Mongo Pojo with Spring boot, how to fetch subset of data in a document

I have a spring boot application setup to work with Mongo data Pojo. I have the following dependencies among other things (I have excluded Jackson, and am using Gson instead) -

<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-data-mongodb</artifactId>
</dependency>
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-web</artifactId>
	<!-- Exclude the default Jackson dependency -->
	<exclusions>
		<exclusion>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-json</artifactId>
		</exclusion>
	</exclusions>
</dependency>

I have 2 classes, both referring to the same collection. The only difference is that ModuleB has some fields missing -

@Document("Module")
class ModuleA extends MongoModel{
	int X = 0;
	String Y = "something";
}


@Document("Module")
class ModuleB extends MongoModel{
	String Y = "something";
}

I am using MongoTemplate to get the data -

protected final MongoTemplate template;
@Override
public MongoModel get(Class<? extends MongoModel> cls, Query query) {
	query.addCriteria(Criteria.where("deleted").is(0));
	return template.findOne(query, cls);
}

Now, when I use spring repository to get an instance/document of ModuleA or ModuleB, is there going to be a performance difference (or query difference) in the select operation that is performed by spring-boot? Or does spring-boot gets all the fields in both cases anyways, and then populates the object with required fields.

The example above may seem trivial, but if I have many fields or DBRef to another document inside the Module, I can save significant processing when fetching a subset of data instead of fetching the entire document.

For maintainability and consistency, you shouldn't use multiple @Document-annotated models referring to the same MongoDB collection.

It can get really messy if one model has e.g. different @Index annotations or datatypes, or you use one model to insert and another model to read or update.

For read-only operations those problem do not occur, but then you don't need a @Document annotation anyway, you can use projections, as mentioned by @prasad_.

You should keep the projections together with the core domain model, so in the future you don't forget to e.g. rename a field in the projection after you renamed it in the core domain model etc.