Preserving output precision with Django DecimalField and PostgreSql Numeric field

Question

I'm saving data to a PostgreSQL backend through Django. Many of the fields in my models are DecimalFields set to arbitrarily high max_digits and decimal_places, corresponding to numeric columns in the database backend. The data in each column have a precision (or number of decimal places) that is not known a priori, and each datum in a given column need not have the same precision.

For example, arguments to a model may look like:

{'dist': Decimal("94.3"), 'dist_e': Decimal("1.2")}
{'dist': Decimal("117"), 'dist_e': Decimal("4")}

where the keys are database column names.

Upon output, I need to preserve and redisplay those data with the precision with which they were read in. In other words, after the database is queried, the displayed data need to look exactly like the data in that were read in, with no additional or missing trailing 0's in the decimals. When queried, however, either in a django shell or in the admin interface, all of the DecimalField data come back with many trailing 0's.

I have seen similar questions answered for money values, where the precision (2 decimal places) is both known and the same for all data in a given column. However, how might one best preserve the exact precision represented by Decimal values in Django and numeric values in PostgreSQL when the precision is not the same and not known beforehand?

EDIT:

Possibly an additional useful piece of information: When viewing the table to which the data are saved in a Django dbshell, the many trailing 0's are also present. The python Decimal value is apparently converted to the maximum precision value specified in the models.py file upon being saved to the PostgreSQL backend.

Answer 1

If you need perfect parity forwards and backwards, you'll need to use a CharField. Any number-based database field is going to interact with your data muxing it in some way or another. Now, I know you mentioned not being able to know the digit length of the data points, and a CharField requires some length. You can either set it arbitrarily high (1000, 2000, etc) or I suppose you could use a TextField, instead.

However, with either approach, you're going to be wasting a lot database resources in most scenarios. I would suggest modifying your approach such that extra zeros at the end don't matter (for display purpose you could always chop them off), or such that the precision is not longer arbitrary.