Let’s start with a few key points about MongoDB itself:
- stores data in JSON-like documents that can have various structures
- uses dynamic schemas, which means that we can create records without predefining anything
- the structure of a record can be changed simply by adding new fields or deleting existing ones
The above-mentioned data model gives us the ability to represent hierarchical relationships, to store arrays and other more complex structures easily.
Terminologies
Understanding concepts in MongoDB becomes easier if we can compare them to relational database structures.
Let’s see the analogies between Mongo and a traditional MySQL system:
- Table in MySQL becomes a Collection in Mongo
- Row becomes a Document
- Column becomes a Field
- Joins are defined as linking and embedded documents
This is a simplistic way to look at the MongoDB core concepts of course, but nevertheless useful. Learn more skills from MongoDB Training
Now, let’s dive into implementation to understand this powerful database.
Maven Dependencies
We need to start by defining the dependency of a Java Driver for MongoDB:
<dependency>
<groupId>org.mongodb</groupId>
<artifactId>mongo-java-driver</artifactId>
<version>3.4.1</version>
</dependency>
Using MongoDB
Now, let’s start implementing Mongo queries with Java. We will follow with the basic CRUD operations as they are the best to start with.
Make a Connection with MongoClient
First, let’s make a connection to a MongoDB server. With version >= 2.10.0, we’ll use the MongoClient:
MongoClient mongoClient = new MongoClient("localhost", 210189);
And for older versions use Mongo class:
Mongo mongo = new Mongo("localhost", 27017);
Connecting to a Database
Now, let’s connect to our database. It is interesting to note that we don’t need to create one. When Mongo sees that database doesn’t exist, it will create it for us:
DB database = mongoClient.getDB("myMongoDb");
Sometimes, by default, MongoDB runs in authenticated mode. In that case, we need to authenticate while connecting to a database. For programming skills MongoDB Course
We can do it as presented below:
MongoClient mongoClient = new MongoClient();
DB database = mongoClient.getDB("myMongoDb");
boolean auth = database.authenticate("username", "pwd".toCharArray());
Show Existing Databases
Let’s display all existing databases. When we want to use the command line, the syntax to show databases is similar to MySQL: Get MongoDB Certification for a bright future.
show databases;
In Java, we display databases using snippet below:
mongoClient.getDatabaseNames().forEach(System.out::println);
Create a Collection
Let’s start by creating a Collection (table equivalent for MongoDB) for our database. Once we have connected to our database, we can make a Collection as:
database.createCollection("customers", null);
Now, let’s display all existing collections for current database:
database.getCollectionNames().forEach(System.out::println);
Save – Insert
The save operation has save-or-update semantics: if an id is present, it performs an update, if not – it does an insert.
When we save a new customer:
DBCollection collection = database.getCollection("customers");
BasicDBObject document = new BasicDBObject();
document.put("name", "Shubham");
document.put("company", "Baeldung");
collection.insert(document);
The entity will be inserted into a database:
{
"_id" : ObjectId("33a52bb7830b8c9b233b4fe6"),
"name" : "Shubham",
"company" : "Baeldung"
}
Save – Update
Let’s now look at save with update semantics, operating on an existing customer:
{
"_id" : ObjectId("33a52bb7830b8c9b233b4fe6"),
"name" : "Shubham",
"company" : "Baeldung"
}
Now, when we save the existing customer – we will update it:
BasicDBObject query = new BasicDBObject();
query.put("name", "Shubham");
BasicDBObject newDocument = new BasicDBObject();
newDocument.put("name", "John");
BasicDBObject updateObject = new BasicDBObject();
updateObject.put("$set", newDocument);
collection.update(query, updateObject);
The database will look like this:
{
"_id" : ObjectId("33a52bb7830b8c9b233b4fe6"),
"name" : "John",
"company" : "Baeldung"
}
Read a Document from a Collection
Let’s search for a Document in a Collection by making a query:
BasicDBObject searchQuery = new BasicDBObject();
searchQuery.put("name", "John");
DBCursor cursor = collection.find(searchQuery);
while (cursor.hasNext()) {
System.out.println(cursor.next());
}
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