Eve: Building RESTful APIs with MongoDB and Flask Transcripts
Chapter: Fine-tuning your REST service
Lecture: Event hooks

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0:00 When a request is received, one or more events are filed by the framework, you can subscribe to these events with multiple callback functions.
0:10 There are essentially two kinds of event hooks. You can attach a custom function to a pre or post request event
0:17 or you can subscribe to a database event. We're now going to give a look at database events here we defined a standard Python function
0:28 it's called inject signature and it accepts the resource name and the response, which is about to be sent back to the client.
0:36 The goal of this function is simply to add a new field to the response with our signature.
0:42 Now, the way this works is that once we have the instance of our Eve app we can attach our function to the on fetched item event.
0:53 If we save and go to our Postman client and try to fetch one item, one document, this is a get request to this specific document,
1:05 we get back the document with the new field. So what we've been doing is essentially inject new fields into a document.
1:14 Now this field is returned to the client, but since we injected it right after reading it from the database, it is not in the database.
1:24 So essentially, we are transforming the document before it is sent back to the client, and we can do that because we are attaching our function
1:33 to the on fetched item, which, as the name implies, is only filed when an item has been fetched from the database
1:41 and is about to be sent back to the server. In this other example I want to show you how you can attach a functional to a post command,
1:52 so every time a post request hits the service and before the documents are sent to the db, we want this function to be invoked.
2:02 As usual, we get the resource and items is an array of documents which are about to be stored, because as you might recall
2:10 a client can submit more than one document with a single request. So what we're doing here is a simply iterate the documents
2:19 and if a born field is available in a document we add an age field that based on the day the person was born.
2:28 We then go and attach our callback function to the on insert event. Let's go and try these out. Here I am trying to post to the people endpoint
2:41 and the guy is called Gimme An Age and was born on 27 August 1970. Let's post this one, everything is fine,
2:53 now let's go back and see what happened to the person, we do a get to the person endpoint,
3:03 we have the last name, we have the born, and the default role but we also have the age field which is 48—
3:11 what happened here is pretty powerful if you think about it we are doing transformation this time, before the document hits the database
3:22 and because our previous inject signature function is still hooked to their own fetched item, when we did the get request
3:30 we also get the signature injected into the document, so while age is permanent on the database, signature is not,
3:39 it is still being injected by our callback function. There is a huge number of events to which you can attach your callback functions,
3:51 there is a table on the website you can go and look it up we have events for fetch, insert, replace, update, delete, you name it
4:00 and also every single command or request has an event before the operation is performed, and after the operation is performed.
4:11 And these are just the database events, then you have all that pre and post request events which are also equally powerful,
4:20 so you want to go and look them up. Finally, let's go back to our code and give a look at the operator we're using here
4:28 we're attaching a function to an event but the operator hits and the fact that we can actually add more than one function to the event
4:38 and this is true, we might have another function doing some fancy stuff, I don't know, performing some computation
4:45 and it will be a different one, isolated, which helps in keeping our code well organized, tidy and clean.

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