I am using Magento 2.3.0

I have migrated products from Magento 1 to Magento 2. I can see related products in product detail page in the front end and it is proper and also I can find related products entry in the database as well but when I checked related products in admin I found that there are some related products missing in some of the products

For Ex. When I checked one product in the front end. I saw three related products there as well as in the database but I can find only 2 products in admin there is one product missing in admin.

I checked two tables in database for related products: "catalog_product_link" and "catalog_product_link_attribute_int".

I have found some values in the "value" column in "catalog_product_link_attribute_int" table which represents the position of the related products in admin and in front end but I found value 0 in "value" column of this table for some of the products and when I checked these products I found that only those products which do have only 0 in "value" column in "catalog_product_link_attribute_int" table are missing related products in admin.

So, How can I solve this issue?

Check my below question as well:

How to override product model files in Magento 2?

I have created a new module to solve the related product issue which I mentioned in this question but I got an error.

Why it happens

This happens when there are inconsistent positions for the related products. This might happen after a migration from magento 1. It was the case for me also.

The problem is cause by the method MagentoCatalogModelProductLinkCollectionProvider::getCollection().

That's the one responsible for retrieving the products.

The part that does not behave as expected is this:

 foreach ($output as $item) 
 $itemPosition = $item['position'];
 if (!isset($sorterItems[$itemPosition])) 
 $sorterItems[$itemPosition] = $item;
 else 
 $newPosition = $itemPosition + 1;
 $sorterItems[$newPosition] = $item;
 
 

Let's take 2 cases.

THe one that works:

Let's say

$output = [
 ['position' => 0],
 ['position' => 2],
 ['position' => 1],

];

(I ignored the other values for the related items as they are not important).

After running the code above this results in

Array
(
 [0] => Array
 (
 [position] => 0
 )

 [2] => Array
 (
 [position] => 2
 )

 [1] => Array
 (
 [position] => 1
 )

)

which is later sorted by the ksort function.

So 3 items on input, 3 on output. So far so good.

But let's take this case:

$output = [
 ['position' => 0], // or null
 ['position' => 1],
 ['position' => 0], // or null
];

The same piece of code returns:

Array
(
 [0] => Array
 (
 [position] => 0
 )

 [1] => Array
 (
 [position] => 0
 )

)

So 3 items in, 2 out.

Solution

I solve this by rewriting the method mentioned above and ensuring there are no position clashes.

To override it, add this in the di.xml file in one of your custom modules

<preference for="MagentoCatalogModelProductLinkCollectionProvider" type="[Vendor][Module]ModelProductLinkCollectionProvider" />

Then create the class [Vendor][Module]ModelProductLinkCollectionProvider in the file [Vendor]/[Module]/Model/ProductLink/CollectionProvider.php

The class should look like this:

<?php
namespace [Vendor][Module]ModelProductLink;

use MagentoFrameworkExceptionNoSuchEntityException;

class CollectionProvider extends MagentoCatalogModelProductLinkCollectionProvider

 /**
 * Get product collection by link type
 *
 * @param MagentoCatalogModelProduct $product
 * @param string $type
 * @return array
 * @throws NoSuchEntityException
 */
 public function getCollection(MagentoCatalogModelProduct $product, $type)
 
 if (!isset($this->providers[$type])) 
 throw new NoSuchEntityException(__('Collection provider is not registered'));
 
 $products = $this->providers[$type]->getLinkedProducts($product);
 $converter = $this->converterPool->getConverter($type);
 $output = [];
 $sorterItems = [];
 foreach ($products as $item) 
 $output[$item->getId()] = $converter->convert($item);
 
 foreach ($output as $item) 
 $itemPosition = (int)$item['position'];
 while (isset($sorterItems[$itemPosition])) 
 $itemPosition++;
 
 $sorterItems[$itemPosition] = $item;
 
 ksort($sorterItems);
 return $sorterItems;