operators to narrow down on the data that you're going to get back.
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And the query operators include things such as greater than less than or less than or equal.
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And you can combine your queries using the logical operators and you have a very fine level of control
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in terms of what data you want to read from your database.
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For example if we were to query our database for a specific record, we could say something like db.
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products to look inside our products collection,
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and we're going to use the find method and inside the find method we're going to specify our query. And
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we're going to use again that key value pair structure.
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So the query that we have is find me all of the documents where the name field is equal to pencil.
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And now if I hit enter you can see that it only brings me the data where this name field matches my
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query criteria which is pencil.
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And now I can read specific documents inside my collection or inside my database through the use of
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a specific query.
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Another query I could have say for example if I had loads of products and I only wanted to find the
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ones that were priced above $1, then I could say db.products.find
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and inside the parentheses I'm going to again add a set of curly braces and specify my key value pair.
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So in this case I'm looking to find the documents where the price is greater than so $
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gt: 1. Here I'm saying my query is based around the price of my records and I'm looking for
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the ones where the price is greater than 1.0.
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So we know we've got pencil that's priced at 0.8 and pen that's priced price one 1.2.
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But if I run this code I only get back my pen document which has a price that is greater than 1.0.
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You can get really fancy with the queries that you make to using the find method.
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And you can drill down to the exact piece of data that you want from your database.
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If we look back at the find method, you can see that there's a second parameter which is called projection.
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And this again is optional which is why even though we didn't include it in our code it didn't really
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matter.
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You don't have to. But you can include it if you wanted to specify the fields to return.
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If we head back to that CRUD documentation you can see that for this particular query they're looking
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for documents inside the users collection where the age field is greater than 18
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and their projection or the fields that they want to return are the name and the address.
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So 1 means true and 0 means false.
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So in our case for example we could say db.products.find and we're going to find all the
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records that have a id of 1.
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But then in our second parameter we're going to put in our projection,
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so the field that we want back. And we only want the name. If we close off this method and hit enter then
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you can see we get back our document with the name and the id.
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Now whenever you use find, id always comes back by default. But you can use the projection to set it to
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be false
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so you can say for example id as 0
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and this will only give you the name. But the second parameter inside this find method is a way for us
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to specify which fields in the data do we want back.
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And in this case we only want the name and nothing else.
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In this case we want the name as well as the id which is included by default
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and if we don't specify one, we get all the fields in that record.