WooCommerce Subscriptions - Get related orders Ids

Is there woocommerce function that will return me all related orders (at least the order ID) for the specific subscription that user has bough?

I have found in this official documentation Subscription Function & Property Reference:

WC_Subscription::get_related_orders( $return_fields, $order_type );

But this does not seem to be for specific subscription?

When ever I try to run it I get a fatal error no mater what I pass in:

Fatal error: Uncaught Error: Using $this when not in object context in C:\xampp\htdocs\mysite.com\wp-content\plugins\woocommerce-subscriptions\includes\class-wc-subscription.php:1413

I am making my own plugin and I select all subscriptions where post status = wc-active from post table. I have looked in the "woocommerce_order_items", "woocommerce_order_itemmeta" and "postmeta" tables but neither of them provide way to get related orders for user bought subscription...

If I only knew where are the relation for user bought subscriptions and its related orders, then i could write some sql but I have no idea and google does not yield any results either.

Any ideas?

My setup:

php version 7.0.4
wordpress version 4.7.3
woocommerce 2.6.8
woocommerce subscriptions: 2.0.18

Updated: Added WooCommerce version 3+ Compatibility

It's very easy to get the order ID from subscription object. I am going to select, just like you, all subscriptions where 'post status' = 'wc-active' from post table.

// Get all customers subscriptions
$customer_subscriptions = get_posts( array(
    'numberposts' => -1,
    // 'meta_key'    => '_customer_user',
    // 'meta_value'  => get_current_user_id(), // Or $user_id
    'post_type'   => 'shop_subscription', // WC orders post type
    'post_status' => 'wc-active' // Only orders with status "completed"
) );

// Iterating through each post subscription object
foreach( $customer_subscriptions as $customer_subscription ){
    // The subscription ID
    $subscription_id = $customer_subscription->ID

    // IMPORTANT HERE: Get an instance of the WC_Subscription Object
    $subscription = new WC_Subscription( $subscription_id );
    // Or also you can use
    // wc_get_order( $subscription_id ); 

    // Getting the related Order ID (added WC 3+ comaptibility)
    $order_id = method_exists( $subscription, 'get_parent_id' ) ? $subscription->get_parent_id() : $subscription->order->id;

    // Getting an instance of the related WC_Order Object (added WC 3+ comaptibility)
    $order = method_exists( $subscription, 'get_parent' ) ? $subscription->get_parent() : $subscription->order;

    // Optional (uncomment below): Displaying the WC_Subscription object raw data
    // echo '<pre>';print_r($subscription);echo '</pre>';
}

You can also uncomment in the post query the 'meta_key' and 'meta_value' array lines to get the subscriptions for one customer… This code is tested and works

The most important thing here is:
$subscription = new WC_Subscription($customer_subscription->ID);
…as you will get the WC_Subscription object in which you can apply all WC_Subscription methods without getting errors, with for example:
$subscription = new WC_Subscription($post_id);
$relared_orders_ids_array = $subscription->get_related_orders();