Adding or editing a price list item should invalidate the cache of the product view

+  /**
+   * {@inheritdoc}
+   */
+  public function save() {
+    // Make sure we invalidate the cache for the variation in question.
+    $variation = $this->getPurchasableEntity();
+    if (!$variation) {
+      // We could not find any entity to collect cache tags from, which means we
+      // can just proceed with the regular saving.
+      return parent::save();
+    }
+    $tags = $variation->getCacheTags();
+    // And if we can resolve the product of it, also invalidate that. Since the
+    // purchasable entity theoretically can be any purchasable entity, we have
+    // to make sure we are actually able to find a product from the variation.
+    if (method_exists($variation, 'getProduct')) {
+      $product = $variation->getProduct();
+      if ($product) {
+        $tags = array_merge($tags, $product->getCacheTags());
+      }
+    }
+    Cache::invalidateTags($tags);
+    return parent::save();
+  }

1. Drupal core tends to invalidate caches in postSave(). Any reason why we can't do the same? Would simplify this code.
2. Instead of the method_exists() call could we check $purchasable_entity->getEntityTypeId()? Seems cleaner.
3. As a Commerce followup it would probably make sense to have $variation->getCacheTags() start returning the product cache tags as well, removing the need for this workaround here.

As a Commerce followup it would probably make sense to have $variation->getCacheTags() start returning the product cache tags as well, removing the need for this workaround here.

  /**
   * {@inheritdoc}
   */
  public function getCacheTagsToInvalidate() {
    $tags = parent::getCacheTagsToInvalidate();
    // Invalidate the variations view builder and product caches.
    return Cache::mergeTags($tags, [
      'commerce_product:' . $this->getProductId(),
      'commerce_product_variation_view',
    ]);
  }

https://git.drupalcode.org/project/commerce/blob/8.x-2.x/modules/product...

We should assert getPurchasableEntity is an instance of \Drupal\commerce\PurchasableEntityInterface and then we can get cache tags.

That postSave can be a lot simpler, by calling the parent only once, on top of the method. And we should keep the variable as $purchasable_entity, there's no need to assume it's a variation.

We can do something like this:

  public function postSave(EntityStorageInterface $storage, $update = TRUE) {
    parent::postSave($storage, $update);

    if ($purchasable_entity = $this->getPurchasableEntity()) {
      Cache::invalidateTags($purchasable_entity->getCacheTagsToInvalidate());
    }
  }

PriceListProductCacheTest can be just ProductCacheTest.

+  /**
+   * A test variation.
+   *
+   * @var \Drupal\commerce_product\Entity\ProductVariationInterface
+   */
+  protected $firstVariation;

Should be just $variation, since we don't have two.

+  /**
+   * Tests creating a price list item.
+   */
+  public function testCreatePriceListItem() {

None of this is true :)

Addressed #17 and committed. Thanks!