Bug? Behavior different with mongo when using $arrayElemAt with index out of range.

[wanmeihuali]

From mongodb's document: https://docs.mongodb.com/manual/reference/operator/aggregation/arrayElemAt/
If idx exceeds the array bounds, $arrayElemAt does not return a result.

However, mongomock seems didn't check the out of range case. if the idx exceeds the array bounds, it throws an IndexError: list index out of range.

A simple range check here(

mongomock/mongomock/aggregate.py

Line 376 in b507ae6

return array[index]

) should help.

Detail:
we have a aggregate pipeline with a stage:

            {'$lookup': {'from': 'security_info', 'localField': 'github_url',   # join the security info table with git
                         'foreignField': 'github_url', 'as': 'security_info'}},
            {'$set': {'security_info': {'$filter': {'input': '$security_info', 'as': 'security_info', 'cond': {
                                '$eq': ['$$security_info.branch', '$branch']}}}}},  # also match with branch
            {'$set': {'security_info': {'$arrayElemAt': ['$security_info', 0]}}},   # turn security field from array to object

It works on our test database on mongo compass, but it fails when using mongomock, with exception

self = <mongomock.aggregate._Parser object at 0x7f7df7ae2b00>
operator = '$arrayElemAt', values = ['$security_info', 0]

    def _handle_project_operator(self, operator, values):
        if operator in _GROUPING_OPERATOR_MAP:
            values = self.parse(values) if isinstance(values, str) else self.parse_many(values)
            return _GROUPING_OPERATOR_MAP[operator](values)
        if operator == '$arrayElemAt':
            key, index = values
            array = self._parse_basic_expression(key)
            index = self.parse(index)
>           return array[index]
E           IndexError: list index out of range

.local/lib/python3.6/site-packages/mongomock/aggregate.py:370: IndexError

[pcorpet]

Thanks for reporting. PRs are welcomed including regression tests to make sure the behavior between pymongo + MongoDB and mongomock stays in sync.

[pcorpet]

This will probably be fixed by #646

[pcorpet]

@wanmeihuali can you check that the fixes work for you please? If so, I'll close this issue.

[wanmeihuali]

The problem is solved, Thanks!