Django snippets

Comments

cpesch (on May 6, 2009):

I have problems loading json (with loaddata) that has been dumped (with dumpdata) from a PostgreSQL database. The to_python method is given a value like:

{u'source': u'web', u'location': u'home'}

and this leads to the error message:

ValueError: Expecting property name: line 1 column 1 (char 1)

loads() seems to expect something like

{"source": "web", "location": "home"}

Inserting something like

value = value.replace("u'", "'").replace("'",'"')

before line 19 cures the problem, but that doesn't look like an elegant solution, isn't it?

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jb0t (on May 13, 2009):

This worked a little better for me. Though I am still not all that happy with it.

# # # # # # # # # # # # # # # # #

from django.db import models
from django.core.serializers.json import DjangoJSONEncoder
from django.utils import simplejson as json

class JSONField(models.TextField):
    """JSONField is a generic textfield that neatly serializes/unserializes
    JSON objects seamlessly

    The json spec claims you must use a collection type at the top level of
    the data structure.  However the simplesjon decoder and Firefox both encode
    and decode non collection types that do not exist inside a collection.
    The to_python method relies on the value being an instance of basestring
    to ensure that it is encoded.  If a string is the sole value at the
    point the field is instanced, to_python attempts to decode the sting because
    it is derived from basestring but cannot be encodeded and throws the
    exception ValueError: No JSON object could be decoded.
    """

    # Used so to_python() is called
    __metaclass__ = models.SubfieldBase

    def to_python(self, value):
        """Convert our string value to JSON after we load it from the DB"""
        if value == "":
            return None

        try:
            if isinstance(value, basestring):
                return json.loads(value)
        except ValueError:
            return value

        return value

    def get_db_prep_save(self, value):
        """Convert our JSON object to a string before we save"""

        if value == "":
            return None

        return super(JSONField, self).get_db_prep_save(json.dumps(value, cls=DjangoJSONEncoder))

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jb0t (on May 13, 2009):

Last sentence should actually read

If a string is the sole value at the point the field is instanced, to_python attempts to decode the sting because it is derived from basestring but cannot be decoded

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Jasber (on May 20, 2009):

jb0t: I updated what you added, thanks.

cpesch: I'm really not sure, I don't have PostgreSQL installed on my machine. Also no, that doesn't look like a very elegant solution. If you can come up with something let me know and I'll add it.

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fmardini (on June 18, 2009):

a hopefully elegant solution :)

value = dict(map(lambda n: map(str, n), value.items()))

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Jasber (on June 24, 2009):

fmardini: I think that may cover most use cases, but simply mapping complex objects to str isn't going to work for everyone.

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chowdere (on August 11, 2009):

I was being thrown for a loop with e.g. "Error binding parameter" (using sqlite) when trying to save a model object that had a JSONField. Turns out I was passing in a list, not a dict (valid JSON), and this JSONField doesn't handle lists. The quick fix has line 32 become:

        if isinstance(value, dict) or isinstance(value, list):

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andrewbadr (on November 9, 2009):

I'm only using JSONField for holding dictionaries, which I suspect is pretty common. So, I took out the other options. Use like "properties = DictField(default={})".

class DictField(models.TextField):
"""DictField is a textfield that contains JSON-serialized dictionaries."""

# Used so to_python() is called
__metaclass__ = models.SubfieldBase

def to_python(self, value):
    """Convert our string value to JSON after we load it from the DB"""
    value = json.loads(value)
    assert isinstance(value, dict)
    return value

def get_db_prep_save(self, value):
    """Convert our JSON object to a string before we save"""
    assert isinstance(value, dict)
    value = json.dumps(value, cls=DjangoJSONEncoder)
    return super(DictField, self).get_db_prep_save(value)

#

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