Question

I have a table that contains a column called region_code with Integer as its datatype. In my Python program I have this line:

region = session.query(Region).filter(Region.region_code/100 == region_prefix).one()

The important part here is the chosen filter method

Region.region_code/100 == region_prefix

where region_code is the integer column in the table and the region_prefix variable holds an integer value

The region_code is the integer column in the table containing values like 1199, 1200, 1300, 1499 while the region_prefix variable (from the program) holds integer values like 11, 12, 13, 14.

What I now want to achive with this filter option is to perform an integer division on region_code (hence cut everything behind the decimal place) to be able to compare it with region_prefix.

Example:

region_prefix = 11

Assume that in the Region table exists a entry with

region_code = 1199

The calculation within the filter should look like this

1199 / 100 = 11

So with this result I am able to filter for region_prefix (11=11).
This works just fine with MSSQL (Microsoft).

Unfortunately (in my case) MySQL division operator / does not do integer divisions. So the result of 1199/100 will be 11.99 and so my query will not find a fitting entry in the DB (because 11.99 != 11).
Nevertheless MySQL actually has the inter division implemented: the DIV operator.

So the following SQL query works just fine for me:

SELECT * FROM Region where region_code DIV 100 = 11;

So my question is:
How do I get the SQLAlchemy filter method to use DIV instead of / operator? Is this possible? Or is the only workaround to use native queries?

Answer 1

Luckily SQLAlchemy allows for custom operators. There are simple ways of doing this (simply specifying in the query) or more complex versions of doing it (fully defining the new operator and making it multi database aware). So it depends on what you’re looking for.

http://docs.sqlalchemy.org/en/latest/core/custom_types.html?highlight=operators#redefining-and-creating-new-operators

region = (session
    .query(Region)
    .filter(Region.region_code.op('div')(region_prefix))
    .one()
)

And the fancy way:

http://docs.sqlalchemy.org/en/rel_0_8/core/types.html#types-operators

from sqlalchemy import Integer

class DivisibleInteger(Integer):
    class comparator_factory(Integer.Comparator):
        def div(self, other):
            return self.op('div')(other)