如何获取相关实体的行数？

Question

我有两个实体，Class 和 Student。一个Class 可以有多个Students (oneToMany)：

# YAML notation for Entity 'Class'
...
oneToMany:
    students:
        targetEntity: MyBundle\Entity\Student
        mappedBy: class

为了获取所有Classes，我正在编写自己的查询，如下所示：

SELECT c
FROM MyBundle:Class c
WHERE c.whatever = :parameter
ORDER BY c.id DESC

现在我正在尝试获取Classes 的列表，按相关Students 的计数排序（DESC）。所以结果看起来像：

Class.id   Class.count(Student)
--------   --------------------
       3                    109
       1                     81
       4                     58
       2                     21

我怎么去那里？我尝试了类似这样的方法：

SELECT
    c,
    COUNT(c.students) AS students
FROM MyBundle:Class c
WHERE c.whatever = :param
GROUP BY c.id
ORDER BY students DESC

（注：我实现了DoctrineExtensions' Date函数）

但我收到一个错误：

[Semantical Error] line 0, col 26 near 'students)': Error: Invalid PathExpression。需要 StateFieldPathExpression 或 SingleValuedAssociationField。

Answer 1

尝试在您的 Class 存储库中使用它：

public function getAllClassesOrderedByNumberOfStudents()
{
    $qb = $this->createQueryBuilder('class');

    $buffer = $qb
        ->select('class, COUNT(students) AS students_per_class')
        ->innerJoin('class.students', 'students')
        ->groupBy('class.id')
        ->orderBy('students_per_class', 'DESC');

    $q = $buffer->getQuery();

    return $q->getResult();
}