WooCommerce Subscription - Allow only one active subscription

Actually a really common case, I thought. But out-of-the-box WooCommerce Subscription does not support this feature of having only one active subscription. So we need to do it manually. I hope you have some experience with WooCommerce and programming. If not maybe you ask a friend to assist you.

So we will now add something to the functions.php of the currently active theme, which should be a child-theme. (If not read about child-themes on Wordpress first).

add_filter('woocommerce_add_to_cart_validation', 'check_num_of_subscriptions', 10, 2);

function check_num_of_subscriptions($valid, $product_id)
{
    $product_to_add = wc_get_product($product_id);
    
    if ($product_to_add instanceof WC_Product_Subscription || $product_to_add instanceof WC_Product_Variable_Subscription) {
        
        // alternative use: $has_sub = wcs_user_has_subscription( '', '', 'active' );
        if (has_active_subscription()) {
            // cart validation must fail, because user is not allowed to have multiple subscriptions
            return false;
        } else {
            return true;
        }
    }
    return $valid;
}

function has_active_subscription()
{
    
    $user_id = get_current_user_id();
    
    $active_subscriptions = get_posts(array(
        'numberposts' => -1,
        'meta_key' => '_customer_user',
        'meta_value' => $user_id,
        'post_type' => 'shop_subscription',
        'post_status' => 'wc-active'
        
    ));
    if (!empty($active_subscriptions))
        return true;
    else
        return false;
}

Ok what does this ?

Okay, so add_filter is a given function that allows us to inject code into the Wordpress system. As you can see it has 4 Parameter. Interesting for us are the first two. The first one is the hookname, in this case woocommerce_add_to_cart_validation. This hook fires and event every time the user wants to add a certain product to the shopping cart. We can register to a hook and when this event is fired the function that we set as the second parameter is called. So check_num_of_subscriptions(name is given by us).

Depending on the hookname our function will get different parameters. This you can find in the documentation of the Plugin, WooCommerce or Wordpress. Here we get $valid and $product_id. So with this product id we check if the product that the user wants to add is a subscription product. If it is a regular product we don't care, but if its a subscription product we now need to check if this user already has active subscriptions.

To do this I encapsulated this functionality in a new function - has_active_subscription(). We check if the user has active subscriptions with the given function get_posts(..) and we filter for user_id, shop_subscription and wc-active. Thats it we return the result and its done.

However you might want to add a notice to the user that he can understand why the product could not be added to the shopping cart. You want to add this directly above the like with the return false;

so:

wc_add_notice( 'You can have only one active subscription.', 'notice' );
    return false;