Designing mongodb schema with searchable nested arrays. - mongodb

Question

I'm new to Mongodb coming from relational databases and I'd also like to point out I'm using SpringBoot with JPA. If I were to build an automotive classified site where I would have thousands of Users and 100s of thousands of listings, how would I go about setting up the schema? I've read some articles that say normalizing nosql data is bad practices.
Anyhow lets say we have the following structure.
User
id
name
email
Cars
id
make
model
year
I would need to be able to list many cars with the User and what i've seen in my examples is it creates a nested array of cars within User. This would work great for user accounts where I'd like to provide the user with all their cars
Where I get a bit confused is with the cars. The cars need to be able to be searched very quickly and would not need the user info right away. In a sql db I would typically do a search against the cars (year, make, model) and grab the user later on if I needed it.
In mongodb, do you create a User document that contains a nested car array? or do you somehow create 2 documents that are both automatically maintained and search against the car document for performance reasons?
Sample code
#Document(collection = "person")
public class Person {
#Id
private String id;
private String firstName;
private String lastName;
// #DBRef(lazy = true)
private List<Listing> listings;
public Person(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
}
#Document(collection = "listing")
public class Listing {
#Id
public String id;
public String year;
public String make;
public String model;
public String trim;
public Listing(String year, String make, String model, String trim) {
this.year = year;
this.make = make;
this.model = model;
this.trim = trim;
}
}
#Override
public void run(String... args) throws Exception {
repository.deleteAll();
List<Listing> listings = new ArrayList<>();
Listing listing = new Listing("2008", "Ford", "Focus", "SE");
//listingRepository.save(listing);
listings.add(listing);
Person person = new Person("Alice", "Smith");
person.setListings(listings);
// save a couple of customers
repository.save(person);
person = new Person("Bob", "Smith");
listings = new ArrayList<>();
listings.add(new Listing("2018", "Chrysler", "300", "S"));
person.setListings(listings);
repository.save(person);
// fetch all customers
System.out.println("Customers found with findAll():");
System.out.println("-------------------------------");
for (Person _person : repository.findAll()) {
System.out.println(_person);
}
System.out.println();
// fetch an individual customer
System.out.println("Person found with findByFirstName('Alice'):");
System.out.println("--------------------------------");
System.out.println(repository.findByFirstName("Alice"));
System.out.println("Persons found with findByLastName('Smith'):");
System.out.println("--------------------------------");
for (Person _person : repository.findByLastName("Smith")) {
System.out.println(_person);
}
List<Listing> _listings = listingRepository.findAll();
System.out.println("listings " + _listings.size());
_listings.forEach(v -> {
System.out.println(v.toString());
});
}

Answer 1

Going by your entity model,I think what you are looking for is analogous to Many to Many/One to Many relationship in a relational database. So you can go for One way Embedding or Two way Embedding in MongoDb.
For One way embedding, You can create a Car collection like below:
db.carCollection.insertMany([{
_id:1,
make: 'porcha',
model:'qwerty',
year:'2018'
},
{
_id:2,
make: 'ferrara',
model:'uiop',
year:'2018'
}])
You can then go on to create user collection as below:
db.userCollection.insert({
_id:1,
user:'Tom',
email:'tom#tom.com',
car_ids:[1,2]
})
The car_ids is an array which will hold the ids of cars that belong to the user.
You can fetch the cars belonging to an user as(using findOne to fetch the user. Search parameter should be an unique id. I am considering email to be unique here.Ideally it should be user's id) :
var user=db.userCollection.findOne({email:'tom#tom.com'})
db.carCollection.find({_id:{$in:user.car_ids}})
This will fetch you all the cars per user
For fetching cars only you can simply do:
db.carCollection.find({})
For Two way embedding you can have similar array (as in user collection) inside cars collection so that each car can be identified to its user.