addExpressionFieldToSelect has to be called after all addFieldToSelect

[dbsdsun]

Preconditions

1. Magento 2.1.12

Steps to reproduce

1. create a collection
2. the following code will only addExpressionFieldToSelect before addFieldToSelect, and the 'stime_part' won't appear in the result.
   $bookingCollection = $this->_bookingCollectionFactory->create();
   $bookingCollection->addExpressionFieldToSelect('stime_part', 'time(start_time)', []);
   $bookingCollection->addFieldToSelect('finish_time', 'finish_date');

Expected result

Field 'stime_part' populated with calculated data.

Actual result

Field 'stime_par' doesn't populated with calculated data.

The bug is in the following code

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

protected function _initSelectFields()
{
    $columns = $this->_select->getPart(\Magento\Framework\DB\Select::COLUMNS);
    $columnsToSelect = [];
    foreach ($columns as $columnEntry) {
        list($correlationName, $column, $alias) = $columnEntry;
        if ($correlationName !== 'main_table') {
            // Add joined fields to select
            if ($column instanceof \Zend_Db_Expr) {
                $column = $column->__toString();
            }
            $key = $alias !== null ? $alias : $column;
            $columnsToSelect[$key] = $columnEntry;
        }
    }

    $columns = $columnsToSelect;

    $columnsToSelect = array_keys($columnsToSelect);

    if ($this->_fieldsToSelect !== null) {
        $insertIndex = 0;
        foreach ($this->_fieldsToSelect as $alias => $field) {
            if (!is_string($alias)) {
                $alias = null;
            }

            if ($field instanceof \Zend_Db_Expr) {
                $column = $field->__toString();
            } else {
                $column = $field;
            }

            if ($alias !== null && in_array(
                $alias,
                $columnsToSelect
            ) ||
                // If field already joined from another table
                $alias === null && isset($alias, $columnsToSelect)
            ) {
                continue;
            }

            $columnEntry = ['main_table', $field, $alias];
            array_splice($columns, $insertIndex, 0, [$columnEntry]);
            // Insert column
            $insertIndex++;
        }
    } else {
        array_unshift($columns, ['main_table', '*', null]);
    }

    $this->_select->setPart(\Magento\Framework\DB\Select::COLUMNS, $columns);

    return $this;
}

This line skips the ExpressionField
if ($correlationName !== 'main_table') {

Working around is
Call all addFieldToSelect before addExpressionFieldToSelect

[magento-engcom-team]

Hi @dbsdsun. Thank you for your report.
To help us process this issue please make sure that you provided the following information:

• Summary of the issue
• Information on your environment
• Steps to reproduce
• Expected and actual results

Please make sure that the issue is reproducible on the vanilla Magento instance following Steps to reproduce. To deploy vanilla Magento instance on our environment, please, add a comment to the issue:

@magento-engcom-team give me {$VERSION} instance

where {$VERSION} is version tags (starting from 2.2.0+) or develop branches (2.2-develop +).
For more details, please, review the Magento Contributor Assistant documentation.

@dbsdsun do you confirm that you was able to reproduce the issue on vanilla Magento instance following steps to reproduce?

• yes
• no

[engcom-backlog-nickolas]

Hello @dbsdsun, thank you for your report.
We've acknowledged the issue and added to our backlog.