Real world applications will almost always have some atypical behavior. 0:00 And our application is no exception. 0:04 Consider the sorting behavior we want for our API. 0:08 When the API serves the answers. 0:11 We'd like them to be ordered by votes first, and 0:14 then their updated app property. 0:16 This means that any time an answer is edited, 0:19 or voted on the order will be affected. 0:22 When an answer is saved to the database. 0:25 We want our application to sort the answers array 0:28 on the parent question document. 0:31 As we saw earlier Mongoose offers us a way to run code before 0:33 any save event with the pre method. 0:37 To save a child document in Mongoose you have to save its parent document. 0:40 Let's implement a pre save hook on the question schema. 0:45 Let's open up the models JS file, and under the question schema. 0:50 Let's set up a pre save callback 0:55 Inside, we'll sort the answers array before calling next. 1:26 There is a problem here though. 1:44 The default sort function supplied by JavaScript doesn't know how 1:46 we want to sort our array of answers. 1:51 If we leave it as is, it will sort the answer objects by the strings. 1:54 Resulting in the calling of two string on each of the answers, 1:59 which would all be identical. 2:03 It would be the string of object. 2:06 In other words, this line as it currently stands, does nothing at all. 2:15 To address this issue, we'll pass in a sorting function into the sorting method. 2:24 There's a link in the teacher's notes to read up on JavaScript arrays sort method. 2:30 In case this is the first time you're encountering it. 2:34 What our sorting function will need to do is compare two unsorted objects and 2:37 say which order they should go in. 2:42 The array sort method will then take that information and sort the array for us. 2:45 Let's put this sort function up at the top with the other variable declarations. 2:59 We'll call it sortAnswers. 3:04 This is a function that will accept two objects, a and b, and compare them. 3:13 It will need to return a negative. 3:29 If A should appear before B. 3:32 A zero to leave the order unchanged. 3:43 And a positive if A is to be sorted after B. 3:49 Let's think about how we want our answers ordered. 4:03 First, let's order them by vote. 4:06 So that the answers with the most votes appear at the top of the list 4:10 if the votes match. 4:15 Let's resort to ordering them by the last time they were updated. 4:16 The most recently updated answers appear above the least recent ones. 4:21 Let's start by ordering answers by votes. 4:27 We can return the value of A's votes subtracted from B's votes. 4:31 If A has more votes. 4:38 The number returned will be negative, and A will be placed before B. 4:40 If B has more votes. 4:46 The difference will be positive, and B will be placed before A. 4:49 If the votes match, 5:00 we want to place the answers with the most recent updated here first. 5:01 Let's first check the equality of votes. 5:20 Now, if the votes match. 5:24 We can compare the a.updatedAt to the b.updatedAt app. 5:26 And return the sorting order based on what we find there. 5:30 As you may or may not know, 5:34 JavaScript thinks about dir objects as numbers of milliseconds. 5:36 That have elapsed since midnight Thursday the 1st of January 1970. 5:40 More recent times are bigger in number than less recent. 5:46 So if a.updatedAt is greater 5:51 than b.updatedAt. 5:57 We'll want A to proceed B which requires a negative number to achieve. 6:04 So we'll return minus one. 6:11 Let's check the reverse of this case and reverse the elements order if it's true. 6:15 If both of these fall through. 6:43 We'll just return zero to leave the ordering unchanged. 6:45 There's a much simpler method we could use for comparing date objects. 6:52 Even though dates are objects in JavaScript. 6:57 You can subtract them. 7:00 When you do, you get the difference in milliseconds. 7:02 So we can return the value of A updatedAt subtracted from BupdatedAt. 7:07 [BLANK AUDIO] Just 7:13 as with votes this will order the larger, or later dates first. 7:20 Now we can supply the source answers function to the sort method. 7:27 Mongoose will now sort the answers right every time it is saved. 7:39 Keeping the state of our database up to date. 7:43 Now let's think about when the answer is updated. 7:53 We'll want to do a couple of things for this to happen. 7:58 We want to apply the updates to unanswered document. 8:03 And we also want the answers updated property to be the current time. 8:07 Let's do this with an instance method. 8:14 So we can call it directly on the answer when we want to update it. 8:16 Although the answer schemas declaration. 8:20 Let's create an instance method called update. 8:23 We want the update method to accept two parameters. 8:44 An update subject and a callback for when the operation is completed. 8:47 Make sure you create this method above the question schema declaration. 8:52 Where the answerschema becomes a sub document, or else you'll get an error. 8:56 Remember that we learned to write instance methods 9:05 by placing the property on the methods property on the answer schema. 9:09 Mongoose gives us an alternative way to create instance methods. 9:13 So let's use that here to see in action. 9:17 Instead of setting a property of methods on the schema we can call the method 9:21 method. 9:26 Passing in two parameters. 9:30 The first, a string of the name of the method you want to create. 9:35 And then the second, a function that you want to become the actual method. 9:42 We can't name our instance method Save. 9:50 That's because there's already an instance method called Saved. 9:53 And overwriting it will conflict with Mongoose internal logic in 9:56 unpredictable ways. 10:00 The first thing we'll want to do is merge the updates into the answer document. 10:03 We can do this with object.assign. 10:08 We'll pass in the document first referenced by this. 10:16 Then the updates object. 10:23 And then we want to set a fresh date on the updated app. 10:29 We can pass that in as a third parameter, and objects assigned will match that too. 10:42 After that, we'll need to save the parent document. 10:48 In other words, the question associated with the answer. 10:52 To access the question, we can use the answers parent method. 10:56 Since the save is the last thing we want to do. 11:04 We can pass in the callback as a parameter. 11:09 There's just one other addition we'll need to make on the answer schema. 11:20 A vote instance method to help with translating strings from 11:25 the URL into math that moves the counts up or down. 11:30 Let's start with the function signature, 11:40 which is almost identical to the last instance method. 11:43 Since we've all ready validated the string that is passed into the vote parameter. 11:52 There are only two possibilities. 11:57 If the vote is equal to up. 12:03 We want to add one to the votes. 12:10 If it's down. 12:17 It's minus one. 12:28 And just like the last method call, we need to save the parent and 12:36 pass in the callback to save. 12:40 Our data modeling seems to be complete. 12:46 It would be nice to test what we've done so far. 12:48 Let's take the final step in building our API, and connect it to the database. 12:51 Then we can use Postman to test our fully functional routes. 12:57