How to Get Distinct Entries from Joined Tables Through Join()

Question

This doubt aroused when I am answering one of the question that posted here. But this question is not a continuation of that question, rather a general doubt.

Basically I need to get log-customer collection. I can't get it directly, since there is no collection defined for log customers. ie

Mage::getModel('log/customer')->getCollection(); //false

However I can get the above collection indirect way. I need to use log-visitor collection for this and then using this collection I can get log-customer collection. This is the code that do that trick.

$collection = Mage::getModel('log/visitor')->getCollection()->addFieldToFilter('store_id', array('eq' => 1));
$collection->getSelect()
            ->join( 
                array('log_customer'=> $collection->getTable('log/customer')),
                'main_table.`visitor_id`= log_customer.`visitor_id`'
            );
foreach ($collection->getItems() as  $item) {
    print_r($item->getData());
}

This code will do the trick if everything came right. But there is a small problem related to this code. In order to show what is the problem, let me show you log_customer table.

+--------+------------+-------------+---------------------+---------------------+----------+
| log_id | `visitor_id` | customer_id | login_at            | logout_at           | store_id |
+--------+------------+-------------+---------------------+---------------------+----------+
|      1 |        105 |           2 | 2014-02-05 18:49:10 | 2014-02-05 13:19:10 |        4 |
|      2 |        105 |           2 | 2014-02-05 13:19:30 | NULL                |        4 |
|      3 |        113 |           3 | 2014-08-05 10:26:08 | NULL                |        1 |
|      4 |        325 |           3 | 2014-09-04 01:32:18 | NULL                |        1 |
+--------+------------+-------------+---------------------+---------------------+----------+

Here you can see that, there are multiple entries for visitor_id = 105. That duplicate entry came because, customer didn't logout properly. (See null value of second duplicate entry). Hence if I filter log-customer collection for store_id = 4. It will throw an error, which seems like this.(the code will work as expected store_id = 1, since there is no duplicate entries)

( ! )Fatal error: Uncaught exception 'Exception' with message 'Item (Mage_Log_Model_Visitor) with the same id "105" already exist' in C:\wamp\www\magento_v1.8\lib\Varien\Data\Collection.php on line 373

( ! ) Exception: Item (Mage_Log_Model_Visitor) with the same id "105" already exist in C:\wamp\www\magento_v1.8\lib\Varien\Data\Collection.php on line 373

So in general, I need to avoid this. So how can I distinct my join query ? Please enlighten me with your beautiful thoughts.

Thanks

Answer 1

What you could consider doing is adding a group by clause for the visitor_id column.

$collection = Mage::getModel('log/visitor')->getCollection()->addFieldToFilter('store_id', array('eq' => 1));
$collection->getSelect()
    ->join(
        array('log_customer'=> $collection->getTable('log/customer')),
        'main_table.`visitor_id`= log_customer.`visitor_id`'
    )->group('main_table.visitor_id');

But you might also need to add an order by to make sure that you get the newest results first.

Answer 2

Zend_Db_Select::distinct()

The above method should help you. You could just call it by:

$collection->getSelect()->distinct();

Answer 3

Distinct and Group, as described by David and Paras, will work on 9 out of 10 queries. If it doesn't, make sure that you aren't reusing the column name of the primary column of the collection.

The following does not work all the time because catalog_product_entity has a column named entity_id which is the primary column of the order collection

$order_collection = Mage::getModel('sales/order')
        ->getCollection();

    $product_collection = Mage::getModel('catalog/product')
        ->getCollection()
        ->addFieldToFilter('attribute_set_id', $attributeSetId)
    ;

    $order_collection->getSelect()
        ->join($sales_order_item_table,
            "$sales_order_item_table.order_id = main_table.entity_id")
        ->join(new Zend_Db_Expr('('.$product_collection->getSelect().')'),
            "t.entity_id = $sales_order_item_table.product_id")
        ->group("main_table.entity_id")
    ;

To fix it, just add an empty array as the third parameter of the join

        $order_collection->getSelect()
        ->join($sales_order_item_table,
            "$sales_order_item_table.order_id = main_table.entity_id",
            array())
        ->join(new Zend_Db_Expr('('.$product_collection->getSelect().')'),
            "t.entity_id = $sales_order_item_table.product_id",
            array())
        ->group("main_table.entity_id")
    ;

Answer 4

While adding the join make sure blank array is passed in the third parameter.

$this->getSelect()->joinLeft(
    'sales_order_item',
    'sales_order_item.order_id = main_table.order_id',
    []
);

After that add following to apply DISTINCT

$this->getSelect()->distinct(true);