How to Write Data to Custom Database Table and Create a Custom Collection in Magento 2

Block-Data.php

<?php

namespace DaanvdB\ProductQty\Block;

use Magento\Framework\View\Element\Template;
use DaanvdB\ProductQty\Model\DataFactory;

class Data extends Template {
	protected $dataFactory;

	public function __construct(
		Template\Context $context,
		DataFactory $dataFactory
	) {
		$this->dataFactory = $dataFactory;
		parent::__construct(
			$context
		);
	}

	public function showData() {
		$collection = $this->dataFactory->create()->getCollection();

		$data = $collection->getData();

		return $data;
	}
}

Model-Data.php

<?php

namespace DaanvdB\ProductQty\Model;

use Magento\Framework\Model\AbstractModel;

class Data extends AbstractModel {
	public function _construct() {
		$this->_init( "\DaanvdB\ProductQty\Model\ResourceModel\Data" );
	}
}

Model-Import.php

<?php

namespace DaanvdB\ProductQty\Model;

use DaanvdB\ProductQty\Model\ResourceModel\Data;

class Import {

	protected $resourceData;

	public function __construct(
		Data $resourceData
	) {
		$this->resourceData = $resourceData;
	}

	public function importData() {
		$connection = $this->resourceData->getConnection();

		$data = [
			'test_sku_1' => 10,
			'test_sku_2' => 20,
			'test_sku_3' => 15
		];

		try {
			$connection->beginTransaction();
			$connection->insertMultiple( $this->resourceData->getTable( 'daanvdb_product_qty' ), $data );
			$connection->commit();
		} catch ( \Exception $e ) {
			return $e;
		}

		return true;
	}
}

Model-ResourceModel-Data-Collection.php

<?php

namespace DaanvdB\ProductQty\Model\ResourceModel\Data;

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

class Collection extends AbstractCollection {
	public function _construct() {
		$this->_init( "DaanvdB\ProductQty\Model\Data", "DaanvdB\ProductQty\Model\ResourceModel\Data" );
	}
}

Model-ResourceModel-Data.php

<?php

namespace DaanvdB\ProductQty\Model\ResourceModel;

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

class Data extends AbstractDb
{
	public function _construct()
	{
		$this->_init('daanvdb_product_qty', 'sku');
	}
}

Setup-UpgradeSchema.php

<?php

namespace DaanvdB\ProductQty\Setup;

use Magento\Framework\Setup\UpgradeSchemaInterface;
use Magento\Framework\Setup\SchemaSetupInterface;
use Magento\Framework\Setup\ModuleContextInterface;

class UpgradeSchema implements UpgradeSchemaInterface {
	public function upgrade( SchemaSetupInterface $setup, ModuleContextInterface $context ) {
		$setup->startSetup();

// Create table and columns.
		$table = $setup->getConnection()->newTable(
			$setup->getTable( 'daanvdb_product_qty' )
		)->addColumn(
			'sku',
			\Magento\Framework\DB\Ddl\Table::TYPE_TEXT,
			null,
			[
				'nullable' => false,
				'primary'  => false
			],
			'Product SKU'
		)->addColumn(
			'qty',
			\Magento\Framework\DB\Ddl\Table::TYPE_INTEGER,
			null,
			[
				'nullable' => false,
				'primary'  => false
			]
		);

// UPDATE OCT 4TH, 2018: And if you want to add a Foreign Key Constraint
		$setup->addForeignKey(
			$setup->getFkName( 'daanvdb_product_qty', 'quote_id', 'other_table', 'other_column' ),
			'quote_id',
			$setup->getTable( 'other_table' ),
			'other_column',
			\Magento\Framework\DB\Ddl\Table::ACTION_CASCADE //onDelete
		);

		$setup->getConnection()->createTable( $table );

// And if you want to add a column to an existing table...

		if ( version_compare( $context->getVersion(), '2.0' < 0 ) ) {
			$table = $setup->getTable( 'daanvdb_product_qty' );

			$setup->getConnection()->addColumn(
				$table,
				'custom_column',
				[
					'type'     => \Magento\Framework\DB\Ddl\Table::TYPE_BOOLEAN,
					'nullable' => false,
					'comment'  => 'Custom Column'
				]
			);
		}

		$setup->endSetup();
	}
}