Add database-specific checks for the maximum supported values of DecimalField max_digits, decimal_places

[anonymous]

For a model field created as

models.DecimalField(max_digits = 200, decimal_places = 100, blank = False, null = False)

While using admin interface to insert a record involving such a DecimalField, the format changes (loss of precision and it uses scientific notation (even in the database)) (Please note - it works properly for low precision values (example - .987654321001234) - but for larger precision values (probably 15 decimal_places or more) it results in loss of precision)

• django version 1.3.1 and 1.4c1 (don't know about older versions);
• python 2.6.6;
• linux;

[anonymous]

The database is sqlite3

The database field (for sqlite3) is 'decimal' (when the field type in the database is changed to say varchar(n) where n > (values to be stored (in this case probably 202)) - everything works perfectly.

The problem seems to be that django emits the column type as decimal for sqlite3 (for other databases this problem is untested) instead of varchar(max_length + probably 2)

[anonymous]

Replying to anonymous:

The database is sqlite3

The database field (for sqlite3) is 'decimal' (when the field type in the database is changed to say varchar(n) where n > (values to be stored (in this case probably 202)) - everything works perfectly.

The problem seems to be that django emits the column type as decimal for sqlite3 (for other databases this problem is untested) instead of varchar(max_length + probably 2)

typo resolution for the above line == instead of varchar(max_digits + probably 2)

[anonymous]

Replying to anonymous:

Replying to anonymous:

The database is sqlite3

The database field (for sqlite3) is 'decimal' (when the field type in the database is changed to say varchar(n) where n > (values to be stored (in this case probably 202)) - everything works perfectly.

The problem seems to be that django emits the column type as decimal for sqlite3 (for other databases this problem is untested) instead of varchar(max_length + probably 2)

typo resolution for the above line == instead of varchar(max_digits + probably 2)

if does not create any side effect(s) a change to this might be the solution === django/db/backends/sqlite3/creation.py

also similar change to respective backends might be the solution (in case a database has limits (unlike python Decimals) on the scale, precision for its corresponding column type)
​http://lockerdome.com/blog

[Taylor Mitchell]

Test illustrating behavior

[Taylor Mitchell]

This is not specific to the admin, as the attached test shows. This patch works under postgres but fails under sqlite3.

[Taylor Mitchell]

Test illustrating problem

[anonymous]

This problem exists for postgres and mysql too (along with sqlite3).

for postgres - limit is up to 131072 digits before the decimal point; up to 16383 digits after the decimal point (from postgres website - ​http://www.postgresql.org/docs/9.1/static/datatype-numeric.html#DATATYPE-NUMERIC-TABLE)

similarly docs on the mysql site mention that a decimal field has a limit of 65 decimal places (but I do not know anything else as I haven't used mysql)

[Ramiro Morales]

Would be acceptable to "fix" this by documenting the limitations of the three database engine/adaptors in the DB notes document?

[Tim Graham]

I think we could add database-specific checks (e.g. django/db/backends/mysql/validation.py) for the maximum values of max_digits, decimal_places supported by each database.