groupBY with identification alias is missing discriminator field

Bug Report

While using groupBy(DQL_ALIAS) for aggregated query resulted SQL with missing discriminator field and this is leading to

[42803] ERROR: column "p0_.type" must appear in the GROUP BY clause or be used in an aggregate function

Q	A
BC Break	no
Version	2.x.x, 3.x.x, 4.x.x

Summary

Suggestion to fix

class SqlWalker implements TreeWalker
{
    ...

    public function walkGroupByItem($groupByItem)
    {

    ...

       // IdentificationVariable
        $sqlParts = [];

        foreach ($this->getMetadataForDqlAlias($groupByItem)->fieldNames as $field) {
            $item       = new AST\PathExpression(AST\PathExpression::TYPE_STATE_FIELD, $groupByItem, $field);
            $item->type = AST\PathExpression::TYPE_STATE_FIELD;

            $sqlParts[] = $this->walkPathExpression($item);
        }

        foreach ($this->getMetadataForDqlAlias($groupByItem)->associationMappings as $mapping) {
            if ($mapping['isOwningSide'] && $mapping['type'] & ClassMetadata::TO_ONE) {
                $item       = new AST\PathExpression(AST\PathExpression::TYPE_SINGLE_VALUED_ASSOCIATION, $groupByItem, $mapping['fieldName']);
                $item->type = AST\PathExpression::TYPE_SINGLE_VALUED_ASSOCIATION;

                $sqlParts[] = $this->walkPathExpression($item);
            }
        }

        ### NEW CODE HERE ###
        if ($this->getMetadataForDqlAlias($groupByItem)->isInheritanceTypeSingleTable()) {
            $field = $this->getMetadataForDqlAlias($groupByItem)->getDiscriminatorColumn()['name'];
            $sqlParts[] = $this->walkIdentificationVariable($groupByItem, $field) . '.' . $field;
        }
        // or add it as part of $this->walkPathExpression($item) with new AST\PathExpression::TYPE_DISCRIMINATOR
        ### END OF NEW CODE ###

        return implode(', ', $sqlParts);

Current behavior

No discriminator field added to GROUP BY clause

How to reproduce

Create aggregated query with group by entity identification alias and entity has discriminator field.

As a result there is no discriminator field in the resulted SQL GROUP BY clause

Expected behavior

All the columns should appear in the SQL GROUP BY clause for aggregated queries

doctrine / orm