[1.8.x] Fixed #24184 -- Prevented automatic soft-apply of migrations

Previously Django only checked for the table name in CreateModel
operations in initial migrations and faked the migration automatically.
This led to various errors and unexpected behavior. The newly introduced
--fake-initial flag to the migrate command must be passed to get the
same behavior again. With this change Django will bail out in with a
"duplicate relation / table" error instead.

Thanks Carl Meyer and Tim Graham for the documentation update, report
and review.

Backport of f287bec583 from master

docs/topics/migrations.txt

@@ -167,6 +167,13 @@ developers (or your production servers) check out the code, they'll
 get both the changes to your models and the accompanying migration at the
 same time.
 
+.. versionadded:: 1.8
+
+If you want to give the migration(s) a meaningful name instead of a generated
+one, you can use the :djadminopt:`--name` option::
+
+    $ python manage.py makemigrations --name changed_my_model your_app_label
+
 Version control
 ~~~~~~~~~~~~~~~
 
@@ -329,10 +336,12 @@ need to convert it to use migrations; this is a simple process::
 
     $ python manage.py makemigrations your_app_label
 
-This will make a new initial migration for your app. Now, when you run
-:djadmin:`migrate`, Django will detect that you have an initial migration
-*and* that the tables it wants to create already exist, and will mark the
-migration as already applied.
+This will make a new initial migration for your app. Now, run ``python
+manage.py migrate --fake-initial``, and Django will detect that you have an
+initial migration *and* that the tables it wants to create already exist, and
+will mark the migration as already applied. (Without the
+:djadminopt:`--fake-initial` flag, the :djadmin:`migrate` command would error
+out because the tables it wants to create already exist.)
 
 Note that this only works given two things:
 
@@ -344,12 +353,11 @@ Note that this only works given two things:
   that your database doesn't match your models, you'll just get errors when
   migrations try to modify those tables.
 
-.. versionadded:: 1.8
+.. versionchanged: 1.8
 
-If you want to give the migration(s) a meaningful name instead of a generated one,
-you can use the :djadminopt:`--name` option::
-
-    $ python manage.py makemigrations --name changed_my_model your_app_label
+    The ``--fake-initial`` flag to :djadmin:`migrate` was added. Previously,
+    Django would always automatically fake-apply initial migrations if it
+    detected that the tables exist.
 
 .. _historical-models:
 
@@ -757,9 +765,10 @@ If you already have pre-existing migrations created with
   ``__init__.py`` - make sure you remove the ``.pyc`` files too.
 * Run ``python manage.py makemigrations``. Django should see the empty
   migration directories and make new initial migrations in the new format.
-* Run ``python manage.py migrate``. Django will see that the tables for the
-  initial migrations already exist and mark them as applied without running
-  them.
+* Run ``python manage.py migrate --fake-initial``. Django will see that the
+  tables for the initial migrations already exist and mark them as applied
+  without running them. (Django won't check that the table schema match your
+  models, just that the right table names exist).
 
 That's it! The only complication is if you have a circular dependency loop
 of foreign keys; in this case, ``makemigrations`` might make more than one
@@ -767,6 +776,12 @@ initial migration, and you'll need to mark them all as applied using::
 
     python manage.py migrate --fake yourappnamehere
 
+.. versionchanged:: 1.8
+
+    The :djadminopt:`--fake-initial` flag was added to :djadmin:`migrate`;
+    previously, initial migrations were always automatically fake-applied if
+    existing tables were detected.
+
 Libraries/Third-party Apps
 ~~~~~~~~~~~~~~~~~~~~~~~~~~