gorus gorus - 4 months ago 15
JSON Question

In Django 1.9, what's the convention for using JSONField (native postgres jsonb)?

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.

Answer

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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