models: Switch from NullBooleanField to BooleanField.

In Django 2.1, the preferred way to express a nullable BooleanField
changed from NullBooleanField to passing null=True to BooleanField.

This updates our codebase to use the preferred API.  Tweaked by
tabbott to update the linter rules.

The migration is a noop for Django accounting only.

Part of #11341.

tools/linter_lib/custom_check.py

@@ -444,7 +444,7 @@ python_rules = RuleList(
          'bad_lines': ['desc = models.CharField(null=True)  # type: Text',
                        'stream = models.ForeignKey(Stream, null=True, on_delete=CASCADE)  # type: Stream'],
          },
-        {'pattern': r' = models[.](?!NullBoolean).*\)  # type: Optional',  # Optional tag, except NullBoolean(Field)
+        {'pattern': r' = models[.].*  # type: Optional',  # Optional tag
          'exclude_pattern': 'null=True',
          'include_only': {"zerver/models.py"},
          'description': 'Model variable annotated with Optional but variable does not have null=true.',

zerver/migrations/0274_nullbooleanfield_to_booleanfield.py

@@ -0,0 +1,53 @@
+# Generated by Django 2.2.12 on 2020-04-26 17:53
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0273_migrate_old_bot_messages'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='stream',
+            name='invite_only',
+            field=models.BooleanField(default=False, null=True),
+        ),
+        migrations.AlterField(
+            model_name='subscription',
+            name='audible_notifications',
+            field=models.BooleanField(default=None, null=True),
+        ),
+        migrations.AlterField(
+            model_name='subscription',
+            name='desktop_notifications',
+            field=models.BooleanField(default=None, null=True),
+        ),
+        migrations.AlterField(
+            model_name='subscription',
+            name='email_notifications',
+            field=models.BooleanField(default=None, null=True),
+        ),
+        migrations.AlterField(
+            model_name='subscription',
+            name='is_muted',
+            field=models.BooleanField(default=False, null=True),
+        ),
+        migrations.AlterField(
+            model_name='subscription',
+            name='push_notifications',
+            field=models.BooleanField(default=None, null=True),
+        ),
+        migrations.AlterField(
+            model_name='subscription',
+            name='wildcard_mentions_notify',
+            field=models.BooleanField(default=None, null=True),
+        ),
+        migrations.AlterField(
+            model_name='userprofile',
+            name='enter_sends',
+            field=models.BooleanField(default=False, null=True),
+        ),
+    ]

zerver/models.py

@@ -944,7 +944,7 @@ class UserProfile(AbstractBaseUser, PermissionsMixin):
     default_all_public_streams: bool = models.BooleanField(default=False)
 
     # UI vars
-    enter_sends: Optional[bool] = models.NullBooleanField(default=False)
+    enter_sends: Optional[bool] = models.BooleanField(null=True, default=False)
     left_side_userlist: bool = models.BooleanField(default=False)
 
     # display settings
@@ -1365,7 +1365,7 @@ class Stream(models.Model):
     # Foreign key to the Recipient object for STREAM type messages to this stream.
     recipient = models.ForeignKey(Recipient, null=True, on_delete=models.SET_NULL)
 
-    invite_only: Optional[bool] = models.NullBooleanField(default=False)
+    invite_only: Optional[bool] = models.BooleanField(null=True, default=False)
     history_public_to_subscribers: bool = models.BooleanField(default=False)
 
     # Whether this stream's content should be published by the web-public archive features
@@ -2088,7 +2088,7 @@ class Subscription(models.Model):
     active: bool = models.BooleanField(default=True)
 
     # Whether this user had muted this stream.
-    is_muted: Optional[bool] = models.NullBooleanField(default=False)
+    is_muted: Optional[bool] = models.BooleanField(null=True, default=False)
 
     DEFAULT_STREAM_COLOR = "#c2c2c2"
     color: str = models.CharField(max_length=10, default=DEFAULT_STREAM_COLOR)
@@ -2097,11 +2097,11 @@ class Subscription(models.Model):
     # These fields are stream-level overrides for the user's default
     # configuration for notification, configured in UserProfile.  The
     # default, None, means we just inherit the user-level default.
-    desktop_notifications: Optional[bool] = models.NullBooleanField(default=None)
-    audible_notifications: Optional[bool] = models.NullBooleanField(default=None)
-    push_notifications: Optional[bool] = models.NullBooleanField(default=None)
-    email_notifications: Optional[bool] = models.NullBooleanField(default=None)
-    wildcard_mentions_notify: Optional[bool] = models.NullBooleanField(default=None)
+    desktop_notifications: Optional[bool] = models.BooleanField(null=True, default=None)
+    audible_notifications: Optional[bool] = models.BooleanField(null=True, default=None)
+    push_notifications: Optional[bool] = models.BooleanField(null=True, default=None)
+    email_notifications: Optional[bool] = models.BooleanField(null=True, default=None)
+    wildcard_mentions_notify: Optional[bool] = models.BooleanField(null=True, default=None)
 
     class Meta:
         unique_together = ("user_profile", "recipient")