[2.2.x] Fixed #30111 -- Fixed AppRegistryNotReady error with django.contrib.postgres in INSTALLED_APPS.

Regression in e192223ed996ed30fe83787efdfa7f2be6b1a2ee.
Backport of 2804b8d2153505ec49b191db2168302dfb92c3af from master.

1 files changed, 22 insertions, 10 deletions

diff --git a/django/db/migrations/recorder.py b/django/db/migrations/recorder.py
index 3a972fe4c6..ad5435d906 100644
--- a/django/db/migrations/recorder.py
+++ b/django/db/migrations/recorder.py
@@ -1,6 +1,7 @@
 from django.apps.registry import Apps
 from django.db import models
 from django.db.utils import DatabaseError
+from django.utils.decorators import classproperty
 from django.utils.timezone import now
 
 from .exceptions import MigrationSchemaMissing
@@ -18,19 +19,30 @@ class MigrationRecorder:
     If a migration is unapplied its row is removed from the table. Having
     a row in the table always means a migration is applied.
     """
+    _migration_class = None
 
-    class Migration(models.Model):
-        app = models.CharField(max_length=255)
-        name = models.CharField(max_length=255)
-        applied = models.DateTimeField(default=now)
+    @classproperty
+    def Migration(cls):
+        """
+        Lazy load to avoid AppRegistryNotReady if installed apps import
+        MigrationRecorder.
+        """
+        if cls._migration_class is None:
+            class Migration(models.Model):
+                app = models.CharField(max_length=255)
+                name = models.CharField(max_length=255)
+                applied = models.DateTimeField(default=now)
 
-        class Meta:
-            apps = Apps()
-            app_label = "migrations"
-            db_table = "django_migrations"
+                class Meta:
+                    apps = Apps()
+                    app_label = 'migrations'
+                    db_table = 'django_migrations'
 
-        def __str__(self):
-            return "Migration %s for %s" % (self.name, self.app)
+                def __str__(self):
+                    return 'Migration %s for %s' % (self.name, self.app)
+
+            cls._migration_class = Migration
+        return cls._migration_class
 
     def __init__(self, connection):
         self.connection = connection