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All right.
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So our goal for this lesson is to be able to sort the messages that we retrieve from our database in
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order of the time that it was sent.
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The first thing that we need to figure out is how do we add a timestamp to the documents that we save
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into our database so that when we create our documents. In addition to the messageSender and the messageBody,
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we also want to add a key-value pair that is K.FStore.dateField, and the value here should
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be the current time,
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so the moment when this message was sent. And then once we have that value, then we'll be able to sort
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it when we get our messages back from Firebase and we'll be able to order our messages chronologically.
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So the way that we would get hold of the current time in Swift is through the use of a Date object.
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So this is a specific point in time.
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Let's initialize this Date object and we're going to tap into one of its methods which is called
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timeIntervalSince1970.
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This is something that programmers use a lot because what it gives us is the number of seconds since
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January the 1st at zero hour in 1970.
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And if you've taken the lesson on the Swift Deep Dive in loops, then you would have already seen me use
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this in order to demonstrate loops.
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So, now that we've got this extra field in our data, we have to go and delete all the current documents
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in our collection because none of these actually have a date field. In order to delete a collection,
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you just have to type in the ID which is messages, and then hit delete, and that empties out our collection.
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And we can now go back over here and create new messages that have three properties: sender, body, and
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date.
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So let's go ahead and run this and create a new message, hit send, and maybe let's try a different message.
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So we've now got two messages in here. And they should if we take a look inside our Firebase Firestore,
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They should have a date field now attached to each of these documents and, indeed, they do.
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So this is the number of seconds since 1970 and we can now use this, instead of the arbitrary ID to
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sort our messages collection.
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So if I go ahead and add some more messages, say, the most recent message, at some point the order breaks
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down like what you saw previously, for example, this terribly auto-corrected Jill alays kajsd,
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actually got inserted in a really random position, right?
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So let's go ahead and sort the collection inside inside our loadMessages method. And the way that we do that
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is, of course, by ordering our data. So we can call a method called order by and provide the key or the
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field name.
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So in this case, it would be that date field. And we can also, say, order by descending or ascending, and
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we can even add other methods like limit the number of results that come back, plus lots of different
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types of queries that Firebase provides.
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Let's try and implement this order by method.
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Notice how our DB dog collection sort of method call is getting lots of methods chained on to each other.
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So I'm gonna go ahead and order the collection that I get back before I add my listener.
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So right here, I'm going to write .order and I'm gonna order by a string which is gonna be the field
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path, right?
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And that is gonna be stored under K.FStore.date field.
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So, now when I get my collection back, I'm going to order it by the datefield in ascending order, and
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then I'm going to add my snapshotlistener to listen for changes.
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Now, if this is getting pretty long for you, then an easy way of formatting this chained method is simply
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by hitting enter on each of these dots. So you can see db.collection, then .order, 
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then .addSnapshotListener.
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Just make sure you don't add anything here in between because, effectively, this is just shorthand for
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them all to be continuous. But this way, it's easier to read on a screen that's not so wide.
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So, now let's run our app again. And you can see our messages are now sorted in the order that they were
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sent.
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So if I were to send a message "A," and then "B," and then "C," you can see that they're being displayed in the
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same order that they were created.
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But when you take a look at our documents in here, for example, B, is way ahead of "A." And we're now ignoring
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the ID of our documents, and instead, sorting by the date when they were created. The last thing to remember
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before you move on is at the very beginning when we created our database under Cloud Firestore,
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we set our rules so that basically anybody can read or write to it for 30 days.
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So if you head over to the rules tab, you can see that we've still got the initial rule set which allows
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anyone to read or write to our database.
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So, now that we're done with testing Firebase, we're ready to update our rules, so that our database can
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be more secure. So we click on that "Learn more" button, you can read about how Cloud Firestore Security
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Rules work, or you can even watch a video on it.
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Now, the version that we're going to go with is the rule that allows anybody who is logged in to be able
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to read and write to our database, that includes all the users who have signed up to our app, so that
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they can access the messages that they've sent before and to be able to send new messages to the database.
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So we're gonna go ahead and copy this rule set from Auth required here, and we're going to select everything
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that's currently in our rules and paste it in here, and then we're gonna go ahead and hit Publish.
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So now we've secured our database.
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It only allows read and write from people who are authenticated and we can now rest safely.
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So, now we've seen how we can use Firebase Firestore to save data, to retrieve data, to listen for updates
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on data, and also to sort and query our data. In the next lesson,
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we're going to be spending some more time on improving our user experience. Because at the moment,
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if you were to run this app on a physical device, there would be no way for you to send a message because
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the keyboard that pops up completely obscures that button.
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So we're going to be solving those problems and more on the next lesson.
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So I hope I'll see you there.