On delete: cascade不起作用

我创建了两个表:

Schema::create('educations', function(Blueprint $table){
    $table->increments('id');
    $table->string('title');
    $table->date('from_date');
    $table->date('to_date');
    $table->string('summary');
    $table->integer('cv_id')->unsigned();
    $table->foreign('cv_id')->references('id')
    ->on('cvs')
    ->onDelete('cascade')
    ->onUpdate('cascade');
    $table->timestamps();
});

它们链接到另一个表:

Schema::create('cvs', function(Blueprint $table){
    $table->increments('id');
    $table->integer('student_id')->unsigned();
    $table->foreign('student_id')->references('id')
    ->on('students')
    ->onDelete('cascade')
    ->onUpdate('cascade');
    $table->string('summary');
    $table->timestamps();
});

问题是，每当我调用这一行时:

$student->cv->delete();

它将删除cv，但不会删除与它相关的表中的其他属性。