Fixed #12990, Refs #27694 -- Added JSONField model field.

Thanks to Adam Johnson, Carlton Gibson, Mariusz Felisiak, and Raphael
Michel for mentoring this Google Summer of Code 2019 project and
everyone else who helped with the patch.

Special thanks to Mads Jensen, Nick Pope, and Simon Charette for
extensive reviews.

Co-authored-by: Mariusz Felisiak <felisiak.mariusz@gmail.com>

tests/model_fields/models.py

@@ -1,3 +1,4 @@
+import json
 import os
 import tempfile
 import uuid
@@ -7,6 +8,7 @@ from django.contrib.contenttypes.fields import (
 )
 from django.contrib.contenttypes.models import ContentType
 from django.core.files.storage import FileSystemStorage
+from django.core.serializers.json import DjangoJSONEncoder
 from django.db import models
 from django.db.models.fields.files import ImageFieldFile
 from django.utils.translation import gettext_lazy as _
@@ -332,6 +334,35 @@ if Image:
                                   width_field='headshot_width')
 
 
+class CustomJSONDecoder(json.JSONDecoder):
+    def __init__(self, object_hook=None, *args, **kwargs):
+        return super().__init__(object_hook=self.as_uuid, *args, **kwargs)
+
+    def as_uuid(self, dct):
+        if 'uuid' in dct:
+            dct['uuid'] = uuid.UUID(dct['uuid'])
+        return dct
+
+
+class JSONModel(models.Model):
+    value = models.JSONField()
+
+    class Meta:
+        required_db_features = {'supports_json_field'}
+
+
+class NullableJSONModel(models.Model):
+    value = models.JSONField(blank=True, null=True)
+    value_custom = models.JSONField(
+        encoder=DjangoJSONEncoder,
+        decoder=CustomJSONDecoder,
+        null=True,
+    )
+
+    class Meta:
+        required_db_features = {'supports_json_field'}
+
+
 class AllFieldsModel(models.Model):
     big_integer = models.BigIntegerField()
     binary = models.BinaryField()