# Save in the bd a field function
**URL:** https://discuss.tryton.org/t/save-in-the-bd-a-field-function/804
**Category:** Developer
**Created:** [October 15, 2018, 1:54pm UTC](https://discuss.tryton.org/t/save-in-the-bd-a-field-function/804 "2018-10-15T13:54:44Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@electro1](https://discuss.tryton.org/u/electro1)
#### Post date: [October 15, 2018, 1:54pm UTC](https://discuss.tryton.org/t/save-in-the-bd-a-field-function/804/1 "2018-10-15T13:54:45Z")
Hello,
I have created the following field function:
t = fields.Function(fields.Float(‘t’),‘get\_t’)
```
def get_t(self,name = None):
m = round ((Decimal(self.quantity) * Decimal(self.gross_unit_price)),2)
return m
```
I’m trying to record it in a field that I created , but without success Please help
d= fields.Float(‘d’)
```
def get_top(cls ,name = None):
if(SaleLine.unit_price):
cls.write( SaleLine, {
'd': SaleLine.t ,
})
```
---
### Author:  [@fransuar](https://discuss.tryton.org/u/fransuar)
#### Post date: [October 16, 2018, 4:09am UTC](https://discuss.tryton.org/t/save-in-the-bd-a-field-function/804/2 "2018-10-16T04:09:27Z")
Hello electro1,
fields.Function’s are not stored on db. These are dynamic fields
Maybe you should try a fields.Float’s instead a fields.Function, and a on\_change\_ or an on\_change\_with\_ function
Something like this maybe:
```
@fields.depends('quantity', 'gross_unit_price')
def on_change_with_t(self, name):
t = round ((Decimal(self.quantity) * Decimal(self.gross_unit_price)),2)
return t
```
That will replace the value of t anytime the value of quantity or gross\_unit\_price change
```
This could help too:
```
> **[on\_change of fields — Tryton server](https://docs.tryton.org/latest/server/topics/models/fields_on_change.html)**
```
Regards
Francisco
```
---
### Author:  [@ced](https://discuss.tryton.org/u/ced)
#### Post date: [October 18, 2018, 9:04am UTC](https://discuss.tryton.org/t/save-in-the-bd-a-field-function/804/3 "2018-10-18T09:04:27Z")
Also the getter method should never modify the data because it is often called in a readonly transaction.
If you want to cache some computed data, the best is to do it at a specific state of the Model, like it is done on `Sale.confirm` with `Sale.store_cache`.