yii\gii\generators\model\Generator::checkJunctionTable PHP Method

    protected function checkJunctionTable($table)
    {
        if (count($table->foreignKeys) < 2) {
            return false;
        }
        $uniqueKeys = [$table->primaryKey];
        try {
            $uniqueKeys = array_merge($uniqueKeys, $this->getDbConnection()->getSchema()->findUniqueIndexes($table));
        } catch (NotSupportedException $e) {
            // ignore
        }
        $result = [];
        // find all foreign key pairs that have all columns in an unique constraint
        $foreignKeys = array_values($table->foreignKeys);
        for ($i = 0; $i < count($foreignKeys); $i++) {
            $firstColumns = $foreignKeys[$i];
            unset($firstColumns[0]);
            for ($j = $i + 1; $j < count($foreignKeys); $j++) {
                $secondColumns = $foreignKeys[$j];
                unset($secondColumns[0]);
                $fks = array_merge(array_keys($firstColumns), array_keys($secondColumns));
                foreach ($uniqueKeys as $uniqueKey) {
                    if (count(array_diff(array_merge($uniqueKey, $fks), array_intersect($uniqueKey, $fks))) === 0) {
                        // save the foreign key pair
                        $result[] = [$foreignKeys[$i], $foreignKeys[$j]];
                        break;
                    }
                }
            }
        }
        return empty($result) ? false : $result;
    }