Form field and database feature

5 min read

I know this post is way overdue. I haven't been feeling well lately and there were some things in life I need to catch up with. Anyway, I still try to make progress in my project whenever I can. In these past two weeks, I've added a form field for my JSONField. I have also done some improvements to the tests and the field itself. I feel like there isn't much to write about this time, since most of the work is documented on the GitHub PR.

Compared to its model fields, Django's form fields are easier to write. This time, the Django documentation isn't very helpful. However, with the JSONField form field available in contrib.postgres, we can reuse most of its code for our cross-DB JSONField's form field.

I won't go into details of each method in the form field, since the form field from contrib.postgres already works out of the box. However, I'll try to add support for custom JSON encoder and decoder that will be used in the field's validation.

It's very easy. I just needed to add the encoder and decoder to the field's constructor like this:

def __init__(self, encoder=None, decoder=None, **kwargs):
    self.encoder, self.decoder = encoder, decoder

and add the cls argument to json.dumps and json.loads calls.

I decided to change the invalid error message from '%(value)s' value must be valid JSON. into Enter a valid JSON value. to be consistent with other form fields in django.forms.

That's it!

Now, we just need to link our model field with the new form field to be used by ModelForm. This can be done by overriding the formfield method in the model field. We basically just need to specify the form class, but since we have added support for custom JSON encoder and decoder in our form field, it's a good idea to pass the encoder and decoder used in the model field to the form field.

# django/db/models/fields/
def formfield(self, **kwargs):
    return super().formfield(**{
        'form_class': forms.JSONField,
        'encoder': self.encoder,
        'decoder': self.decoder,

On the other hand, I finally touched the backend code to add a new database feature in Django, named supports_json. It's a flag that determines if the currently used database backend supports JSON. I used it to prevent the use of JSONField if the backend does not support it. It may or may not be useful for web developers, I'm not really sure ¯\_(ツ)_/¯.

For most backends, I just write it as a boolean variable with True as its value if the minimum version required by Django for that backend already supports JSON. Otherwise, I write it as a method that checks the version and return True or False accordingly. The method is decorated with cached_property as with other database features that use methods. Here's an example:

# django/db/backends/mysql/
def supports_json(self):
    if self.connection.mysql_is_mariadb:
        return self.connection.mysql_version >= (10, 2, 7)
    return self.connection.mysql_version >= (5, 7, 8)

For SQLite, it was a bit tricky. JSON support for SQLite is provided through the JSON1 extension, which was introduced with the release of SQLite 3.9.0. However, it's a loadable extension and there's the possibility that it's not included in the installation (since it's optional).

There is no command to check for enabled extensions in SQLite. One way I could think of is to execute an SQL query like SELECT JSON('"test"') and see if it works. If it does, then JSON1 is enabled, and vice versa. To do this, I execute the query in the feature method, wrapped with transaction.atomic() to avoid breaking transactions if the query raises an exception.

# django/db/backends/sqlite3/
def supports_json(self):
        with self.connection.cursor() as cursor, transaction.atomic():
            cursor.execute("SELECT JSON('\"test\"')")
    except OperationalError:
        return False
        return True

Now that I've touched the backend code, I might end up removing db_type and db_check in our JSONField and defining them in the backend by adding JSON to data_types and adding the constraints to data_type_check_constraints in the DatabaseWrapper. I'll need opinions to see if this is the right approach, though.

I guess that's it for this post. The first GSoC evaluation is happening right now until June 29, 2019 at 00:59 (UTC+07:00). Let's hope I'll make it through!