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>

django/forms/fields.py

@@ -4,6 +4,7 @@ Field classes.
 
 import copy
 import datetime
+import json
 import math
 import operator
 import os
@@ -21,8 +22,8 @@ from django.forms.widgets import (
     FILE_INPUT_CONTRADICTION, CheckboxInput, ClearableFileInput, DateInput,
     DateTimeInput, EmailInput, FileInput, HiddenInput, MultipleHiddenInput,
     NullBooleanSelect, NumberInput, Select, SelectMultiple,
-    SplitDateTimeWidget, SplitHiddenDateTimeWidget, TextInput, TimeInput,
-    URLInput,
+    SplitDateTimeWidget, SplitHiddenDateTimeWidget, Textarea, TextInput,
+    TimeInput, URLInput,
 )
 from django.utils import formats
 from django.utils.dateparse import parse_datetime, parse_duration
@@ -38,7 +39,8 @@ __all__ = (
     'BooleanField', 'NullBooleanField', 'ChoiceField', 'MultipleChoiceField',
     'ComboField', 'MultiValueField', 'FloatField', 'DecimalField',
     'SplitDateTimeField', 'GenericIPAddressField', 'FilePathField',
-    'SlugField', 'TypedChoiceField', 'TypedMultipleChoiceField', 'UUIDField',
+    'JSONField', 'SlugField', 'TypedChoiceField', 'TypedMultipleChoiceField',
+    'UUIDField',
 )
 
 
@@ -1211,3 +1213,66 @@ class UUIDField(CharField):
             except ValueError:
                 raise ValidationError(self.error_messages['invalid'], code='invalid')
         return value
+
+
+class InvalidJSONInput(str):
+    pass
+
+
+class JSONString(str):
+    pass
+
+
+class JSONField(CharField):
+    default_error_messages = {
+        'invalid': _('Enter a valid JSON.'),
+    }
+    widget = Textarea
+
+    def __init__(self, encoder=None, decoder=None, **kwargs):
+        self.encoder = encoder
+        self.decoder = decoder
+        super().__init__(**kwargs)
+
+    def to_python(self, value):
+        if self.disabled:
+            return value
+        if value in self.empty_values:
+            return None
+        elif isinstance(value, (list, dict, int, float, JSONString)):
+            return value
+        try:
+            converted = json.loads(value, cls=self.decoder)
+        except json.JSONDecodeError:
+            raise ValidationError(
+                self.error_messages['invalid'],
+                code='invalid',
+                params={'value': value},
+            )
+        if isinstance(converted, str):
+            return JSONString(converted)
+        else:
+            return converted
+
+    def bound_data(self, data, initial):
+        if self.disabled:
+            return initial
+        try:
+            return json.loads(data, cls=self.decoder)
+        except json.JSONDecodeError:
+            return InvalidJSONInput(data)
+
+    def prepare_value(self, value):
+        if isinstance(value, InvalidJSONInput):
+            return value
+        return json.dumps(value, cls=self.encoder)
+
+    def has_changed(self, initial, data):
+        if super().has_changed(initial, data):
+            return True
+        # For purposes of seeing whether something has changed, True isn't the
+        # same as 1 and the order of keys doesn't matter.
+        return (
+            json.dumps(initial, sort_keys=True, cls=self.encoder) !=
+            json.dumps(self.to_python(data), sort_keys=True, cls=self.encoder)
+        )