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/invalid_models_tests/test_models.py

@@ -5,7 +5,7 @@ from django.core.checks.model_checks import _check_lazy_references
 from django.db import connection, connections, models
 from django.db.models.functions import Lower
 from django.db.models.signals import post_init
-from django.test import SimpleTestCase, TestCase
+from django.test import SimpleTestCase, TestCase, skipUnlessDBFeature
 from django.test.utils import isolate_apps, override_settings, register_lookup
 
 
@@ -1349,6 +1349,40 @@ class OtherModelTests(SimpleTestCase):
         ])
 
 
+@isolate_apps('invalid_models_tests')
+class JSONFieldTests(TestCase):
+    @skipUnlessDBFeature('supports_json_field')
+    def test_ordering_pointing_to_json_field_value(self):
+        class Model(models.Model):
+            field = models.JSONField()
+
+            class Meta:
+                ordering = ['field__value']
+
+        self.assertEqual(Model.check(databases=self.databases), [])
+
+    def test_check_jsonfield(self):
+        class Model(models.Model):
+            field = models.JSONField()
+
+        error = Error(
+            '%s does not support JSONFields.' % connection.display_name,
+            obj=Model,
+            id='fields.E180',
+        )
+        expected = [] if connection.features.supports_json_field else [error]
+        self.assertEqual(Model.check(databases=self.databases), expected)
+
+    def test_check_jsonfield_required_db_features(self):
+        class Model(models.Model):
+            field = models.JSONField()
+
+            class Meta:
+                required_db_features = {'supports_json_field'}
+
+        self.assertEqual(Model.check(databases=self.databases), [])
+
+
 @isolate_apps('invalid_models_tests')
 class ConstraintsTests(TestCase):
     def test_check_constraints(self):