Fix fetch_val for raw queries in Postgres backend

[pvanliefland]

As of now, fetch_val does not work for raw queries in Postgres. Possibly related to the discussion in #101.

This is a simple fix that will distinguish between row[0] and row['foo'] in the case of raw queries.

await database.fetch_val("SELECT foo FROM bar")

in core.py:

async def fetch_val(
    self,
    query: typing.Union[ClauseElement, str],
    values: dict = None,
    column: typing.Any = 0,
) -> typing.Any:
    async with self._query_lock:
        row = await self._connection.fetch_one(self._build_query(query, values))
    return None if row is None else row[column]

in backends/postgres.py:

def __getitem__(self, key: typing.Any) -> typing.Any:
    if len(self._column_map) == 0:  # raw query
        return self._row[tuple(self._row.keys()).index(key)]

In this case, key is 0, while tuple(self.row.keys()) is ('id',), which raises ValueError: tuple.index(x): x not in tuple

[gvbgduh]

@pvanliefland asyncpg actually has fetchval method, in contradiction to mysql and sqlite, so instead of
return self._row[key if type(key) is int else tuple(self._row.keys()).index(key)]
it's possible to implement it
in postgres.py

    async def fetch_val(self, query: ClauseElement) -> typing.Optional[typing.Mapping]:
        assert self._connection is not None, "Connection is not acquired"
        query, args, result_columns = self._compile(query)
        row = await self._connection.fetchval(query, *args)
        if row is None:
            return None
        return row

and in core.py

    async def fetch_val(
        self,
        query: typing.Union[ClauseElement, str],
        values: dict = None,
        column: typing.Any = 0,
    ) -> typing.Any:
        async with self._query_lock:
            row = await self._connection.fetch_val(self._build_query(query, values))
        return row

it also requires implementation of fetch_val in other backends in a way it's implemented in core.py now.
A bit more work, but it gets rid of this overhead for postgres as asyncpg just returns the value.

[pvanliefland]

@gvbgduh good idea, will do :)

[gvbgduh]

awesome!

[blueyed]

For reference: #132