Join Fetch

Filter related entities while selecting specific columns

In a POJO, you can use @JoinFetch on a field or class to filter associated entities. The difference from @Join is that this allows fetching all lazy-related data at once.

Single Join Fetch

For example, consider a Customer entity that has a one-to-many relationship with an Order entity:


@Entity
class Customer {

  @OneToMany(fetch = LAZY, cascade = ALL)
  @JoinColumn(name = "order_id")
  private Collection<Order> orders;
}

@Entity
class Order {
  
  private String itemName;
}

If you want to fetch Order data when retrieving Customer, you can do:


@Data
@JoinFetch(path = "orders")
class CustomerOrderCriteria {

  @Spec
  String name;
}

The executed SQL will be like:


select distinct 
  customer0_.* ...,
  orders1_.* ...
from customer customer0_ 
inner outer join orders orders1_ on customer0_.id=orders1_.order_id 
where customer0_.name=?

You can see that orders1_.* is also included in the select fields.

Join Behavior

To better align with most usage scenarios, the default behavior is as follows:

The default join type is INNER.
Duplicate results are removed (distinct).

You can modify the default behavior by configuring @JoinFetch#joinType or @JoinFetch#distinct, for example:


@JoinFetch(joinType = JoinType.RIGHT, distinct = false)

With Clause

@JoinFetch also allows filtering data. For example, if you want to filter both customer names and order names, define a POJO as follows:


@Data
@JoinFetch(path = "orders", alias = "o")
public class CustomerOrderCriteria {

  @Spec
  String name;

  @Spec(path = "o.itemName", value = In.class)
  Collection<String> items;
}

The executed SQL will be like:


select distinct 
  customer0_.* ...,
  orders1_.* ...
from customer customer0_ 
inner outer join orders orders1_ on customer0_.id=orders1_.order_id 
where customer0_.name=? and orders1_.item_name in (?)

@JoinFetch can also be used at the field level, for example:


@Data
public class CustomerOrderCriteria {

  @Spec
  String name;

  @JoinFetch(path = "orders", alias = "o")
  @Spec(path = "o.itemName", value = In.class)
  Collection<String> items;
}

Multi Join Fetches

You can use @JoinFetches to define multi-level fetches. For example, if the Order entity has a many-to-many relationship with a Tag entity:


@Entity
class Customer {

  @OneToMany(cascade = ALL, fetch = LAZY)
  @JoinColumn(name = "order_id")
  private Set<Order> orders;
}

@Entity
class Order {
    
  @ManyToMany(cascade = ALL, fetch = LAZY)
  private Set<Tag> tags;
}

@Entity
class Tag {

  private String name;
}

If you want to query customers who purchased items belonging to specific categories, define a POJO as follows:


@Data
class CustomerOrderTagCriteria {

  @JoinFetches({
    @JoinFetch(path = "orders", alias = "o"),
    @JoinFetch(path = "o.tags", alias = "t")
  })
  @Spec(path = "t.name", value = In.class)
  Collection<String> tags;
}

The executed SQL will be like:


select distinct 
  customer0_.* ...,
  orders1_.* ...,
  tag3_.* ...
from customer customer0_ 
inner join orders orders1_ on customer0_.id=orders1_.order_id 
inner join orders_tags tags2_ on orders1_.id=tags2_.order_id 
inner join tag tag3_ on tags2_.tags_id=tag3_.id 
where tag3_.name in (?)

@JoinFetches can also be used at the class level, making the aliases available across fields in the same object. For example:


@Data
@JoinFetches({
  @JoinFetch(path = "orders", alias = "o"),
  @JoinFetch(path = "o.tags", alias = "t")
})
public class CustomerOrderCriteria {

  @Spec(path = "o.itemName", value = In.class)
  Collection<String> items;

  @Spec(path = "t.name", value = In.class)
  Collection<String> tags;
}

Fetches Order

Annotations are processed in order, so @JoinFetches must be defined in the correct sequence.

For example, in the previous scenario, the following order is incorrect:


@Data
class CustomerOrderTagCriteria {

  @JoinFetches({
    @JoinFetch(path = "o.tags", alias = "t"), // "o" alias will be not exist during processing this @Join
    @JoinFetch(path = "orders", alias = "o")
  })
  @Spec(path = "t.name", value = In.class)
  Collection<String> tagNames;
}

Alias

The usage rules for @JoinFetch#alias are as follows:

It is shared within the same POJO
It cannot be declared multiple times within the same POJO
If not provided, the default alias is @JoinFetch#path
If it contains ., it will be replaced with _

For example:


@JoinFetches({
  @JoinFetch(path = "orders"), // default alias is "orders"
  @JoinFetch(path = "orders.tags") // default alias is "orders_tags"
})
@Spec(path = "orders_tags.name", value = In.class)