Django highly suggests not to use null=True
for CharField and TextField string-based fields in order not to have two possible values for "no data" (assuming you're allowing empty strings with blank=True
). This makes total sense to me and I do this in all my projects.
Django 1.9 introduces JSONField, which uses the underlying Postgres jsonb
data type. Does the suggestion above carry over to JSONField (i.e. blank=True
should be used instead of null=True
)? Or, should null=True
be used instead? Or, should default=dict
be used instead? Or, ..? Why?
In other words, what is the convention for the new native JSONField, when you want to allow only one "no data" value? Please support your answer because I did a lot of research and couldn't find anything official. Thanks in advance.
The convention implied from the Django code seems to be to store null JSON values as NULL as opposed to as an empty string (as is the convention for the CharField
). I say this because of the following:
The empty_strings_allowed
is inherited from Field
in CharField
, and is set to True
:
django/db/models/fields/__init__.py#L96
class Field(RegisterLookupMixin): """Base class for all field types""" # Designates whether empty strings fundamentally are allowed at the # database level. empty_strings_allowed = True ...
JSONField
, however, overrides it with False
:
django/contrib/postgres/fields/jsonb.py#L13
class JSONField(Field): empty_strings_allowed = False ...
This causes CharField
's to default to ""
and JSONField
's to None
when you instantiate a model without explicitly passing the values for these fields.
django/db/models/fields/init.py#L791
def get_default(self): """ Returns the default value for this field. """ if self.has_default(): if callable(self.default): return self.default() return self.default if (not self.empty_strings_allowed or (self.null and not connection.features.interprets_empty_strings_as_nulls)): return None return ""
Therefore, if you want to make a JSONField
optional, you have to use:
json_field = JSONField(blank=True, null=True)
If you use only blank=True
, as you would for CharField
, you'll get an IntegrityError
when trying to run MyModel.objects.create(...)
without passing a json_field
argument explicitly.
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