Make QueryBuilder.one return single value instead of list if only single projection

[sphuber]

When projecting a single property in a query, one() and first() will still return a list, forcing one to still index the returned list to get the value of interest. This leads to a lot of occurrences of the following code:

result = builder.one()[0]

If there is only a single projection in the query, then the QueryBuilder knows that the user most likely just wants the single result and could return that instead of the pointless list. This would simplify the code to

result = builder.one()

The difference in type of the result value is acceptable, I find, because the caller defines the projections and so is guaranteed to know what the type of the return value is going to be. However, this would be a backwards incompatible change, so this could only be integrated in v2.0.0 at the earliest.

[chrisjsewell]

completely agree, @sphuber for back-compatibility, perhaps first we could add an option like:

result = builder.one(True)

single (or another name?) would be None by default, but would emit a warning if not set; that eventually single=True will be the default.
does that make sense? e.g.

def one(single: Optional[bool] = None):
    if single is None:
       warn('`single` option not set, defaulting to False, in v3 this will default to True')
       single = False
    ...
    if single:
       return result[0]

[sphuber]

Would be great to add this and have a deprecation pathway as you suggested. Maybe just a different name. Something like unpack? Since that is the name for unpacking a tuple into independent variables no?

[sphuber]

This was added for first in #5410