<?php
 namespace MyModuleModelSave;
 use MyModuleModelResourceModelSaveCollectionFactory;
 class DataProvider extends MagentoUiDataProviderAbstractDataProvider
{
/**
 * @param string $name
 * @param string $primaryFieldName
 * @param string $requestFieldName
 * @param CollectionFactory $contactCollectionFactory
 * @param array $meta
 * @param array $data
 */

public function __construct(
    $name,
    $primaryFieldName,
    $requestFieldName,
    CollectionFactory $contactCollectionFactory,
    array $meta = [],
    array $data = []
) {
    $this->collection = $contactCollectionFactory->create();

    parent::__construct($name, $primaryFieldName, $requestFieldName, $meta, $data);
}

public function getData()
{
    if (isset($this->loadedData)) {
        return $this->loadedData;
    }

    $items = $this->collection->getItems();
    $this->loadedData = array();

    /** @var Customer $customer */
    foreach ($items as $contact) {
        // notre fieldset s'apelle "contact" d'ou ce tableau pour que magento puisse retrouver ses datas :
        $this->loadedData[$contact->getId()]['contact'] = $contact->getData();
    }


    return $this->loadedData;

}
}

<?php

 namespace MyModuleModelResourceModelSave;

 use MagentoFrameworkModelResourceModelDbCollectionAbstractCollection;

 class Collection extends AbstractCollection
{
public function _construct()
{
    $this->_init('MyModuleModelSave', 'MyModuleModelResourceModelSave');

}
}

public function _initSelect()
     {
        parent::_initSelect();

        $this->getSelect()->joinLeft(
        ['secondTable' => $this->getTable('SECOND_TABLE_NAME')],
        'main_table.COULMN_NAME_TO_JOIN_TABLE1 = secondTable.COULMN_NAME_TO_JOIN_TABLE2',
        ['COLUMN_U_WANT_TO_ADD'=>'COLUMN_NAME_OF_NEW_COLUMN']
        );
        $this->addFilterToMap('COLUMN_U_WANT_TO_ADD', 'secondTable.COLUMN_NAME_OF_NEW_COLUMN');
     }