Charfield variable length
Odile Lambert
Hello
I have problems with the Charfield max_length. In the source
code, the possibility exists to have max_length = None but it does
not pass Django check when models.py. contains a charfield = None
I can understand that there is a need for such a max_length form
the database point of view .
My need is the following:
I have a set of fields whose content in the database is a binary
code of maximum 180 bits. Most of the time it will be much shorter
and there is in the form a field (max size) giving this maximum
size.
The user needs to input this field in the database as a Character string of 0 and 1. using hexadecimal fields would be very unpractical. Last but not least, I am using the admin to enter these data in the database.
In a first shot, I used a Charfield and I overwrote the field
in the init to set the maximum length to the value chosen by the
user. I also modifed the attributes of the widget. But at the
end the HTML contains a texinput with the max length set in the
model.
Is this an admin feature or did I miss something?
. Any other suggestion on how to handle these fields would be
welcomed.
Mike Dewhirst
>
> Hello
>
> I have problems with the Charfield max_length. In the source code, the
> possibility exists to have max_length = None but it does not pass
> Django check when models.py. contains a charfield = None
>
> I can understand that there is a need for such a max_length form the
> database point of view .
>
database*. Therefore it can only be set once per migration.
> My need is the following:
>
> I have a set of fields whose content in the database is a binary code
> of maximum 180 bits. Most of the time it will be much shorter and
> there is in the form a field (max size) giving this maximum size.
>
> The user needs to input this field in the database as a Character
> string of 0 and 1. using hexadecimal fields would be very unpractical.
> Last but not least, I am using the admin to enter these data in the
> database.
>
> modifed the attributes of the widget. But at the end the HTML contains
> a texinput with the max length set in the model.
> Is this an admin feature or did I miss something?
>
A binary field requires bytes not text. I think if I needed binary data
entered by a user I would use a CharField with a reasonable max_length
and in the model.save() method I would convert/interpret the data
entered into precisely the binary format I needed and then
programmatically store it into a "parallel" binary field. I would
probably make it readonly in the Admin or just not display it.
I'm not sure about using binary fields with a choice attribute but that
might be another thing to research.
You can certainly use choices in a CharField to totally restrict the
data entered.
> . Any other suggestion on how to handle these fields would be welcomed.
>
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Odile Lambert
Hello thank you very much for your answer.
Your answer about the chafield max length is clear.
I do not know how to clarify my use case because it is very special : the binary codes (there are 4) in the database will be used to set pushers on a knitting machine!...So what has a meaning for the user is really the position of the 0 and 1. Thus the idea of entering 0 and 1 characters. And the admin who creates the database must enter those fields !...
He (she) also decides what is the max length of these binary
codes, as this is a repeating factor. But physically the machine
does not support more than 180 pushers, thus the admin may not
enter more than 180 bits (0 and 1 characters) !... which is rather
good news for him(she)
Entering 180 0 and 1 characters is very unpractical but most of
the time the repeating factor (max length of the binary codes)
will be in the order of 10.
There will be no search whatsoever in the database on this field which is just saved and will be used by a GUI to display information to the user and to do calculations.
I hope this clarifies the need!....
I am now aiming to define a custom smallIntergerfield in the model, with a custom formfield and a textinput widget. As I am rather new to Django this is a fairly high challenge for which I will probably require more help.
If you believe this is a crazy path, please let me know!....
