Aggregate strategies 

The aggregate strategy defines how to construct an entity aggregate from an arbitrary SELECT statement. It provides a structured way to map relational query results into hierarchical entity structures by specifying how entities should be linked together.

Aggregate strategy definition 

An aggregate strategy is defined by annotating an interface with @AggregateStrategy. This annotation specifies how an entity aggregate is reconstructed from a query result.

@AggregateStrategy(root = Department.class, tableAlias = "d")
interface DepartmentAggregateStrategy {
    ...
}

The root element specifies the entity class that serves as the root of the aggregate.
The tableAlias element defines the alias for the table corresponding to the root entity class. This alias must be used in the SELECT statement to correctly map query results to entity properties.

Association linker definition 

An aggregate strategy must contain at least one field of type BiConsumer or BiFunction, annotated with @AssociationLinker. These functions are responsible for dynamically associating two entity instances. Use a BiFunction when associating immutable entities. For mutable entities, you may use either a BiConsumer or a BiFunction.

@AggregateStrategy(root = Department.class, tableAlias = "d")
interface DepartmentAggregateStrategy {
  @AssociationLinker(propertyPath = "employees", tableAlias = "e")
  BiConsumer<Department, Employee> employees =
      (d, e) -> {
        d.getEmployees().add(e);
        e.setDepartment(d);
      };

  @AssociationLinker(propertyPath = "employees.address", tableAlias = "a")
  BiFunction<Employee, Address, Employee> address =
      (e, a) -> {
        e.setAddress(a);
        return e;
      };
}

The first type parameter of a BiConsumer or BiFunction represents the type of the property owner, and the second type parameter represents the type of the property. The third type parameter of a BiFunction must be the same as the first one and represents the type of the entity after the association is applied.
The propertyPath element specifies the name of the target property as a dot-separated path from the root entity class.
The tableAlias element specifies the alias for the table corresponding to the entity class used as the second type parameter of the BiConsumer or BiFunction. This alias must be used in the SELECT statement.

Example 

The DepartmentAggregateStrategy described above is based on the following entity definitions:

@Entity(naming = NamingType.SNAKE_LOWER_CASE)
public class Department {
  @Id Integer id;
  String name;
  @Association List<Employee> employees = new ArrayList<>();

  // getter, setter
}

@Entity(naming = NamingType.SNAKE_LOWER_CASE)
public class Employee {
  @Id Integer id;
  String name;
  Integer departmentId;
  Integer addressId;
  @Association Department department;
  @Association Address address;

  // getter, setter
}

@Entity(naming = NamingType.SNAKE_LOWER_CASE)
public class Address {
  @Id Integer id;
  String street;

  // getter, setter
}

In entity classes, association properties must be annotated with @Association. These properties can be linked using @AssociationLinker.

Using an aggregate strategy 

DepartmentAggregateStrategy is used by specifying it in the aggregateStrategy element of @Select:

@Dao
interface DepartmentDao {
  @Select(aggregateStrategy = DepartmentAggregateStrategy.class)
  Department selectById(Integer id);
}

The selectById method requires a SELECT statement like the following:

select
    d.id as d_id,
    d.name as d_name,
    a.id as a_id,
    a.street as a_street,
    e.id as e_id,
    e.name as e_name,
    e.department_id as e_department_id,
    e.address_id as e_address_id
from
    department d
    left outer join
    employee e on (d.id = e.department_id)
    left outer join
    address a on (e.address_id = a.id)
where
    d.id = /* id */0

Note

The SELECT list must include the IDs of all entities that form the aggregate.

Column aliasing rules 

Table aliases must match those defined in DepartmentAggregateStrategy.
Column aliases must begin with the table alias followed by an underscore (_). For example, d.id should be aliased as d_id and e.id as e_id.

Using the expansion directive 

By using the expansion directive, the above SELECT statement can be written more concisely:

select
    /*%expand*/*
from
    department d
    left outer join
    employee e on (d.id = e.department_id)
    left outer join
    address a on (e.address_id = a.id)
where
    d.id = /* id */0

How expansion works 

The /*%expand*/* directive automatically expands into a column list following predefined aliasing rules.
By default, all columns from all tables are included in the result set.

To selectively expand only specific tables, pass a comma-separated list of table aliases:

select
    /*%expand "e, d" */*,
    a.id as a_id,
    a.street as a_street
from
    department d
    left outer join
    employee e on (d.id = e.department_id)
    left outer join
    address a on (e.address_id = a.id)
where
    d.id = /* id */0

Here, only columns from tables e (employee) and d (department) are expanded.
The columns from table a (address) are explicitly specified.