JPA implemntation many to many parent child relation of using one table and one mapping table

 JPA implemntation many to many parent child relation of using single table and one mapping table, with save api and fetch api by the root id.

In JPA (Java Persistence API), a many-to-many relationship on a single class can be modeled using a self-referencing join table. This is done by creating two entities, one for the class and another for the join table, and using the @ManyToMany annotation on the class with the @JoinTable annotation on the join table. The @JoinTable annotation is used to specify the name and columns of the join table, and the @ManyToMany annotation is used to specify the mapping between the class and the join table.

Here is an example of implementing a many-to-many parent-child relationship using JPA, with a Category table and a mapping table, as well as a save API and a fetch API by root ID:

1. Create Category and Mapping entities.

@Entity  public class Category {
@Id@GeneratedValue(strategy = GenerationType.IDENTITY)

  private Long id;

  private String name;

  @ManyToMany(cascade = CascadeType.ALL)

  @JoinTable(

      name = "category_mapping",

      joinColumns = @JoinColumn(name = "category_id"),

      inverseJoinColumns = @JoinColumn(name = "child_id")

  )

  private Set<Category> children;

  // getters and setters

}

@Entity

public class CategoryMapping {

  @Id

  @GeneratedValue(strategy = GenerationType.IDENTITY)

  private Long id;

  @ManyToOne

  @JoinColumn(name = "category_id")

  private Category category;

  @ManyToOne

  @JoinColumn(name = "child_id")

  private Category child;

  // getters and setters

}

2. Save API:

public void save(Category parent, Category child) {

  parent.getChildren().add(child);

  CategoryMapping mapping = new CategoryMapping();

  mapping.setCategory(parent);

  mapping.setChild(child);

  entityManager.persist(mapping);

}

3. Fetch API by root ID:

public Category findByRootId(Long rootId) {

  return entityManager.createQuery(

      "SELECT c " +

      "FROM Category c " +

      "LEFT JOIN FETCH c.children " +

      "WHERE c.id = :rootId", Category.class)

      .setParameter("rootId", rootId)

      .getSingleResult();

}

you could implement the fetch API in a JPA repository class:

With this implementation, you can use the findByRootIdWithChildren method in your service class to fetch a Category entity by its ID, along with its children, in a single query.

@Repository public interface CategoryRepository extends JpaRepository<Category, Long> { @Query("SELECT c FROM Category c LEFT JOIN FETCH c.children WHERE c.id = :rootId") Category findByRootIdWithChildren(@Param("rootId") Long rootId); }

@Service

public class CategoryService { private final CategoryRepository categoryRepository; public CategoryService(CategoryRepository categoryRepository) { this.categoryRepository = categoryRepository; } public Category findByRootId(Long rootId) { return categoryRepository.findByRootIdWithChildren(rootId); } }

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Here is an example of a JPA repository method that fetches a Category entity by its root ID, including all children at the leaf level:

@Repository public interface CategoryRepository extends JpaRepository<Category, Long> { @Query(value = "WITH RECURSIVE category_tree(id, name, parent_id) AS ( " + " SELECT id, name, NULL as parent_id " + " FROM category " + " WHERE id = :rootId " + " UNION ALL " + " SELECT c.id, c.name, c.parent_id " + " FROM category_tree t " + " JOIN category_mapping m ON t.id = m.parent_id " + " JOIN category c ON m.child_id = c.id " + ")" + "SELECT id, name, parent_id " + "FROM category_tree", nativeQuery = true) List<Object[]> findAllByRootIdWithChildren(@Param("rootId") Long rootId); }

This method uses a common table expression (CTE) and the WITH RECURSIVE syntax to build a recursive query that fetches all the descendants of the Category entity with the specified root ID. The query results are returned as an array of objects, which you can then map to Category entities if desired.

Note that this method requires a database that supports the WITH RECURSIVE syntax, such as PostgreSQL. If you are using a different database, you may need to use a different approach.

Yes, it is possible to create the query without using the WITH RECURSIVE syntax. Here is an example of how you could achieve this using multiple JOIN statements:

@Repository public interface CategoryRepository extends JpaRepository<Category, Long> { @Query(value = "SELECT c1.id, c1.name, c2.id as parent_id " + "FROM category c1 " + "LEFT JOIN category_mapping m ON c1.id = m.child_id " + "LEFT JOIN category c2 ON m.parent_id = c2.id " + "WHERE c1.id = :rootId " + "UNION ALL " + "SELECT c1.id, c1.name, c2.id as parent_id " + "FROM category_mapping m " + "JOIN category c1 ON m.child_id = c1.id " + "JOIN category c2 ON m.parent_id = c2.id " + "WHERE m.parent_id = :rootId", nativeQuery = true) List<Object[]> findAllByRootIdWithChildren(@Param("rootId") Long rootId); }

This query uses two separate JOIN statements to fetch the Category entity with the specified root ID, as well as all of its descendants. The results are combined using the UNION ALL operator, and returned as an array of objects.

This approach may be more suitable for databases that do not support the WITH RECURSIVE syntax. However, it may not be as efficient as the WITH RECURSIVE approach, as it requires multiple JOIN operations.

Oracle does not support the WITH RECURSIVE syntax for common table expressions (CTEs). Oracle has its own syntax for defining recursive subqueries, which is similar to the syntax used in other databases. However, if you need to perform recursive queries in Oracle, you will need to use a different approach, such as connecting the results of multiple subqueries using the CONNECT BY clause.

Here is an example of how you could write the query in Oracle to fetch a Category entity by its root ID, including all children at the leaf level:

@Repository public interface CategoryRepository extends JpaRepository<Category, Long> { @Query(value = "SELECT id, name, parent_id " + "FROM ( " + " SELECT c1.id, c1.name, c2.id as parent_id, " + " LEVEL as lvl " + " FROM category c1 " + " LEFT JOIN category_mapping m ON c1.id = m.child_id " + " LEFT JOIN category c2 ON m.parent_id = c2.id " + " WHERE c1.id = :rootId " + " UNION ALL " + " SELECT c1.id, c1.name, c2.id as parent_id, " + " lvl + 1 " + " FROM category_mapping m " + " JOIN category c1 ON m.child_id = c1.id " + " JOIN category c2 ON m.parent_id = c2.

here is an example of how you could write the query in Oracle using the CONNECT BY clause as a replacement for WITH RECURSIVE:

@Repository public interface CategoryRepository extends JpaRepository<Category, Long> { @Query(value = "SELECT c.id, c.name, m.parent_id " + "FROM category c " + "JOIN category_mapping m ON c.id = m.child_id " + "START WITH c.id = :rootId " + "CONNECT BY PRIOR m.child_id = m.parent_id", nativeQuery = true) List<Object[]> findAllByRootIdWithChildren(@Param("rootId") Long rootId); }

In this query, the JOIN clause is used to join the category and category_mapping tables based on the child_id column. The START WITH clause specifies the root node of the hierarchy, and the CONNECT BY PRIOR clause defines the relationship between parent and child nodes using the parent_id column from the category_mapping table. The results are returned as an array of objects, where each object represents a Category entity, its name, and its parent ID.

Here is an example of the JSON output for a sample hierarchy of categories:

[ { "id": 1, "name": "Root Category", "parent_id": null }, { "id": 2, "name": "Child Category 1", "parent_id": 1 }, { "id": 3, "name": "Child Category 2", "parent_id": 1 }, { "id": 4, "name": "Grandchild Category 1", "parent_id": 2 }, { "id": 5, "name": "Grandchild Category 2", "parent_id": 2 }, { "id": 6, "name": "Great-grandchild Category", "parent_id": 4 } ]

In this example, the hierarchy starts with a root category with an ID of 1 and two child categories with IDs of 2 and 3. The child category with ID 2 has two grandchild categories with IDs of 4 and 5, and the grandchild category with ID 4 has a great-grandchild category with ID 6. The parent ID of each category is listed in the parent_id field of the JSON object.

another method in repository class for oracle using connect by to get the parent child data for category in a hiararchy

You can use a recursive function to process the query results and create the parent-child structure. Here's an example of how you can do this in Java:

public class CategoryNode { private Long id; private String name; private Long parentId; private List<CategoryNode> children; public CategoryNode(Long id, String name, Long parentId) { this.id = id; this.name = name; this.parentId = parentId; this.children = new ArrayList<>(); } public void addChild(CategoryNode child) { this.children.add(child); } } @Repository public interface CategoryRepository extends JpaRepository<Category, Long> { @Query(value = "SELECT c.id, c.name, m.parent_id " + "FROM category c " + "JOIN category_mapping m ON c.id = m.child_id " + "START WITH c.id = :rootId " + "CONNECT BY PRIOR m.child_id = m.parent_id", nativeQuery = true) List<Object[]> findAllByRootIdWithHierarchy(@Param("rootId") Long rootId); default List<CategoryNode> getCategoryHierarchy(Long rootId) { List<Object[]> queryResult = findAllByRootIdWithHierarchy(rootId); Map<Long, CategoryNode> categoryNodeMap = new HashMap<>(); for (Object[] row : queryResult) { Long id = (Long) row[0]; String name = (String) row[1]; Long parentId = (Long) row[2]; categoryNodeMap.putIfAbsent(id, new CategoryNode(id, name, parentId)); } for (CategoryNode node : categoryNodeMap.values()) { if (node.parentId != null) { CategoryNode parent = categoryNodeMap.get(node.parentId); parent.addChild(node); } } return new ArrayList<>(categoryNodeMap.values()); } }

In this example, the CategoryNode class represents a node in the category hierarchy, with an ID, a name, a parent ID, and a list of children. The getCategoryHierarchy method retrieves the hierarchy for categories using the findAllByRootIdWithHierarchy query, and then processes the query results to create a map of CategoryNode objects. The method then adds each node to its parent node's list of children, and finally returns the list of all nodes in the hierarchy.

The output of the getCategoryHierarchy method will be a list of CategoryNode objects that represent the category hierarchy, starting from the root node specified by the rootId parameter.

Here's an example of what the output might look like for a hierarchy of categories:

[ { "id": 1, "name": "Root Category", "parentId": null, "children": [ { "id": 2, "name": "Child Category 1", "parentId": 1, "children": [ { "id": 3, "name": "Grandchild Category 1", "parentId": 2, "children": [] } ] }, { "id": 4, "name": "Child Category 2", "parentId": 1, "children": [] } ] } ]

In this example, the first element in the list represents the root category with ID 1, which has two children (Child Category 1 and Child Category 2), and Child Category 1 has one grandchild (Grandchild Category 1).