Magento – SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails

Well I think your issue is due to the products which are creating hindrance in the Index process.

You have to run the below queries for re-indexing to work:

delete FROM `catalog_product_entity_datetime` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_decimal` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_gallery` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_group_price` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_int` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_media_gallery` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_text` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_tier_price` where entity_id not in (select entity_id from catalog_product_entity);
delete FROM `catalog_product_entity_varchar` where entity_id not in (select entity_id from catalog_product_entity);
#originals with formatting fixed:
delete from `catalog_category_product` WHERE product_id not in(select entity_id from catalog_product_entity);
delete from `catalog_category_product` WHERE category_id not in(select entity_id from catalog_category_entity); 
delete from `catalog_product_website` WHERE product_id not in(select entity_id from catalog_product_entity); 
delete from `catalog_product_index_eav_idx` WHERE entity_id not in(select entity_id from catalog_product_entity); 
delete from `catalog_product_index_eav` WHERE entity_id not in(select entity_id from catalog_product_entity); 
delete from `catalog_product_link` WHERE product_id not in(select entity_id from catalog_product_entity); 
delete from `catalog_product_relation` WHERE parent_id not in(select entity_id from catalog_product_entity);

Hope this helps you out

I ran into the same problem when i tried to change data on quote items with the checkout_cart_product_add_after event. This event is fired after every product add, but before the cart is saved. Check the Magento\Checkout\Controller\Cart\Add line 108-113 for reference. So if you want to save the quote item on a new session there is no quote yet and there is no quote_id to reference to.

In my case, i just left the $item->save(); out, because it will be saved afterwards automatically.