Answers for "how prevent deleting foreign key laravel"

larael drop foreign key

Schema::table('posts', function (Blueprint $table) {
	$table->dropForeign(['category_id']);
});

remove foreign key constraint laravel

Schema::table('table_name', function (Blueprint $table) {
    $table->dropForeign(['foreign_key']);
    $table->dropColumn('column_key');
});

PS: usually foreign_key = column_key

ex: 

Schema::table('despatch_discrepancies', function (Blueprint $table) {
    $table->dropForeign(['pick_detail_id']);
    $table->dropColumn('pick_detail_id');
});