如何在DQL查询（Doctrine 2）中访问相关实体的属性？

I have set up a many to one relationship between a schools entity and students entity, I've linked the two together using a manyToOne relationship;

Students entity:

/**
 * @var \Doctrine\Common\Collections\Collection
 * @ManyToOne(targetEntity="Schools", inversedBy="Students")
 * @JoinColumn(name="school_id", referencedColumnName="school_id")
 */
private $Schools;

Schools entity:

/**
 * @var \Doctrine\Common\Collections\Collection
 * @OneToMany(targetEntity="Students", mappedBy="Schools")
 */
private $Students;

This works as expected.

I am trying to write a query for the Students table which specifies the school_id but I don't know how to specify this within the query;

$qb->select('t.position')
    ->from('\Entities\Students', 't')
    ->where($qb->expr()->eq('t.studentId', ':student_id'))
    ->andWhere($qb->expr()->eq('t.Schools.schoolId', ':school_id')) // doesn't work
    ->setParameters(array('student_id' => $this->studentId, 'school_id' => $school_id));

I've attempted to access the school id through $Schools property I set up in the student identity but this throws an error.

As a temporary fix I've created a $schoolId property in the Students entity and am using this instead but surely this is not the correct way to do this.

Appreciate the help.

UPDATE

I got it to work by changing the line;

->andWhere($qb->expr()->eq('t.Schools.schoolId', ':school_id')) // doesn't work

To:

->andWhere($qb->expr()->eq('t.Schools', ':school_id'))

However the same approach doesn't work with the following query:

$qb->select('DISTINCT t.age, s.Schools') // Doesn't work
    ->from('\Entities\Schools', 't')
    ->innerJoin('t.Students', 's')

The following error is displayed:

[Semantical Error] line 0, col 28 near 'Schools FROM': Error: Invalid PathExpression. Must be a StateFieldPathExpression.