BaseCrudController

BaseCrudController.java

package com.example.api;

import org.springframework.data.domain.Page;
import org.springframework.data.domain.Pageable;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;

public abstract class BaseCrudController<T> {

    // Each concrete controller must provide its own repository
    protected abstract JpaRepository<T, Integer> getRepository();

    // LIST (pagination + sorting)
    @GetMapping
    public Page<T> list(Pageable pageable) {
        return getRepository().findAll(pageable);
    }

    // FIND BY ID
    @GetMapping("/{id}")
    public ResponseEntity<T> findById(@PathVariable Integer id) {
        return getRepository().findById(id)
                .map(ResponseEntity::ok)
                .orElse(ResponseEntity.notFound().build());
    }

    // CREATE
    @PostMapping
    public ResponseEntity<T> create(@RequestBody T entity) {
        T saved = getRepository().save(entity);
        return ResponseEntity.status(HttpStatus.CREATED).body(saved);
    }

    // UPDATE
    @PutMapping("/{id}")
    public ResponseEntity<T> update(@PathVariable Integer id, @RequestBody T entity) {
        if (!getRepository().existsById(id)) {
            return ResponseEntity.notFound().build();
        }
        T saved = getRepository().save(entity);
        return ResponseEntity.ok(saved);
    }

    // DELETE
    @DeleteMapping("/{id}")
    public ResponseEntity<Void> delete(@PathVariable Integer id) {
        if (!getRepository().existsById(id)) {
            return ResponseEntity.notFound().build();
        }
        getRepository().deleteById(id);
        return ResponseEntity.noContent().build();
    }
}

ParticipationController.java

@RestController
@RequestMapping("/api/participations")
public class ParticipationController extends BaseCrudController<Participation> {

    private final ParticipationRepository participationRepository;

    public ParticipationController(ParticipationRepository participationRepository) {
        this.participationRepository = participationRepository;
    }

    @Override
    protected JpaRepository<Participation, Integer> getRepository() {
        return participationRepository;
    }

    @GetMapping("/by-student/{id}")
    public List<Participation> byStudent(@PathVariable Integer id) {
        return participationRepository.findByStudentId(id);
    }
}

ParticipationRepository.java

package com.example.repository;

import com.example.model.Participation;
import com.example.model.Status;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Query;
import org.springframework.data.repository.query.Param;

import java.util.List;

public interface ParticipationRepository extends JpaRepository<Participation, Integer> {

    // Derived Query Method
    List<Participation> findByStudentId(Integer studentId);

    List<Participation> findByProjectId(Integer projectId);

    List<Participation> findByStatus(Status status);

    // Custom JPQL query
    @Query("select p from Participation p where p.student.id = :studentId and p.status = :status")
    List<Participation> findByStudentAndStatus(
            @Param("studentId") Integer studentId,
            @Param("status") Status status
    );

    // Query with sorting inside
    List<Participation> findByStudentIdOrderByCreatedAtDesc(Integer studentId);

}