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Week 8 Tutorial: MongoDB Basic Queries

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COMP5347: Web Application Development
Week 8 Tutorial: MongoDB Basic Queries
Learning Objectives
Practice basic MongoDB features with given JSON document:
• import data into mongodb from JSON file
• basic read query
• basic aggregation query
• indexing
• get familiar with MongoDB query reference document
Part 1: Start MongoDB server on lab machine
Note: a guide for installing MongoDB and RoboMongo on your machine is available on Canvas
> Modules > Week 8
MongoDBisinstalledonallLabPCsunderthedirectoryC:/Program Files/MongoDB/. It
requires a data directory to store all data. It is recommended that you create this data
directory on U: drive. This way you can access your data from any lab machine. mongod
is the primary demon process of the MongoDB database engine. It can be started by a
regular user with the command mongod.exe.
Task: Below are the basic steps for starting MongoDB database engine the first time:
• Create a directory comp5347/mongodbon your U drive.
• Openacommandwindoworpower shellwindowandchangetodirectoryC:/Program
Files/MongoDB/Server/3.4/bin
• run the following command
mongod.exe –dbpath U:/comp5347/mongodb –smallfiles
If the database engine starts correctly, you will see a few lines of initialization message
and the last line should be “waiting for connections on port 27017”. Leave this window
open unless you are ready to shut down the server.
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Part 2: Start Robomongo as client shell GUI
The JavaScript interactive shell can be invoked directly from another command window.
There are a lot of third party client side tools making it much easier to type in command and to view the results. All lab machines have Robomongo installed. You can find
Robomongo from the query window and start it. Once started, you will see a prompt window
similar to figure 1:
Figure 1: Robomongo start screen
Task: Click Create to create a new connection and the default address would be
localhost:27017. Youcan keep it and give a new name such as comp5347, see figure 2.
Save the connection and select it to make the connection.
Figure 2: Robomongo create connection
Once connected, you will see the main screen of Robomongo. It consists of a top menu
bar, left and right panels. The left panel shows all databases and tables in the current
system. The right panel is used for query command and result display.
Any MongoDB server contains a System database. This database is used by the database
engine to store system wide data such as users and global functions. You are not
recommended to directly change the content of this database.
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You should create and work on your own database. To create a database, right click the
connection name and select create database. You can call the new database wikipedia.
see figure 3.
After the database is created, expend it and you will see three items: Collections,
Functions and Users. Right click Collections to create acollections: revisions. We
will use this collection to store the wiki revision and user data.
Figure 3: Robomongo create database
Part 3: Import json file to MongoDB collection
Download the week8-data directory from Canvas (Modules > Week 8) and extract the
two json files: BBC.json and CNN.json. You can inspect the content of the files using
Notepad++. They contain revision data of the two wikipedia articles: BBC and CNN
respectively. Data about each revision is stored as an object with a number of properties:
title, timestamp, user and so on. The collection of revisions is stored as a large
array.
MongoDB provides a simple tool mongoimport to import JSON, CSV or TSV files to
mongo collection. We will use it to import the revision data in the newly created collection
revisions.
Task: Openanother commandwindoworpower shellwindowandchangetodirectory
C:/Program Files/MongoDB/Server/3.4/bin.
Run the following command to import a file to revisionscollection.
mongoimport –jsonArray –db wikipedia –collection
revisions –file <full-path-to-your-downloaded-revision-file>
If the command executes successfully, you will see information such as how many documents are imported. Each object in the json file will be imported as a document in the
specified collection.
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Part 4: Simple MongoDB read queries
Go back to the robomongo main screen and double click the collection revisions on the
left panel. This will invoke a select-all query on the collection. Both the query command
and the results will be displayed on the right panel in a tab, see figure 4. You may notice
that an id field is automatically created for each document with type ObjectId. This is
the unique identifier of each document. The import command is able to infer simple data
type, such as string and integer, for each field. You will notice that all fields in the imported
documentsareeitherassignedaStringorInt32type.Thisincludesthetimestampfield.
Figure 4: Robomongo query and result
You can have many tabs on the right panel, each with a text box at the top for queries and a
view pane to display the corresponding query results at the bottom. You can slightly modify the automatically generated query to see how many documents are in each collection.
The following command find out the number of documents in revisions collection.
db.revisions.find({}).count()
Task: Here are a few other queries you can try:
1. Find distinct users in the revisions collection:
db.getCollection(‘revisions’).distinct(‘user’)
2. Find distinct user from article ‘BBC’:
db.getCollection(‘revisions’).distinct(‘user’, {title:‘CNN’})
3. Find the first revision of article CNN
db.getCollection(‘revisions’).find({title:‘CNN’}).sort({timestamp:1}).limit(1)
This query starts by retrieving all documents with title ‘CNN’ in the revisionscollection
(db.getCollection(‘revisions’).find({title:‘CNN’})), then sort them bythe field
‘timestamp’ in ascending order ( .sort({timestamp:1})) and output only the first document in the results ( .limit(1) ). The ‘timestamp’ field currently is of type String with
format ‘YYYY-MM-DDTHH:MM:SS’, sorting by string results would be similar to sorting
by Date result.
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Part 5: Aggregation
Aggregation framework allows us to run more complex queries on the collection by grouping
documents based on certain criteria and summarizing the results in different ways.
Task: Run the following aggregation to find out which five editors made the most revisions
on CNN page:
db.getCollection(“revisions”).aggregate([
{$match:{title:”CNN”}},
{$group:{_id:”$user”, numOfEdits: {$sum:1}}},
{$sort:{numOfEdits:-1}},
{$limit:5}
])
The aggregation has four stages: the first stage ( $match ) finds all revision documents
belong to article ‘CNN’; the second stage groups ( $group ) the documents based on the
“user” field value, and create a new field called “numOfEdits”, the value of which is set to
the number of document with that particular “user” value; the third stage ( $sort ) sorts the
resulting documents by the filed “numOfEdits” in descending order; the last stage ( $limit )
limits the output to the first five documents.
Part 6: JavaScript shell scripting
The mongodb shell is written in JavaScript and simple JavaScript statements/functions
canbeusedtoqueryormanipulatedata.Inparticular,areadquerydb.collection.find()
always returns a cursor object to the results. The interactive JavaScript shell by default
iterates through the cursor for up to 50 items by calling the next() method and prints the
matching documents out. In addition to the next() method, a lot of other methods can be
used to traverse or manipulate the result set. The reference document for all cursor
methods can be found from
https://docs.mongodb.com/manual/reference/method/#cursor
In the following example, we use simple script to update the data type of existing
documents in revisions.
db.revisions.find().forEach(function(doc){
doc.timestamp = new ISODate(doc.timestamp);
db.revisions.save(doc)
});
Westart bya read query db.revisions.find() to find all documents in the revisions
collection. This returns a cursor object. We use the forEach() method of the cursor to apply a JavaScript function to each document returned ( .forEach(function(doc){…})).
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The function is an anonymous function defined right at the spot with two statements: the
first statement doc.timestamp = new ISODate(doc.timestamp); converts the timestamp
field to type ISODate; the second statement db.revisions.save(doc) saves the updated
document back in the collection. Because both documents have the same ID, the new
one will override the old one. (https://docs.mongodb.com/manual/reference/method/
db.collection.save/)
JavaScript statements also allows you run a query with subsequent ones built on results
from previous ones. The following example uses two queries to find out set of registered
users who have made revisions on both articles. The first query finds out distinct regular
users in article “CNN” and save the results as in variable users. The second queryuses
the variable to finds out the users also appear in article “BBC”.
users = db.revisions.distinct(“user”,{title:”CNN”, anon:{$exists:false}})
common_users = db.revisions.distinct(“user”,{title:”BBC”, user:{$in:users}})
It is possible to save script as a file and use the shell command mongo to run it (https:
//docs.mongodb.com/manual/tutorial/write-scripts-for-the-mongo-shell/).
Part 7: Setting up indexes
Run the following command to set up an index on revisions collection.
db.revisions.createIndex({timestamp:1})
The explain method on cursor object can be used to see how those indexes are used in
query execution. Run the following command to see how index is used.
db.getCollection(“revisions”).find(
{title:”CNN”, timestamp:{$gte: new Date(“2016-01-01”)}})
.explain(“executionStats”);
The query tries to find the revisions of article “CNN” made in 2016. The query condition
contains two fields: title andtimestamp. There is an index on the timestamp field. The
executionStatsshows thatthenReturnedis123,andthis is theresultofexamining518
documents. Figure 5 shows a sample output. The query runs in two stages: an input
stage and a filter stage. The timpstamp index is used in the input stage and returns 518
documents satisfy the timestamp condition. The filter stages examines the 518 documents by comparing their titles with “CNN”, which returns 123 Documents as the query
results.
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Figure 5: Mongodb Query Execution Statistics
Part 8: Write your own query/aggregation
Task: Write your own query or aggregation to find out:
• All revisions after 2016 in article ’BBC’.
The number of minor revisions in the revisions collection. A minor revision has an
minor field with no value. you can use the $exist operator to check if certain field
exists (https://docs.mongodb.com/manual/reference/operator/query/exists/)
• The number of minor revisions belonging to each article: ‘BBC’ and ‘CNN’

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