[MUSIC] Hello and welcome back. In this lecture we will look at the "R"ead in the CRUD operations. "R"ead can be accomplished by the keyword find. And we will also look at cursor iterations, pretty printing, and projections using the find by examples. Now what is a find command? Well, we've seen example in the previous lecture when we did a simple insert, and we used the find command to see if the insert went through or not. But we will look at more examples here and also upcoming lectures. So find returns a cursor object, which will allow us to iterate over the selected document. Now, the find can come back with one, or it can come back with multiple documents, depending on the kind of query criteria that you send in. So that leads us to the last point here. You can use find with a wide variety of query criteria. And you can selectively get the data from the database. So here's an example of a basic find. So this will go and find the city Baltimore from the MongoDB, right? So, let's go and try that command on the IRB shell. So this is the command which will actually go and find the city. #rocket BALTIMORE or city equal to Baltimore. And that's what we get back, which is the view representation of the response that we got back. Now, if we add a dot first, I should see the first document in the collection where city is equal to Baltimore. Now what you saw on the first query is a view, which means we've got a view representation. But this is when it actually made a call or actually queried the database to get the data out. So this is the first document in the collection where this condition was met, okay. Now with find you can also send additional query criterias like for example I can do find distinct state. Now we have 30,000 documents approximately as you know in the collection. And each of this is a zip collection, right, so it has city, state and latitude and longitude and all that. So almost all of the documents in the collection has a state. So you should see, when you just do a regular find state, you should get 29,000 documents, right, or 30,000 documents, approximately. So when I do a distinct state, what happens is you'll only get one value back. Right, one distinct. It's similar to a distinct in SQL. So once it finds a city called MA or Massachusetts, it will ignore any other state. That is Massachusetts right, it'll skip everything else because it already has one response coming back here. So distinct is very useful. Distinct will give you again a distinct data from the database, so no replication or new duplication of data. So this will give me the count of city hash rocket GERMANTOWN or city equal to GERMANTOWN. So we have eight cities in the entire collection that has city equal to GERMANTOWN. Now let's take this command and try a couple different things. Now let's say we want to send in more hash into our fund. So for example, we were doing city = "GERMANTOWN". I also want to send in state equal to New York, right. So now I want to filter it down from eight to it give me the documents where it's not only GERMANTOWN city but I also want to get a state. Notice how I have a pp here which stands for pretty printing. Now when I do with the very first time I get a message called undefined method pp right. Basically the console is saying the shell is coming back and saying hey I don't understand what pp is. Because you need to do a require pp or you need to load that gem right. So let's go and do that. Okay, so that is successful. And so now if I go into this, I should see the data come back on the screen, and it's also pp, or pretty-printed. And it looks nice, right? Now let me do the same thing for a different state. So now what I get back here is for Maryland. So look at the location that's different, one for New York and one for Maryland. So this is again an interesting combination where I can send in more than one hash. Now you can send more, we can send three or four or five. This is basically doing the find with additional parameters. Now in the previous few slides ago we saw that Mongoose approach, well the find can come back with the cursor object, right? The cursor reiterations. Now when you have more than one document in the collection that meets the criteria, what comes back is a collection. Right. A collection of data. So if you want to print that or parse that or process that, then you will have to to go through the collection and pull each document out and process it or print it in our case. So you can accomplish that by doing the .each. The .each will actually print out, in this example, as you can see, it's printing out the r or the result. So, if you run this command we should get the 29,000 plus documents. Almost 30,000 documents will get displayed on the screen. So let's go and try that command. Now, this is the command that will go and pull every single document and print on the screen. Now, I'm going to warn you that we will see a lot of documents get printed on the screen, obviously every single one, so I'm going to do a Ctrl+C once we see the first few documents get printed. But that's the command, db[:zips].find().each, and then we will do a print of the r, which is a result in our case. So as I said once I hit Enter, you will see a lot of documents come up on the screen. So give this a try and you will see all this documents coming up on the screen. So I'm going to stop this or abort this because we select 14,000 there. So I'm going to do a system.clear, right, that'll clear the screen. So just to recall. This is the command that's going to print everything on the screen. And again if you have data that you want to go through, you can type this command to get all the data. So the next we'll look at in find is Projections. Now what is projections? Projections is similar to select in relational database. It will limit the fields to return from all the matching documents, right. So you can specify the inclusion or exclusion. So you want to say select first name last name and middle name from the employee and ignore everything else right, that's what you do in a relational database. So here, what you would do is you will include which is true or 1 and exclude which is false or 0. So either you will make a property inclusive or exclusive and this is how you will filter out the data. Now a good example is this, remember id is a primary key in Mongo. So Mongo by default, whether you ask or not right, it will automatically include _id by default as part of your select or part of your find query. Now there might be instance where you don't rally need the id to come back then you can go and turn this off by making this again, by inclusive or exclusive you can control the showing of id. Now again id's default, right, but by default it's shown, but you can turn this off by setting it to false or to 0. So let's see an example of this. Okay so let's see the first example where we will do a simple find the state and state colon true. So, this is saying, okay, show me the state. Now, notice we are not doing anything with ID, right? We're not projecting ID at all, and guess what? We get ID by default, because Mongo automatically sensed this. Now if we turn this off, like if we don't really care about the id, then we need to set the id as false. So let's give that a try. So it's the same example, except we go and say id:false. And I do an Enter. And notice how id is gone and all we see here is state, is state is equal to Maryland. So, very handy, when you want to filter something off then you just have to take, whether it's id or any other field in your document. I need to set the document to be false. So just to summarize this topic, we look at find and we saw a couple of examples using the find operation and we also looked at projection. Which is selecting only the necessary data, rather than selecting the entire document? Now, again you might have a need to get the whole document and which case you will not need to do this but, sometimes you need to filter out? Then the projection will come very handy. So, the next topic we'll go and look at paging, sorting, and limit, okay? Thank you.
