Conditional relations with doctrine Symfony

I have need to create this database scheme in Symfony Entities :

ClassA:

• id
• name
• ... some other attributes only for class A

ClassB:

• id
• name
• ... some other attributes only for class B

ClassC:

• id
• name
• ... some other attributes only for class C

Course:

• id
• name
• type
• class_id
• class_type [A, B, C]
• ...

I need to create relations between Course Entity (using the class_id) with the other entities (ClassA, ClassB, ClassC) (using the id) and using the class_type (A, B, C).

A Course is not a class and a class is not a course. There is no inheritance here. In my project I'm using this concept (id and type to map different entities). So I need to solve this problem with a simple example like this one

I thought about using @ORM\DiscriminationMap here:

/**
 * Course
 *
 * @ORM\Table(name="Course")
 * @ORM\InheritanceType("SINGLE_TABLE")
 * @ORM\DiscriminatorColumn(name="course_type", type="string")
 * @ORM\DiscriminatorMap({"A" = "ClassA", "B" = "ClassB", "C" = "ClassC"})
 */
abstract class Course
{
 //...
}

class ClassA extends Course
{
 //...
}

class ClassB extends Course
{
 //...
}

class ClassC extends Course
{
 //...
}

Or something like this.

But I don't want to copy the Course's attributes in the ClassA, ClassB, ClassC entities because "extends" will copy the parent entity's attributes to the child.

It's not about inheritence. I need to keep the database scheme described above without adding any additional attributes from the other entities.

Thank you