Question

I have this function for comparing version numbers:

def version_gt(left, right):
    """
    Compare two version numbers telling which is greater    

    Returns True if left is greater, False if right is greater.
    """
    left = map(int, filter(None, str(left).split('.')))
    right = map(int, filter(None, str(right).split('.')))
    for i in range(max([len(left), len(right)])):
        if i > len(left)-1:
            # left ran out of version numbers, therefore right is greater
            return False
        elif i > len(right)-1:
            # right ran out of version numbers therefore left is greater
            return True
        else:
            # compare versions only if the version number is different
            if left[i] != right[i]:
                return left[i] > right[i]
    return False # if all is equal, return False since left is not greater

How can I use this in SQLAlchemy to compare version numbers in a select statement?

I couldn’t figure out the online documentation. Thanks for your help.
-Sunjay03

[EDIT] Here is how I have defined the columns:

Table("updates", metadata,
    Column("u_id", Integer, primary_key=True),
    Column("appname", String, nullable=False),
    Column("version", String, nullable=False),
    Column("uploaded", DateTime, default=func.now()),
)

The answer has something to do with this:
http://www.sqlalchemy.org/docs/orm/mapper_config.html#custom-comparators

I couldn’t understand it very well, maybe you can.

Answer 1

The sqlite database engine is not quite expressive enough to do that kind of comparison on a single column, even in raw SQL. The best option will be to split out the version column into several, or to simply accept a simple string comparison (something to the effect of updates1.version > updates2.version. If none of those are an option, you will need to extend the sqlite engine (via sqlite_create_function()) with the function you need.