JPQL: How to join via #ElementCollection with #MapKeyJoinColumn - jpa

Question

I have problems creating the correct JPQL query for joining through the following tables:
While between GROUPS and USERS there is a conventional #ManyToMany mapping table, DOCUMENTS_GROUPS is what causes the trouble. As you can see in the following entity, I want the relationship between DOCUMENTS and GROUPS to be mapped as a Map containing the access_mode (which works just fine except for the query):
#Entity
#Table(name = "DOCUMENTS")
#NamedQueries({
#NamedQuery(
name = "Documents.findAccessibleByUser",
query = "SELECT d FROM Document d INNER JOIN d.groups g INNER JOIN KEY(g).members m WHERE m.id = :userId"
)
})
public class Document {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private int id;
#ElementCollection
#CollectionTable(name = "DOCUMENTS_GROUPS", joinColumns = {#JoinColumn(name = "document_id")})
#MapKeyJoinColumn(name = "group_id")
#Column(name = "access_mode")
#Enumerated(EnumType.STRING)
private Map<Group, AccessMode> groups = new HashMap<>();
/* ... */
}
With Group being rather normal:
#Entity
#Table(name = "GROUPS")
public class Group {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private int id;
#Column(length = 255)
private String name;
#ManyToMany
#JoinTable(name = "USERS_GROUPS", //
joinColumns = {#JoinColumn(name = "group_id")}, //
inverseJoinColumns = {#JoinColumn(name = "user_id")} //
)
private Set<User> members = new HashSet<>();
/* ... */
}
My question is now: How do I need to modify the second JOIN in my JPQL query?
SELECT d FROM Document d
INNER JOIN d.groups g
INNER JOIN KEY(g).members m
WHERE m.id = :userId
is syntactically wrong (unexpected KEY after INNER JOIN).
Of course, I have already tried a plain INNER JOIN g.members m, but since we're dealing with a Map<Group, AccessMode>, this fails with cannot dereference scalar collection element: members.

Answer 1

I was facing the same problem with a simple key-value Map<String, String> like:
#Entity Item.java
#ElementCollection
#MapKeyColumn(name = "name")
#Column(name = "value")
#CollectionTable(indexes = #Index(columnList = "value"))
private Map<String, String> attributes = new HashMap<>();
Joining the attributes was possible:
Query query = em.createQuery("SELECT i FROM Item i INNER JOIN i.attributes attr");
but not querying fields:
Query query = em.createQuery("SELECT i FROM Item i INNER JOIN i.attributes attr WHERE attr.value = 'something'");
I debugged the Hibernate internals and found out that the alias attr is already resolved to the value (e.attributes.value), so the only thing you can do here is:
Query query = em.createQuery("SELECT i FROM Item i INNER JOIN i.attributes attr WHERE attr = 'something'");
But I did not find any documentation or JPQL examples pointing that out. The behaviour is is useless in my case, because I want to have conditions for both key and value. Thats why I migrated to a foreign entity collection with key mapping and composite primary key. Its way more complicated but works as expected.
The composite key entity to prevent single primary keys
#Embeddable
public class ItemAttributeName implements Serializable {
private String name;
#ManyToOne
#JoinColumn(nullable = false)
private Item item;
// Empty default constructor is important
public ItemAttributeName() {
}
public ItemAttributeName(Item item, String name) {
this.item = article;
this.name = name;
}
}
The real attribute entity
#Entity
public class ItemAttribute {
#EmbeddedId
private ItemAttributeName id;
private String value;
// Empty default constructor is important
public ItemAttribute() {
}
public ItemAttribute(Item item, String name) {
this.id = new ItemAttributeName (item, name);
}
public String getValue() {
return value;
}
}
#Entity Item.java
#OneToMany(mappedBy = "id.item",cascade = CascadeType.PERSIST)
#MapKeyColumn(name = "name")
public Map<String, ItemAttribute> attributes = new HashMap<>();
Creating entities:
Item item = new Item ();
ItemAttribute fooAttribute = new ItemAttribute(item, "foo");
fooAttribute.setValue("356");
item.attributes.put("foo", fooAttribute);
Querying entities:
Query query = em.createQuery("SELECT i FROM Item i JOIN i.attributes attr WHERE attr.id.name = 'foo' AND attr.value='bar'");
List<Item> resultList = query.getResultList();
System.out.println(resultList.get(0).attributes.get("foo").getValue());
Prints out: bar