MongoDB is a NoSQL database that works really well with Kotlin and Spring Boot. MongoDB is incredibly useful in situations where the structure of data isn’t known prior to writing the application. For example, picture a blogging website where users can enter any number of comments or response. Modeling such a data structure would be difficult in a relational database, but it’s much easier with Mongo.

In this example application, we are going to use MongoDB to document Restaurants with any number of employees (of course, a simple example such as this can be done in a relational database, but let’s go with this for simplicity sake). The cool part using Mongo with Spring Boot is that there is zero configuration providing you are using default settings. This let’s us jump right into our code.

Let’s begin by creating a couple of data classes to store in our database.

//Create a document class
//that persists to the DB
@Document
data class Restaurant(
        //Mark this field as the document id
        @field: Id var name : String = "",
        //Unstructured Data Here
        var employees : List = mutableListOf())

//This class embeds directly into Restaurant
//without any annotations
data class Employee(var name : String = "",
                    var position : String = "")

Our Restaurant class is annotated with @Document to mark it as a persistable class. We also annotate the name field with the Id annotation to mark it as the document id. This value has to be unique in the database. The other class is Employee which does not have any annotations at all. It’s used as a property in the Employees database and the persistence provide is able store all of employee objects embedded in Restaurant.

Our next class is a repository class which Spring will generate the implementation for us. Before this can happen, we have to enable mongo repositories. All we need to do is annotate a configuration class to make this happen.

@Configuration
@EnableMongoRepositories //Allow Spring to Generate Mongo Repositories
class Config

Once we have enabled the mongo repositories, we just need to define an interface that extends MongoRespository.

//Spring will implement our interface for us!
interface RestaurantRepository : MongoRepository

Now let’s make a controller class to test our application. See this post for an explanation of Spring MVC.

//Example Controller class for demonstration purposes
@Controller
@RequestMapping("/")
class IndexController(
        //We can inject our RestaurantRepository class, Spring will
        //provide an implementation
        @Autowired private val restaurantRepository: RestaurantRepository){

    @RequestMapping(method = arrayOf(RequestMethod.GET))
    fun doGet(model : Model) : String {
        model.apply {
            addAttribute("restaurant", Restaurant())
            //Query all Restaurants
            addAttribute("allRestaurants", restaurantRepository.findAll())
        }
        return "index"
    }

    @RequestMapping(method = arrayOf(RequestMethod.POST))
    fun doPost(@RequestParam("name") name : String,
               @RequestParam("employees") employees : String,
               model : Model) : String {
        val restaurant = Restaurant(name = name,
                                    employees = parseEmployees(employees))
        //Save the new restaurant
        restaurantRepository.save(restaurant)
        model.apply {
            addAttribute("restaurant", Restaurant())
            //Query all Restaurants
            addAttribute("allRestaurants", restaurantRepository.findAll())
        }
        return "index"
    }

    fun parseEmployees(employees : String) : List {
        val employeeList = mutableListOf()
        val parts = employees.split('\n')

        parts.forEach {
            val subParts = it.split(",")
            employeeList.add(
                    Employee(name = subParts[0],
                            position = subParts[1]))
        }
        return employeeList.toList()
    }
}

Notice that we can directly inject RestaurantRepository into our controller. Spring does the work of providing an implementation for our controller class. In our doPost() method, we call restaurantRepository.save() to save our new document. In both doGet() and doPost(), we call restaurantRepository.findAll() to pull back all of our restaurants stored in the database.

Now we just need an HTML template to provide us with front end code.

Conclusion

Here is an example of the application when run.

As you can see, Spring Boot combined with Kotlin makes it really easy to persist data into MongoDB. We only need to define a few data classes and allow Spring to make our Repository classes for us in order to get started.

You can view the code for this project at my GitHub page at this link.