Special Products Collection

I’ve been trying to get a special price collection of products for Magento 2 but I keep on failing.

I’ve got the following example that works(almost):

    protected function _getSaleCategoryCollection()
    {
        if ($this->_productCollection === null) {
            /** @var MagentoCatalogModelResourceModelProductCollection $collection */
            $collection = $this->_productCollectionFactory->create();
            $this->_catalogLayer->prepareProductCollection($collection);

            $todayStart = $this->_localeDate->date()->setTime(0, 0, 0)->format('Y-m-d H:i:s');
            $todayEnd = $this->_localeDate->date()->setTime(23, 59, 59)->format('Y-m-d H:i:s');

            $collection
                ->addStoreFilter()
                ->addAttributeToFilter(
                    'special_from_date',
                    [
                        'or' => [
                            ['date' => true, 'to' => $todayEnd],
                            ['is' => new Zend_Db_Expr('null')]
                        ]
                    ],
                    'left'
                )->addAttributeToFilter(
                    'special_to_date',
                    [
                        'or' => [
                            ['date' => true, 'from' => $todayStart],
                            ['is' => new Zend_Db_Expr('null')]
                        ]
                    ],
                    'left'
                )->addAttributeToFilter(
                    [
                        ['attribute' => 'special_price', 'is' => new Zend_Db_Expr('not null')]
                    ]
                );





            $this->_productCollection = $collection;
        }

        return $this->_productCollection;
    }
}

Alright, so the above example shows me all the products that has the special price ‘value’ not empty and also if the product is between the 2 dates.

The problem comes when I have a product where the special price is not empty, but equal to the normal price(therefore the product does not have a special price).

So in conclusion I need to add an ‘and’ condition to the above collection so the final price < smaller price

I’ve added following after researching different forums:

...
 )->addAttributeToFilter(
                    [
                        ['attribute' => 'special_price', 'is' => new Zend_Db_Expr('not null')]
                    ]
                );


            $collection->getSelect()->where(
                    'price_index.final_price < price_index.price'
            );


            $this->_productCollection = $collection;
....

After adding that getSelect->where() , I’m only getting 4 simple products so I’m thinking this is not the right approach for me.
Also, I’m using elasticsearch(I don’t know if relevant).

I’d appreciate if anyone has an idea.
Thank you!