$arrayElemAt - doesn't work when index is an expresion

[mgodkowicz]

With query:

{
    "$project": {
        "outputkey": {"$arrayElemAt": ["$input", "$idx"]}
    }
}

I get the following error:

self = <mongomock.aggregate._Parser object at 0x1357fe1c0>
operator = '$arrayElemAt', values = ['$themes', '$idx']

    def _handle_project_operator(self, operator, values):
        if operator in _GROUPING_OPERATOR_MAP:
            return _GROUPING_OPERATOR_MAP[operator](self.parse_many(values))
        if operator == '$arrayElemAt':
            key, index = values
            array = self._parse_basic_expression(key)
           return array[index]
           TypeError: list indices must be integers or slices, not str

Simple if statement that checks if index is an integer and if not parsing expression fixed the issue:

    def _handle_project_operator(self, operator, values):
        if operator in _GROUPING_OPERATOR_MAP:
            return _GROUPING_OPERATOR_MAP[operator](self.parse_many(values))
        if operator == '$arrayElemAt':
            key, index = values
            array = self._parse_basic_expression(key)
            if not isinstance(index, int):
                index = self._parse_basic_expression(index)
            return array[index]
        raise NotImplementedError("Although '%s' is a valid project operator for the "
                                  'aggregation pipeline, it is currently not implemented '
                                  'in Mongomock.' % operator)