Week 9 Tutorial: Mongoose and the model layer

COMP5347: Web Application Development
Week 9 Tutorial: Mongoose and the model layer
Learning Objectives
• Understand basic mongoose concepts and objects
• Integrate mongoose with Expressjs
Part 1: Add mongoose module
Task: Download the provided week9-src.zip from Canvas and extract the content in
a directory. Start Eclipse, select “open projects from file system” under file menu, go to
the directory you just extracted the content and click open. In this case, all the
necessary node modules are specified in package.json. Right click it on the project
explorer panel, select Run as then npm install.
Optional: Just keep in mind, in your own project, you need to run the following
command in the terminal to install install mongoose manually.
npm install mongoose --save
Alternatively, you may open file “package.json” and add the following line at the end of
the “dependencies”
"mongoose": "^4.3.0",
Save the file and right click it on the project explorer panel to Run As then npm update.
Part 2: Simple mongoose query
Task: Start the mongodb server using the following command, replace the “path” with a
path to the local mongodb working directory where you create the wikipedia
database with the revisions collections.
mongod --dbpath path
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Run the code by right click the /app/models/mongoose.revisions.js on project
explorer panel and run as node.js application.
Optional: line 3, the connection to mongodb is configured. If you give a different name
to the Wikipedia data set when you import the data, the configuration might be different.
In this case, you can replace the “dbname” in the following path with the name you
created before. If you are not sure about the dbname, you may open the robomongo db
browser and check it there.
‘mongodb:://localhost/dbname’
You need to uncomment the lines representing the update statement to try the update
example. You may notice that writing queries, updates and aggregation statements in
mongoose follow the same syntax as mongodb shell commands.
Try to write your own code to find the user who made the most non-minor revisions on
article “CNN” and run mongoose.revisions.js again to test that you have obtained
the correct results. You can also compare the results you get from mongoose to the
results you get from respective shell command.
Part 3: Full MVC Express Application
This part will build a small application with mongodb as backend database server. It
allows an end user to type in an article’s title to find its latest revision data from the
revisions collection.
Task: Understand the project structure
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The view layer contains two views: one for displaying the form for entering title. The
other for displaying revision data. Both views are provided as PUG files:
titleForm.pug and revision.pug.
The model layer is supposed to handle data of the application. Database related code
should be put in the model layer. Our simple application only needs to handle revision
data, it has a module represent revision mode called revision.js. It also has a
module called db.js which contains database initialization code.
Optional: Same as before, line 3 of db.js, the connection to mongodb is configured. If
you give a different name to the Wikipedia data set when you import the data, the
configuration might be different. In this case, you can replace the “dbname” in the
following path with the name you created before. If you are not sure about the dbname,
you may open the robomongo db browser and check it there.
‘mongodb:://localhost/dbname’
The revision.js script provides a static method findTitleLatestRev. This
method will find an article’s latest revision given a title. It does not define its own
callback function.
The application also contains a controller revision.server.controller.js, which
calls findTitleLatestRev and provides a callback function when the result is back.
The revserver.js contains code to create and start the application.
Task: Implement the router
The only part missing is a route file that will bind controller methods to urls. Inspect all
relevant code to determine the content of the route file and implement it in
revision.server.routes.js under app/routes
Task: Run the application
Run the start file as node.js application after you have implemented the route file. Once
started, points your browser to localhost:3000/revision. It should display a form.
When you type in a title “BBC”, the request will be sent, and the server will query
mongodb to find the latest revision on article BBC and send it back. See below
screenshots for sample output.
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(b) Result
Part 4: Introduction to Promise Library
Node.js naturally do not block IO as other mainstream programming languages do. In
that case, if you want to write some code based on the output of previous function, you
cannot just put the statement at the following line. Alternatively, you should use callback
function to chain the following codes with the previous function. Most node core
modules such as ‘fs’ module embrace this style in the nutshell.
When your logic gain complex, there will be a lot of layers of callback function. The term
“callback hell” is coined to describe this situation. Promise is one of various remedial
solutions, supported by various npm packages such as ‘mongoose’ and ‘request.js’.
Bluebird1 promise library is taken as an example due to its popularity among node.js
ecosystem.
Bluebird can be used to turn this:
fs.readFile("BBC.json", function (err, val) {
 if (err) {
 console.error("unable to read file");
 }
 else {
 try {
 val = JSON.parse(val);
 console.log(val);
 }
 catch (e) {
 console.error("invalid json in file");
 }
 }
});

1 http://bluebirdjs.com/docs/getting-started.html
(a) A simple form
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Into this:
fs.readFileAsync("BBC.json")
 .then(JSON.parse)
 .then(function (val) {
 console.log(val);
 })
 .catch(SyntaxError, function (e) {
 console.error("invalid json in file");
 })
 .catch(function (e) {
 console.error("unable to read file");
 });
Task: Download Promise.zip from Canvas and extract it to some local directory. Import
the promise.js and BBC.json to the project root directory. Execute promise.js file
by right clicking the file name on project explorer panel and run as node.js
application. Uncomment the fs.readFileAsync function to compare the
outcome of ‘callback’ style and ‘promise’ style code segments. Of course, the outcome
should be the same.