Spring Data MongoDB is part of the Spring Data family. It provides an easy way to interact with MongoDB databases using Spring Boot or Spring Framework. Instead of manually writing a lot of boilerplate code for queries, connections, and data conversions, Spring Data MongoDB automates and simplifies most of that

Here’s how it works at a high level:

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1. MongoTemplate (Lower-level API)

• MongoTemplate is the core class that handles interactions with MongoDB.
• It provides methods like save(), find(), findOne(), remove(), etc.
• You can manually construct queries if you need full control.

Example:

@Autowired
private MongoTemplate mongoTemplate;

public List<User> getAllUsers() {
    return mongoTemplate.findAll(User.class);
}

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2. MongoRepository (Higher-level abstraction)

• If you don’t want to manually write queries, you can create a Repository interface that extends MongoRepository.
• Spring will automatically generate implementations for common database operations.

Example:

public interface UserRepository extends MongoRepository<User, String> {
    List<User> findByName(String name);
}

Usage:

@Autowired
private UserRepository userRepository;

public List<User> findUsersByName(String name) {
    return userRepository.findByName(name);
}

You don’t need to implement anything — just declare the method signatures, and Spring generates the queries based on method names (this is called Query Method Derivation).

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3. Domain Model (Entity Classes)

• You create simple Java classes annotated with @Document (to mark them as MongoDB documents).
• Fields can be annotated with @Id (primary key) and other annotations like @Field.

Example:

@Document(collection = "users")
public class User {
    @Id
    private String id;
    private String name;
    private int age;
    
    // getters and setters
}

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4. Configuration

• In Spring Boot, configuration is mostly automatic if you include the right dependencies (spring-boot-starter-data-mongodb).
• You just set a few properties in application.properties or application.yml:

spring.data.mongodb.uri=mongodb://localhost:27017/mydatabase

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5. Advanced Features

• Custom queries with @Query annotations.
• Aggregation pipelines for complex data transformations.
• Validation with Java Bean Validation (e.g., @NotNull).
• Reactive programming (with Spring WebFlux) if you want non-blocking MongoDB operations using ReactiveMongoRepository.

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Summary Diagram (quick mental picture):

Your Code (UserRepository, User class)
    ↓
Spring Data MongoDB (automatic query building, templates)
    ↓
MongoDB Java Driver
    ↓
MongoDB Server

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Now, let’s dive into how Spring Data MongoDB parses repository interfaces and generates code behind the scenes.

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🔥 How Spring Mongo Repositories Work Internally

1. At startup: Spring scans repository interfaces

When your Spring Boot app starts, Spring looks for interfaces that extend a base interface like:

MongoRepository<User, String>

It finds these interfaces because of:

• Classpath scanning (@EnableMongoRepositories or auto-configuration).
• Marker interfaces and packages.

Spring doesn’t look for concrete classes — it looks for interfaces and automatically generates implementations.

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2. Spring creates proxy instances

• Spring uses dynamic proxies or CGLIB proxies to create runtime classes that “implement” your repository interface.
• These proxies intercept your method calls and handle them dynamically.

The real class you talk to is something like:

SimpleMongoRepository (for standard methods)
CustomMongoRepositoryImpl (for custom implementations)

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3. Spring parses method names

When you declare methods like:

List<User> findByNameAndAgeGreaterThan(String name, int age);

Spring parses the method name based on a predefined naming convention:

• findBy → means a find query.
• NameAndAgeGreaterThan → means where name = ? AND age > ?.

The parsing engine tokenizes the method name, maps it to MongoDB fields, and understands operations like:

• And, Or, GreaterThan, LessThan, IsNull, etc.

Spring uses internal classes like:

• PartTree
• QueryMethod
• MongoQueryCreator

Example:
findByNameAndAgeGreaterThan → parsed into a MongoDB query document:

{ "name": "John", "age": { "$gt": 25 } }

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4. Spring generates the MongoDB query

Once Spring has the parsed structure, it:

• Builds a Query object (from org.springframework.data.mongodb.core.query.Query).
• Converts your method arguments into query parameters.
• Sends the query to MongoTemplate internally.

Example (behind the scenes):

Query query = new Query();
query.addCriteria(Criteria.where("name").is(name).and("age").gt(age));
mongoTemplate.find(query, User.class);

You never see this code, but that’s what’s happening inside.

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5. Custom Queries (@Query)

If you want, you can override the method parsing by using @Query:

@Query("{ 'name' : ?0, 'age' : { $gt : ?1 } }")
List<User> customFind(String name, int age);

In this case, Spring directly uses your query string instead of trying to parse the method name.

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🔥 Quick Visual Summary:

Your Repository Interface
    ↓
Spring Boot scans interfaces
    ↓
Proxy instance created (SimpleMongoRepository)
    ↓
Method call intercepted
    ↓
Method name parsed → Query object built
    ↓
MongoTemplate used to run query
    ↓
Result returned

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✨ Important Internals:

Feature	Internal class
Parsing method name	PartTree, Part
Building Mongo queries	MongoQueryCreator, Criteria
Executing query	MongoTemplate
Creating repository proxy	RepositoryFactorySupport, MongoRepositoryFactory

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⚡ Bonus Tip:

If you want to see the generated queries at runtime, add this to application.properties:

logging.level.org.springframework.data.mongodb.core.MongoTemplate=DEBUG

Spring will log actual MongoDB queries to the console!

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