From 5d25804eaf81795c7d457e5a2a9f0b9b0989136c Mon Sep 17 00:00:00 2001
From: Sanyam Khurana <sanyam.khurana01@gmail.com>
Date: Thu, 13 Dec 2018 01:41:50 +0530
Subject: [PATCH] Fixed #20098 -- Added a check for model Meta.db_table
 collisions.

---
 django/core/checks/model_checks.py         | 14 ++++
 docs/ref/checks.txt                        |  2 +
 tests/check_framework/test_model_checks.py | 75 ++++++++++++++++++++++
 3 files changed, 91 insertions(+)
 create mode 100644 tests/check_framework/test_model_checks.py

diff --git a/django/core/checks/model_checks.py b/django/core/checks/model_checks.py
index 2397fe3bb2..8514fb4d12 100644
--- a/django/core/checks/model_checks.py
+++ b/django/core/checks/model_checks.py
@@ -1,5 +1,6 @@
 import inspect
 import types
+from collections import defaultdict
 from itertools import chain
 
 from django.apps import apps
@@ -8,12 +9,15 @@ from django.core.checks import Error, Tags, register
 
 @register(Tags.models)
 def check_all_models(app_configs=None, **kwargs):
+    db_table_models = defaultdict(list)
     errors = []
     if app_configs is None:
         models = apps.get_models()
     else:
         models = chain.from_iterable(app_config.get_models() for app_config in app_configs)
     for model in models:
+        if model._meta.managed and not model._meta.proxy:
+            db_table_models[model._meta.db_table].append(model._meta.label)
         if not inspect.ismethod(model.check):
             errors.append(
                 Error(
@@ -25,6 +29,16 @@ def check_all_models(app_configs=None, **kwargs):
             )
         else:
             errors.extend(model.check(**kwargs))
+    for db_table, model_labels in db_table_models.items():
+        if len(model_labels) != 1:
+            errors.append(
+                Error(
+                    "db_table '%s' is used by multiple models: %s."
+                    % (db_table, ', '.join(db_table_models[db_table])),
+                    obj=db_table,
+                    id='models.E028',
+                )
+            )
     return errors
 
 
diff --git a/docs/ref/checks.txt b/docs/ref/checks.txt
index acd860065f..6ec3028c35 100644
--- a/docs/ref/checks.txt
+++ b/docs/ref/checks.txt
@@ -302,6 +302,8 @@ Models
 * **models.E026**: The model cannot have more than one field with
   ``primary_key=True``.
 * **models.W027**: ``<database>`` does not support check constraints.
+* **models.E028**: ``db_table`` ``<db_table>`` is used by multiple models:
+  ``<model list>``.
 
 Security
 --------
diff --git a/tests/check_framework/test_model_checks.py b/tests/check_framework/test_model_checks.py
new file mode 100644
index 0000000000..2e55ad637d
--- /dev/null
+++ b/tests/check_framework/test_model_checks.py
@@ -0,0 +1,75 @@
+from django.core import checks
+from django.core.checks import Error
+from django.db import models
+from django.test import SimpleTestCase
+from django.test.utils import (
+    isolate_apps, modify_settings, override_system_checks,
+)
+
+
+@isolate_apps('check_framework', attr_name='apps')
+@override_system_checks([checks.model_checks.check_all_models])
+class DuplicateDBTableTests(SimpleTestCase):
+    def test_collision_in_same_app(self):
+        class Model1(models.Model):
+            class Meta:
+                db_table = 'test_table'
+
+        class Model2(models.Model):
+            class Meta:
+                db_table = 'test_table'
+
+        self.assertEqual(checks.run_checks(app_configs=self.apps.get_app_configs()), [
+            Error(
+                "db_table 'test_table' is used by multiple models: "
+                "check_framework.Model1, check_framework.Model2.",
+                obj='test_table',
+                id='models.E028',
+            )
+        ])
+
+    @modify_settings(INSTALLED_APPS={'append': 'basic'})
+    @isolate_apps('basic', 'check_framework', kwarg_name='apps')
+    def test_collision_across_apps(self, apps):
+        class Model1(models.Model):
+            class Meta:
+                app_label = 'basic'
+                db_table = 'test_table'
+
+        class Model2(models.Model):
+            class Meta:
+                app_label = 'check_framework'
+                db_table = 'test_table'
+
+        self.assertEqual(checks.run_checks(app_configs=apps.get_app_configs()), [
+            Error(
+                "db_table 'test_table' is used by multiple models: "
+                "basic.Model1, check_framework.Model2.",
+                obj='test_table',
+                id='models.E028',
+            )
+        ])
+
+    def test_no_collision_for_unmanaged_models(self):
+        class Unmanaged(models.Model):
+            class Meta:
+                db_table = 'test_table'
+                managed = False
+
+        class Managed(models.Model):
+            class Meta:
+                db_table = 'test_table'
+
+        self.assertEqual(checks.run_checks(app_configs=self.apps.get_app_configs()), [])
+
+    def test_no_collision_for_proxy_models(self):
+        class Model(models.Model):
+            class Meta:
+                db_table = 'test_table'
+
+        class ProxyModel(Model):
+            class Meta:
+                proxy = True
+
+        self.assertEqual(Model._meta.db_table, ProxyModel._meta.db_table)
+        self.assertEqual(checks.run_checks(app_configs=self.apps.get_app_configs()), [])