Magento – the best practice way in Magento 2 for creating Many To Many relationships

I've looked around the core and seen a few examples of many to many relationships between models, but I can't see a definitive answer on this.

As an example, say we create a new model and we want to have a many to many relationship with existing products table.

So we have our new Model – Stockist, and we create 2 tables as such, one to store the Stockist name, the other to store the many to many relationship with products.

Truncated version of setup classes:

$table = $setup->getConnection()
        ->newTable($installer->getTable('stockist'))
        ->addColumn('stockist_id',
            \Magento\Framework\DB\Ddl\Table::TYPE_INTEGER,
            null,
            ['identity' => true, 'unsigned' => true, 'nullable' => false, 'primary' => true],
            'Stockist Id')
        ->addColumn('name',
            \Magento\Framework\DB\Ddl\Table::TYPE_TEXT,
            null,
            ['nullable' => false],
            'Stockist Name');

 $table = $installer->getConnection()
            ->newTable($installer->getTable('stockist_product'))
            ->addColumn(
                'entity_id',
                \Magento\Framework\DB\Ddl\Table::TYPE_INTEGER,
                null,
                ['identity' => true, 'nullable' => false, 'primary' => true],
                'Entity ID'
            )
            ->addColumn(
                'stockist_id',
                \Magento\Framework\DB\Ddl\Table::TYPE_INTEGER,
                null,
                ['unsigned' => true, 'nullable' => false, 'primary' => true, 'default' => '0'],
                'Stockist ID'
            )
            ->addColumn(
                'product_id',
                \Magento\Framework\DB\Ddl\Table::TYPE_INTEGER,
                null,
                ['unsigned' => true, 'nullable' => false, 'primary' => true, 'default' => '0'],
                'Product ID'
            )
            ->addIndex(
                $installer->getIdxName('stockist_product', ['product_id']),
                ['product_id']
            )
            ->addIndex(
                $installer->getIdxName(
                    'stockist_product,
                    ['stockist_id', 'product_id'],
                    \Magento\Framework\DB\Adapter\AdapterInterface::INDEX_TYPE_UNIQUE
                ),
                ['stockist_id', 'product_id'],
                ['type' => \Magento\Framework\DB\Adapter\AdapterInterface::INDEX_TYPE_UNIQUE]
            )
            ->addForeignKey(
                $installer->getFkName('stockist_product', 'product_id', 'catalog_product_entity', 'entity_id'),
                'product_id',
                $installer->getTable('catalog_product_entity'),
                'entity_id',
                \Magento\Framework\DB\Ddl\Table::ACTION_CASCADE
            )
            ->addForeignKey(
                $installer->getFkName('stockist_product', 'stockist_id', 'stockist', 'stockist_id'),
                'stockist_id',
                $installer->getTable('stockist'),
                'stockist_id',
                \Magento\Framework\DB\Ddl\Table::ACTION_CASCADE
            )
            ->setComment('Stockist to Product Many to Many');

Then we create a standard Model/ResourceModel/Collection for Stockist as so:

namespace OurModule\Stockist\Model;

use Magento\Framework\Model\AbstractModel;

class Stockist extends AbstractModel
{

    protected function _construct()
    {
        $this->_init('OurModule\Stockist\Model\ResourceModel\Stockist');
    }

}

namespace OurModule\Stockist\Model\ResourceModel;

use Magento\Framework\Model\ResourceModel\Db\AbstractDb;

class Stockist extends AbstractDb
{

    protected function _construct()
    {
        $this->_init('stockist', 'stockist_id');
    }

}

namespace OurModule\Stockist\Model\ResourceModel\Stockist;

use Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection;

class Collection extends AbstractCollection
{

    public function _construct()
    {
        $this->_init('OurModule\Stockist\Model\Stockist', 'OurModule\Stockist\Model\ResourceModel\Stockist');
    }

}

This is where we come to how to handle the table with the many to many relationship. So far I've come up with something along the lines of this.

Create a model to represent StockistProduct

namespace OurModule\Stockist\Model;

use Magento\Framework\Model\AbstractModel;

class StockistProduct extends AbstractModel
{

protected function _construct()
{
    $this->_init('OurModule\Stockist\Model\ResourceModel\StockistProduct');
}

/**
 * @param array $productIds
 */
public function getStockists($productIds)
{
    return $this->_getResource()->getStockists($productIds);
}

/**
 * @param array $stockistIds
 */
public function getProducts($stockistIds)
{
    return $this->_getResource()->getProducts($stockistIds);
}
}

Here defining 2 methods that will take in either an array of stockist Ids, returning an array of matching Product Ids and vice-versa.

This uses a Resource Model for the stockist_product table containing the many to many relationship:

/**
 * Class StockistProduct
 */
class StockistProduct extends AbstractDb
{
    /**
     * Model initialization
     *
     * @return void
     */
    protected function _construct()
    {
        $this->_init('stockist_product', 'entity_id');
    }

    /**
     * Retrieve product stockist Ids
     *
     * @param array $productIds
     * @return array
     */
    public function getStockists(array $productIds)
    {
        $select = $this->getConnection()->select()->from(
            $this->getMainTable(),
            ['product_id', 'stockist_id']
        )->where(
            'product_id IN (?)',
            $productIds
        );
        $rowset = $this->getConnection()->fetchAll($select);

        $result = [];
        foreach ($rowset as $row) {
            $result[$row['product_id']][] = $row['stockist_id'];
        }

        return $result;
    }


    /**
     * Retrieve stockist product Ids
     *
     * @param array $stockistIds
     * @return array
     */
    public function getProducts(array $stockistIds)
    {
        $select = $this->getConnection()->select()->from(
            $this->getMainTable(),
            ['product_id', 'stockist_id']
        )->where(
            'stockist_id IN (?)',
            $stockistIds
        );
        $rowset = $this->getConnection()->fetchAll($select);

        $result = [];
        foreach ($rowset as $row) {
            $result[$row['product_id']][] = $row['stockist_id'];
        }

        return $result;
    }
}

Then using this StockistProduct model when you need to retrieve a set of either model as so, assuming we have a Product Model in $product, and $stockistProduct is an instance of \OurModule\Stockist\Model\StockistProduct

$stockists = $stockistProduct->getStockists([$product->getId()]);

We can then create each model in turn by looping the list of Ids returned as so, where $stockistFactory is an instance of \OurModule\Stockist\Model\StockistFactory

$stockist = $this->stockistFactory->create();
$stockist->load($stockistId);

This all works fine, and is based on some similar code within the Core of Magento 2, but I can't help but wonder if there is a better way?