How i can work with extension attributes using collection?

[VitaliyMain]

Hello i have to add extension attributes to 

Magento\Catalog\Api\Data\ProductInterface

So i added this one to extension_attrributes.xml:

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
  <extension_attributes for="Magento\Catalog\Api\Data\ProductInterface">
    <attribute code="custom_attribute" type="int"/>
  </extension_attributes>
</config>

And to di.xml added:

<type name="Magento\Framework\EntityManager\Operation\ExtensionPool">
        <arguments>
            <argument name="extensionActions" xsi:type="array">
                <item name="Magento\Catalog\Api\Data\ProductInterface" xsi:type="array">
                    <item name="read" xsi:type="array">
                        <item name="customAttributeReader" xsi:type="string">Vendor\Custom\Model\Product\CustomAttribute\ReadHandler</item>
                    </item>
                    <item name="create" xsi:type="array">
                        <item name="customAttributeCreator" xsi:type="string">Vendor\Custom\Model\Product\CustomAttribute\SaveHandler</item>

                    </item>
                    <item name="update" xsi:type="array">
                        <item name="customAttributeSaver" xsi:type="string">Vendor\Custom\Model\Product\CustomAttribute\SaveHandler</item>
                    </item>
                </item>
            </argument>
        </arguments>
    </type>

So it`s work fine. When i get product entity using ProductRepository or load() function in Product model, all my extension attributes loads successful. But i have some issue when i work with Collection:

When i do something like:

$productCollection->addFieldToFilter('custom_attribute', 2);

My code don't wok and it's expected because my model not contains this attribute.

So i have a question. How i can filter by my extension attribute using collection ?

Also if i do:

$productCollection->getFirstItem()->getExtensionAttributes()

I get empty array.

So i need your help. I'll be very grateful!!!

[Patel_Chirag]

  @VitaliyMain 

I am not sure this is fit for your requirement. but i think you have to go through this.

The system uses a join directive to add external attributes to a collection and to make the collection filterable. The join element in the extension_attributes.xml file defines which object fields and the database table/column to use as the source of a search.

In the following example, an attribute named stock_item of type Magento\CatalogInventory\Api\Data\StockItemInterface is being added to the Magento\Catalog\Api\Data\ProductInterface.

<extension_attributes for="Magento\Catalog\Api\Data\ProductInterface">
    <attribute code="stock_item" type="Magento\CatalogInventory\Api\Data\StockItemInterface">
        <join reference_table="cataloginventory_stock_item" reference_field="product_id" join_on_field="entity_id">
            <field>qty</field>
        </join>
    </attribute>
</extension_attributes>

When getList() is called, it returns a list of ProductInterfaces. When it does this, the code populates the stock_item with a joined operation in which the StockItemInterface’s qty property comes from the cataloginventory_stock_item table where the Product’s entity_Id is joined with the cataloginventory_stock_item.product_id column.  

See the detail documentation

I hope it helps!

[VitaliyMain]

@Patel_Chirag Thank for your answer. 

Using getList() of repository is well and work pretty nice. But i have to get list of ProductInterfaces via Magento\Catalog\Model\ResourceModel\Product\Collection. 

So when i do: 

$productCollection->getItems();

I get products items  but my extension attributes are empty. 

So how i can populate them when i work with collection?

[ankit811824]

Did you get a solution for above problem? I also want to know if magento is providing anything to achieve this otherwise I think we need to use plugin for that.