Fetching Polymorphic Objects with Spring Data MongoDB: How to Query Inherited Types in One Go

Learn how to efficiently fetch multiple inherited types from an abstract class in Spring Data MongoDB with just one query. This guide breaks down the necessary steps for achieving polymorphic behavior.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How do I fetch specific types inheriting from an abstract class from spring-mongodb with one query?

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Fetching Polymorphic Objects with Spring Data MongoDB

In the world of Spring Data MongoDB, managing class hierarchies can sometimes pose a challenge, especially when it comes to retrieving documents that involve both parent and child classes. If you've ever found yourself grappling with the need to fetch a mix of inherited objects—say from an abstract base class—you're not alone. In this post, we'll explore how to effectively fetch specific types inheriting from an abstract class with a single query in Spring Data MongoDB.

Understanding the Problem

Let's clarify the scenario. You might have a class structure like this:

Class C (Abstract Class)

Class A (Inherits from C)

Class B (Inherits from C)

Both classes A and B have a _class field that MongoDB uses to distinguish between different types. You might want to retrieve a list containing both A and B objects but are concerned about how to accomplish this using the MongoTemplate within Spring Data MongoDB.

Your concern is valid because standard find methods with MongoTemplate typically return a single type of entity. So how can we fetch both types and have them mapped to their respective Java objects?

The Solution: Using the Query Class

1. Querying with MongoTemplate

Spring Data MongoDB offers powerful querying capabilities through the MongoTemplate class, which allows you to work with your MongoDB collections. Although you provide a single type parameter in the find methods, it doesn't restrict document selection based on types. Instead, it defines how the results should be mapped.

2. Restricting Query Results by Type

To effectively fetch documents of both class A and class B, you can harness the Query class. Specifically, you will utilize the restrict(Class, Class...) method. Here’s how you can implement this:

Example Implementation

[[See Video to Reveal this Text or Code Snippet]]

3. Breakdown of the Code

Query Creation: A new Query object is instantiated to construct your query dynamically.

Criteria Definition: Using the Criteria class, we specify that we want to match documents whose _class field corresponds to the class names of A and B.

Conclusion

Fetching documents that belong to multiple inherited classes from an abstract class using Spring Data MongoDB is straightforward using the Query class and its ability to restrict results by type. By utilizing the criteria for the _class field, you can effortlessly retrieve and map these documents to their corresponding Java objects.

If you're dealing with multiple inherited types in your MongoDB collections, incorporating this method in your Spring Data setup can save you both time and effort! Happy coding!