Handle loading invalid JSON from db (#248)

src/jsonfield/fields.py

@@ -1,20 +1,26 @@
 import copy
 import json
+import warnings
 
 from django.db import models
 from django.forms import ValidationError
 from django.utils.translation import gettext_lazy as _
 
 from . import forms
 from .encoder import JSONEncoder
-from .json import checked_loads
+from .json import JSONString, checked_loads
 
 DEFAULT_DUMP_KWARGS = {
     'cls': JSONEncoder,
 }
 
 DEFAULT_LOAD_KWARGS = {}
 
+INVALID_JSON_WARNING = (
+    '{0!s} failed to load invalid json ({1}) from the database. The value has '
+    'been returned as a string instead.'
+)
+
 
 class JSONFieldMixin(models.Field):
     form_class = forms.JSONField
@@ -44,7 +50,12 @@ def to_python(self, value):
     def from_db_value(self, value, expression, connection):
         if value is None:
             return None
-        return checked_loads(value, **self.load_kwargs)
+
+        try:
+            return checked_loads(value, **self.load_kwargs)
+        except json.JSONDecodeError:
+            warnings.warn(INVALID_JSON_WARNING.format(self, value), RuntimeWarning)
+            return JSONString(value)
 
     def get_prep_value(self, value):
         """Convert JSON object to a string"""

tests/test_jsonfield.py

@@ -1,3 +1,4 @@
+import warnings
 from collections import OrderedDict
 from decimal import Decimal
 
@@ -305,6 +306,51 @@ def test_mti_deserialization(self):
         self.assertEqual(child.parent_data, {'parent': 'data'})
         self.assertEqual(child.child_data, {'child': 'data'})
 
+    def test_load_invalid_json(self):
+        # Ensure invalid DB values don't crash deserialization.
+        from django.db import connection
+
+        with connection.cursor() as cursor:
+            cursor.execute('INSERT INTO tests_jsonnotrequiredmodel (json) VALUES ("foo")')
+
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
+            instance = JSONNotRequiredModel.objects.get()
+
+        self.assertEqual(len(w), 1)
+        self.assertIs(w[0].category, RuntimeWarning)
+        self.assertEqual(str(w[0].message), (
+            'tests.JSONNotRequiredModel.json failed to load invalid json (foo) '
+            'from the database. The value has been returned as a string instead.'
+        ))
+
+        self.assertEqual(instance.json, 'foo')
+
+    def test_resave_invalid_json(self):
+        # Ensure invalid DB values are resaved as a JSON string.
+        from django.db import connection
+
+        with connection.cursor() as cursor:
+            cursor.execute('INSERT INTO tests_jsonnotrequiredmodel (json) VALUES ("foo")')
+
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
+            instance = JSONNotRequiredModel.objects.get()
+
+        self.assertEqual(len(w), 1)
+        self.assertEqual(instance.json, 'foo')
+
+        # Save instance and reload from the database.
+        instance.save()
+
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
+            instance = JSONNotRequiredModel.objects.get()
+
+        # No deserialization issues, as 'foo' was saved as a serialized string.
+        self.assertEqual(len(w), 0)
+        self.assertEqual(instance.json, 'foo')
+
 
 class QueryTests(TestCase):
     def test_values_deserializes_result(self):