On Sunday, January 13, 2019 at 10:24:03 AM UTC-5, mostwanted wrote:
>
> How can achieve decremental subtraction of a value in a database field?
>
> *MODEL CODE*
>
> db.define_table('fuelLogging',
> Field('Quantity', 'integer', requires=IS_NOT_EMPTY()),
> Field('Rate', 'integer', requires=IS_NOT_EMPTY()),
> Field('Total', compute=lambda r:r['Quantity']*r['Rate']),
> Field('ODO_Reading', 'integer', requires=IS_NOT_EMPTY()),
> * Field('Delta', compute=lambda
> r:r['ODO_Reading']-r['ODO_Reading'], requires=IS_NOT_EMPTY()),*
> Field('Fuel_Usage', compute=lambda r: r['Delta']/r[
> 'Quantity']))
> To get the delta field i have to subtract the previously entered
> ODO_Reading value from the newly entered ODO_Reading value but what i have
> done here is not working, any suggestions?
>
To use an existing field value in a query, you can use the field object
itself (i.e., db.fuelLogging.ODO_Reading). However, for the DAL to treat
the expression as an Expression object, the field object must come first,
so you cannot do r.ODO_Reading - db.fuelLogging.ODO_Reading. You must
instead use (db.fuelLogging.ODO_Reading * -1) + r.ODO_Reading.
Another problem you will have is that the compute function will not work on
inserts because there is no value in the ODO_Reading field in that case. A
workaround is to set the compute attribute conditionally only on updates:
def fuelLogging_before_update(*args, **kwargs):
db.fuelLogging.compute = lambda r: (db.fuelLogging.ODO_Reading * -1) + r
.ODO_Reading
db.define_table('fuelLogging',
Field('ODO_Reading', 'integer'),
Field('Delta', 'integer', default=0),
before_update=fuelLogging_before_update)
Anthony
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