Webhook: Payment Failed!

Hey guys,

maybe you can help me :) At the moment I do setup my webhooks, and I'm following this lecture.

I understand the logic that we query only for invoices where actually a subscription was involved.

But why do we look for $stripeEvent->data->object->subscription?

In Stripe I created a test webhook and it returned at this field a value of null. Contrary, it returned within the invoice items array a subscription item.

This is just an excerpt of the line item's array

      lines: {                                  // The individual line items that make up the invoice. lines is sorted as follows: invoice items in reverse chronological order, followed by the subscription, if any.
        data: [
          {
            id: "sli_00000000000000",             // Unique identifier for the object.
            object: "line_item",                  // String representing the object’s type. Objects of the same type share the same value.
            amount: 2000,                         // The amount, in cents.
            currency: "eur",                      // Three-letter ISO currency code, in lowercase. Must be a supported currency.
            description: "1 × Gold Special (at €20.00 / month)", // An arbitrary string attached to the object. Often useful for displaying to users.
            discountable: true,                   // If true, discounts will apply to this line item. Always false for prorations.
            livemode: false,                      // Whether this is a test line item.
            metadata: {
            },
            period: {                             // The timespan covered by this invoice item.
              end: 1541424401,                    // End of the line item’s billing period
              start: 1538746001                   // Start of the line item’s billing period
            },
plan: {// https://stripe.com/docs/api#plan_object  // The plan of the subscription, if the line item is a subscription or a proration.
              id: "gold_00000000000000",          // Unique identifier for the object.
              object: "plan",                     // String representing the object’s type. Objects of the same type share the same value.
              active: true,                       // Whether the plan is currently available for new subscriptions.
              aggregate_usage: null,
              amount: 2000,                       // The amount in cents to be charged on the interval specified.
              billing_scheme: "per_unit",
              created: 1394752996,                // Time at which the object was created. Measured in seconds since the Unix epoch.
              currency: "eur",                    // Three-letter ISO currency code, in lowercase. Must be a supported currency.
              interval: "month",                  // One of day, week, month or year. The frequency with which a subscription should be billed.
              interval_count: 1,                  // The number of intervals (specified in the interval property) between subscription billings. For example, interval=month and interval_count=3 bills every 3 months.
              livemode: false,                    // Has the value true if the object exists in live mode or the value false if the object exists in test mode.
              metadata: {
              },
              nickname: null,                     // A brief description of the plan, hidden from customers.
              product: "prod_00000000000000",     // The product whose pricing this plan determines.
              tiers: null,
              tiers_mode: null,                   // Defines if the tiering price should be graduated or volume based. In volume-based tiering, the maximum quantity within a period determines the per unit price, in graduated tiering pricing can successively change as the quantity grows.
              transform_usage: null,
              trial_period_days: null,            // Default number of trial days when subscribing a customer to this plan using trial_from_plan=true.
              usage_type: "licensed"
            },

And further below in the invoice event:

      subscription: null,
      subtotal: 0,                                // Total of all subscriptions, invoice items, and prorations on the invoice before any discount is applied.
      tax: null,                                  // The amount of tax included in the total, calculated from tax_percent and the subtotal. If no tax_percent is defined, this value will be null.
      tax_percent: null,                          // This percentage of the subtotal has been added to the total amount of the invoice, including invoice line items and discounts. This field is inherited from the subscription’s tax_percent field, but can be changed before the invoice is paid. This field defaults to null.
      total: 0,                                   // Total after discount.
      webhooks_delivered_at: null                 // The time at which webhooks for this invoice were successfully delivered (if the invoice had no webhooks to deliver, this will match date). Invoice payment is delayed until webhooks are delivered, or until all webhook delivery attempts have been exhausted.
    }
  }

When I look into the API https://stripe.com/docs/api#invoice_object I don't understand the field description of event.data.object.subscription: 'The subscription that this invoice was prepared for, if any.'

According to our setup the subscription filter would not trigger, although within the line Items there is a subscription.