magento admin grid show data in column from model method

Question

I have a custom module that implements an adminhtml grid in magento admin. Currently the column that I have added gets the data from the model's table. Following is the code to Grid.php.

<?php

class Beyondroid_Subscribepro_Block_Adminhtml_Customersubscriptions_Grid extends Mage_Adminhtml_Block_Widget_Grid {

    protected function _prepareCollection() {
        $collection = Mage::getModel('subscribepro/customersubscriptions')->getCollection();
        $this->setCollection($collection);

        return parent::_prepareCollection();
    }

    public function getRowUrl($row) {
        return $this->getUrl(
            'adminhtml/customersubscriptions/edit',
            array('id' => $row->getId())
        );
    }

    protected function _prepareColumns() {

        $this->addColumn(
            'id', array(
                'header' => 'ID',
                'type' => 'number',
                'index' => 'id'
            )
        );
        $this->addColumn(
            'subscription_id', array(
                'header' => 'Subscription ID',
                'type' => 'number',
                'index' => 'subscription_id'
            )
        );
        $this->addColumn(
            'order_id', array(
                'header' => 'Order ID',
                'type' => 'number',
                'index' => 'order_id'
            )
        );
        $this->addColumn(
            'start_date', array(
                'header' => 'Start Date',
                'type' => 'date',
                'index' => 'start_date'
            )
        );
        $this->addColumn(
            'end_date', array(
                'header' => 'End Date',
                'type' => 'date',
                'index' => 'end_date'
            )
        );
        $this->addColumn(
            'is_enabled', array(
                'header' => 'Enabled',
                'type' => 'text',
                'index' => 'is_enabled',
                'renderer' => 'Beyondroid_Subscribepro_Block_EnableDisableRenderer'
            )
        );
        $this->addColumn(
            'deliveries_made', array(
                'header' => 'Deliveries Made',
                'type' => 'number',
                'index' => 'deliveries_made'
            )
        );
        $this->addColumn(
            'wallet_amount', array(
                'header' => 'Wallet Amount',
                'type' => 'number',
                'index' => 'wallet_amount'
            )
        );

        return parent::_prepareColumns();
    }

    protected function _getHelper() {
        return Mage::helper('beyondroid_subscribepro');
    }
}

in my Customsubscription Model I have a method defined as follows:

public function getDeliveryDates() {
    $dates = array();
    $deliveries = Mage::getModel('subscribepro/customerdeliveries')
        ->getCollection()
        ->addFieldToFilter('subscription_id', $this->getId());
    foreach ($deliveries as $delivery) {
        $date = Mage::getModel('core/date')->date('m/d/Y',$delivery->getData('date'));
        array_push($dates, $date);
    }

    return $dates;
}

The above method returns an array of delivery dates. I want to show a comma separated list of delivery dates in a column in the above mentioned Grid. Is that possible at all ? If yes, how do I define a column that picks up data from this function ?

Update#1

So I have tried joining the customerdelivery tables date values to the subscription collection as follows:

protected function _prepareCollection() {
    $collection = Mage::getModel('subscribepro/customersubscriptions')->getCollection();
    $collection->getSelect()->join(
        'subscribepro_customerdeliveries',
        '`subscribepro_customerdeliveries`.subscription_id=`main_table`.id',
        array(
            'dates' => new Zend_Db_Expr('group_concat(`subscribepro_customerdeliveries`.date SEPARATOR ",")'),
        )
    );
    $this->setCollection($collection);

    return parent::_prepareCollection();
}

and added the column as follows:

    $this->addColumn(
        'dates', array(
            'header' => 'Delivery Dates',
            'type' => 'number',
            'index' => 'dates'
        )
    );

But it just shows 2015 in the column, where as I want a comma separated list of dates:

I have 3 delivery date rows in the DB for this customer subscription (id=31)

Update#2

The type value in add_column should be text, So if I change it to follows, it shows me the list but for all subscription_ids.

    $this->addColumn(
        'dates', array(
            'header' => 'Delivery Dates',
            'type' => 'text',
            'index' => 'dates'
        )
    );

Answer 1

Ok, so finally resolved. Just used a join and grouped on id and it works :)

protected function _prepareCollection() {
    $collection = Mage::getModel('subscribepro/customersubscriptions')->getCollection();
    $collection->getSelect()->join(
        'subscribepro_customerdeliveries',
        '`subscribepro_customerdeliveries`.subscription_id=`main_table`.id',
        array(
            'dates' => new Zend_Db_Expr('group_concat(`subscribepro_customerdeliveries`.date SEPARATOR ", \r")'),
        )
    );
    $collection->getSelect()->group('main_table.id');
    $this->setCollection($collection);

    return parent::_prepareCollection();
}