I am trying your formula but my Delta field is not getting updated. It is
supposed to change as i enter a different value in ODO_Reading.
On Monday, January 14, 2019 at 6:26:09 AM UTC+2, Anthony wrote:
>
> 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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