Fixed #31157 -- Fixed displaying read-only JSONField values in admin.

django/contrib/admin/utils.py

@@ -398,6 +398,11 @@ def display_for_field(value, field, empty_value_display):
         return formats.number_format(value)
     elif isinstance(field, models.FileField) and value:
         return format_html('<a href="{}">{}</a>', value.url, value)
+    elif isinstance(field, models.JSONField) and value:
+        try:
+            return field.get_prep_value(value)
+        except TypeError:
+            return display_for_value(value, empty_value_display)
     else:
         return display_for_value(value, empty_value_display)
 

tests/admin_utils/tests.py

@@ -179,6 +179,20 @@ class UtilsTests(SimpleTestCase):
         display_value = display_for_field(None, models.JSONField(), self.empty_value)
         self.assertEqual(display_value, self.empty_value)
 
+    def test_json_display_for_field(self):
+        tests = [
+            ({'a': {'b': 'c'}}, '{"a": {"b": "c"}}'),
+            (['a', 'b'], '["a", "b"]'),
+            ('a', '"a"'),
+            ({('a', 'b'): 'c'}, "{('a', 'b'): 'c'}"),  # Invalid JSON.
+        ]
+        for value, display_value in tests:
+            with self.subTest(value=value):
+                self.assertEqual(
+                    display_for_field(value, models.JSONField(), self.empty_value),
+                    display_value,
+                )
+
     def test_number_formats_display_for_field(self):
         display_value = display_for_field(12345.6789, models.FloatField(), self.empty_value)
         self.assertEqual(display_value, '12345.6789')