How to use Predicate with Collection

Question

I have three tables.

1. parent
2. parent_child_mapping
3. child

I want to filter by child name using predicate. I am trying this without passing child name as argument. I want to return the Parent & Child details if child name only "abc". This is my sample code

Parent entity

@Entity
@Table(name = "parent")
public class Parent {

    private static final long serialVersionUID = 1L;
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "parent_id")
    private Long parentId;
    
    ....

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "mappingParentId")
    private Collection<ParentChildMapping> parentChildMappingCollection;
}

Parent child mapping entity

@Entity
@Table(name = "parent_child_mapping")
public class ParentChildMapping{

    private static final long serialVersionUID = 1L;
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "mapping_id")
    private Long mappingId;
       
    @JoinColumn(name = "mapping_child_id")
    @ManyToOne(optional = false)
    private ChildEntity mappingChildId;
    
    @JoinColumn(name = "mapping_parent_id")
    @ManyToOne(optional = false)
    private ParentEntity mappingParentId;
}

Child entity

@Data
@Entity
@Table(name = "child")
public class Child implements Serializable 
{
    private static final long serialVersionUID = 1L;
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "child_id")
    private Long childId;
    
    @Basic(optional = false)
    @NotNull
    @Size(min = 1, max = 255)
    @Column(name = "child_name")
    private String childName;
}

current implementation logic

Child child = new Child(); 
        child.setChildName("abc"); //setting the required child name
        
        ParentChildMapping parentChildMapping = new ParentChildMapping();
        parentChildMapping.setMappingChildId(child); //setting the child to mapping
        
        List<ParentChildMapping> parentChildMappingArray = new ArrayList<>();
        parentChildMappingArray.add(parentChildMapping); //setting mapping to mapping collection
        
        Parent parent =  new Parent();
        parent.setParentChildMappingCollection(parentChildMappingArray); //finally set mapping collection to parent
        
        CriteriaQuery<Parent> criteriaQuery = criteriaBuilder.createQuery(Parent.class);
        Root<Parent> root = criteriaQuery.from(Parent.class);
        criteriaQuery.select(root);
        
        List<Predicate> predicateList = new ArrayList<>();  
    
  System.out.println(((List<ParentChildMapping>)parent.getParentChildMappingCollection()).get(0).getMappingChildId().getChildName().trim().toLowerCase()); //abc
    
        //not working, null pointer exception
        predicateList.add(criteriaBuilder.equal(criteriaBuilder.lower(root.<ParentChildMapping>get("parentChildMappingCollection").<ChildEntity>get("mappingChildId").<String>get("childName")), ((List<ParentChildMapping>)parent.getParentChildMappingCollection()).get(0).getMappingChildId().getChildName().trim().toLowerCase()));
    
        criteriaQuery.where(predicate);

I want to return the Parent & Child if child name only "abc".

I am trying to implement this

  SELECT p.parent_id
  FROM parent p
  JOIN parent_child_mapping pcmap on p.parent_id = pcmap.mapping_parent_id
  JOIN child c on pcmap.mapping_child_id = c.child_id
  WHERE c.child_name = 'abc'

I hope you understand this question.

Answer 1

You will have to use an EXISTS predicate to only return Parent if one of the children names match. See How to check if entity in set contains a certain value using criteriaBuilder? for details.