Скачать или смотреть Why Your Entity Can't Map the Columns Table in Spring Data JPA

Discover the common pitfalls in mapping entity columns with Spring Data JPA and learn how to properly set up your entity for success.
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This video is based on the question https://stackoverflow.com/q/63549813/ asked by the user 'recoba78' ( https://stackoverflow.com/u/12648069/ ) and on the answer https://stackoverflow.com/a/63549940/ provided by the user 'Sagar Gangwal' ( https://stackoverflow.com/u/5809720/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Entity Mapping in Spring Data JPA

When working with Spring Data JPA, developers often face challenges in mapping entity attributes to database table columns properly. One common issue is when an entity isn't returning any results, even though you believe the mapping is correctly set up. This post explores why an entity like customCountry may not successfully map its columns from the MySQL database table named countries and provides a straightforward solution.

The Problem: No Results Returned

Scenario: You have a MySQL table called countries with columns such as id, name, code, and others. You created an entity class customCountry and a corresponding repository. However, upon calling a service method to load the custom countries, you find that the size of the returned list is 0.

Entity and Repository Setup

Your setup includes an entity class and a repository interface, as shown below:

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Method Call and the Result

When you call the method loadcustomcountries() from your service class, you receive a size of 0, indicating that no entity instances were retrieved from the database.

The Root Cause: Redundant Method Definition

What's Happening?

The primary issue here lies in the redundant declaration of the findAll() method in your repository interface. The JpaRepository that customCountryRepository extends already provides an implementation for the findAll() method, which means there is no need to declare it again.

Why the Redundancy Causes Issues

By declaring List<customCountry> findAll(); in your repository, the application expects a user-defined implementation of this method. However, since you haven't provided an implementation, it defaults to an empty result, leading to the observed behavior of receiving a size of 0.

The Solution: Remove the Redundant Method

To resolve this issue, simply remove the redundant declaration of the findAll() method in your custom repository interface. Here’s how your repository should look after the change:

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Re-check Your Service Method

Once the redundant method has been removed, your service method should now correctly return the list of customCountry entities when called. Implement the following check in your service class:

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Conclusion

By understanding the structure of your repository and utilizing the features provided by JpaRepository, you can effectively eliminate issues related to entity mappings. The key takeaway here is to avoid unnecessary method declarations in your repository that can lead to confusion and errors. With these adjustments, your spring data JPA setup should work seamlessly, returning the expected results.

Feel free to ask any questions or share your own experiences with Spring Data JPA in the comments below!