Fixed #13711 -- Model check added to ensure that auto-generated column name is within limits of the database.

Thanks russellm for report and Tim Graham for review.

django/db/backends/__init__.py

@@ -520,9 +520,6 @@ class BaseDatabaseFeatures(object):
     # at the end of each save operation?
     supports_forward_references = True
 
-    # Does the backend allow very long model names without error?
-    supports_long_model_names = True
-
     # Is there a REAL datatype in addition to floats/doubles?
     has_real_datatype = False
     supports_subqueries_in_group_by = True

django/db/backends/mysql/base.py

@@ -171,7 +171,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     has_select_for_update = True
     has_select_for_update_nowait = False
     supports_forward_references = False
-    supports_long_model_names = False
     # XXX MySQL DB-API drivers currently fail on binary data on Python 3.
     supports_binary_field = six.PY2
     supports_microsecond_precision = False

django/db/models/base.py

@@ -12,7 +12,7 @@ from django.conf import settings
 from django.core import checks
 from django.core.exceptions import (ObjectDoesNotExist,
     MultipleObjectsReturned, FieldError, ValidationError, NON_FIELD_ERRORS)
-from django.db import (router, transaction, DatabaseError,
+from django.db import (router, connections, transaction, DatabaseError,
     DEFAULT_DB_ALIAS, DJANGO_VERSION_PICKLE_KEY)
 from django.db.models.deletion import Collector
 from django.db.models.fields import AutoField, FieldDoesNotExist
@@ -1068,6 +1068,7 @@ class Model(six.with_metaclass(ModelBase)):
         if not cls._meta.swapped:
             errors.extend(cls._check_fields(**kwargs))
             errors.extend(cls._check_m2m_through_same_relationship())
+            errors.extend(cls._check_long_column_names())
             clash_errors = cls._check_id_field() + cls._check_field_name_clashes()
             errors.extend(clash_errors)
             # If there are field name clashes, hide consequent column name
@@ -1451,6 +1452,76 @@ class Model(six.with_metaclass(ModelBase)):
                 )
         return errors
 
+    @classmethod
+    def _check_long_column_names(cls):
+        """
+        Check that any auto-generated column names are shorter than the limits
+        for each database in which the model will be created.
+        """
+        errors = []
+        allowed_len = None
+        db_alias = None
+
+        # Find the minimum max allowed length among all specified db_aliases.
+        for db in settings.DATABASES.keys():
+            # skip databases where the model won't be created
+            if not router.allow_migrate(db, cls):
+                continue
+            connection = connections[db]
+            max_name_length = connection.ops.max_name_length()
+            if max_name_length is None:
+                continue
+            else:
+                if allowed_len is None:
+                    allowed_len = max_name_length
+                    db_alias = db
+                elif max_name_length < allowed_len:
+                    allowed_len = max_name_length
+                    db_alias = db
+
+        if allowed_len is None:
+            return errors
+
+        for f in cls._meta.local_fields:
+            _, column_name = f.get_attname_column()
+
+            # Check if auto-generated name for the field is too long
+            # for the database.
+            if (f.db_column is None and column_name is not None
+                    and len(column_name) > allowed_len):
+                errors.append(
+                    checks.Error(
+                        'Autogenerated column name too long for field "%s". '
+                        'Maximum length is "%s" for database "%s".'
+                        % (column_name, allowed_len, db_alias),
+                        hint="Set the column name manually using 'db_column'.",
+                        obj=cls,
+                        id='models.E018',
+                    )
+                )
+
+        for f in cls._meta.local_many_to_many:
+            # Check if auto-generated name for the M2M field is too long
+            # for the database.
+            for m2m in f.rel.through._meta.local_fields:
+                _, rel_name = m2m.get_attname_column()
+                if (m2m.db_column is None and rel_name is not None
+                        and len(rel_name) > allowed_len):
+                    errors.append(
+                        checks.Error(
+                            'Autogenerated column name too long for M2M field '
+                            '"%s". Maximum length is "%s" for database "%s".'
+                            % (rel_name, allowed_len, db_alias),
+                            hint=("Use 'through' to create a separate model "
+                                "for M2M and then set column_name using "
+                                "'db_column'."),
+                            obj=cls,
+                            id='models.E019',
+                        )
+                    )
+
+        return errors
+
 
 ############################################
 # HELPER FUNCTIONS (CURRIED MODEL METHODS) #

docs/ref/checks.txt

@@ -58,6 +58,11 @@ Models
 * **models.E016**: ``index_together/unique_together`` refers to field
   ``<field_name>`` which is not local to model ``<model>``.
 * **models.E017**: Proxy model ``<model>`` contains model fields.
+* **models.E018**: Autogenerated column name too long for field ``<field>``.
+  Maximum length is ``<maximum length>`` for database ``<alias>``.
+* **models.E019**: Autogenerated column name too long for M2M field
+  ``<M2M field>``. Maximum length is ``<maximum length>`` for database
+  ``<alias>``.
 
 Fields
 ~~~~~~

docs/releases/1.8.txt

@@ -305,6 +305,28 @@ Now to implement the same behavior, you have to create an
 ``parser.add_argument`` to add any custom arguments, as parser is now an
 :py:class:`argparse.ArgumentParser` instance.
 
+Model check ensures auto-generated column names are within limits specified by database
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A field name that's longer than the column name length supported by a database
+can create problems. For example, with MySQL you'll get an exception trying to
+create the column, and with PostgreSQL the column name is truncated by the
+database (you may see a warning in the PostgreSQL logs).
+
+A model check has been introduced to better alert users to this scenario before
+the actual creation of database tables.
+
+If you have an existing model where this check seems to be a false positive,
+for example on PostgreSQL where the name was already being truncated, simply
+use :attr:`~django.db.models.Field.db_column` to specify the name that's being
+used.
+
+The check also applies to the columns generated in an implicit
+``ManyToManyField.through`` model. If you run into an issue there, use
+:attr:`~django.db.models.ManyToManyField.through` to create an explicit model
+and then specify :attr:`~django.db.models.Field.db_column` on its column(s)
+as needed.
+
 Miscellaneous
 ~~~~~~~~~~~~~
 

tests/backends/models.py

@@ -4,7 +4,7 @@ from django.contrib.contenttypes.fields import (
     GenericForeignKey, GenericRelation
 )
 from django.contrib.contenttypes.models import ContentType
-from django.db import models, connection
+from django.db import models
 from django.utils.encoding import python_2_unicode_compatible
 
 
@@ -31,10 +31,8 @@ class SchoolClass(models.Model):
     day = models.CharField(max_length=9, blank=True)
     last_updated = models.DateTimeField()
 
-# Unfortunately, the following model breaks MySQL hard.
+
-# Until #13711 is fixed, this test can't be run under MySQL.
+class VeryLongModelNameZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ(models.Model):
-if connection.features.supports_long_model_names:
-    class VeryLongModelNameZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ(models.Model):
     class Meta:
         # We need to use a short actual table name or
         # we hit issue #8548 which we're not testing!

tests/backends/tests.py

@@ -379,26 +379,23 @@ class LongNameTest(TestCase):
     check it is. Refs #8901.
     """
 
-    @skipUnlessDBFeature('supports_long_model_names')
     def test_sequence_name_length_limits_create(self):
         """Test creation of model with long name and long pk name doesn't error. Ref #8901"""
-        models.VeryLongModelNameZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ.objects.create()
+        models.VeryLongModelNameZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ.objects.create()
 
-    @skipUnlessDBFeature('supports_long_model_names')
     def test_sequence_name_length_limits_m2m(self):
         """Test an m2m save of a model with a long name and a long m2m field name doesn't error as on Django >=1.2 this now uses object saves. Ref #8901"""
-        obj = models.VeryLongModelNameZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ.objects.create()
+        obj = models.VeryLongModelNameZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ.objects.create()
         rel_obj = models.Person.objects.create(first_name='Django', last_name='Reinhardt')
         obj.m2m_also_quite_long_zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz.add(rel_obj)
 
-    @skipUnlessDBFeature('supports_long_model_names')
     def test_sequence_name_length_limits_flush(self):
         """Test that sequence resetting as part of a flush with model with long name and long pk name doesn't error. Ref #8901"""
         # A full flush is expensive to the full test, so we dig into the
         # internals to generate the likely offending SQL and run it manually
 
         # Some convenience aliases
-        VLM = models.VeryLongModelNameZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
+        VLM = models.VeryLongModelNameZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
         VLM_m2m = VLM.m2m_also_quite_long_zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz.through
         tables = [
             VLM._meta.db_table,

tests/invalid_models_tests/test_models.py

@@ -1,13 +1,36 @@
 # -*- encoding: utf-8 -*-
 from __future__ import unicode_literals
 
+import unittest
+
+from django.conf import settings
 from django.core.checks import Error
-from django.db import models
+from django.db import models, connections
 from django.test.utils import override_settings
 
 from .base import IsolatedModelsTestCase
 
 
+def get_max_column_name_length():
+    allowed_len = None
+    db_alias = None
+
+    for db in settings.DATABASES.keys():
+        connection = connections[db]
+        max_name_length = connection.ops.max_name_length()
+        if max_name_length is None:
+            continue
+        else:
+            if allowed_len is None:
+                allowed_len = max_name_length
+                db_alias = db
+            elif max_name_length < allowed_len:
+                allowed_len = max_name_length
+                db_alias = db
+
+    return (allowed_len, db_alias)
+
+
 class IndexTogetherTests(IsolatedModelsTestCase):
 
     def test_non_iterable(self):
@@ -252,6 +275,124 @@ class FieldNamesTests(IsolatedModelsTestCase):
         ]
         self.assertEqual(errors, expected)
 
+    max_column_name_length, column_limit_db_alias = get_max_column_name_length()
+
+    @unittest.skipIf(max_column_name_length is None,
+                    "The database doesn't have a column name length limit.")
+    def test_M2M_long_column_name(self):
+        """
+        #13711 -- Model check for long M2M column names when database has
+        column name length limits.
+        """
+        allowed_len, db_alias = get_max_column_name_length()
+
+        # A model with very long name which will be used to set relations to.
+        class VeryLongModelNamezzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz(models.Model):
+            title = models.CharField(max_length=11)
+
+        # Main model for which checks will be performed.
+        class ModelWithLongField(models.Model):
+            m2m_field = models.ManyToManyField(
+                VeryLongModelNamezzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz,
+                related_name="rn1"
+            )
+            m2m_field2 = models.ManyToManyField(
+                VeryLongModelNamezzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz,
+                related_name="rn2", through='m2msimple'
+            )
+            m2m_field3 = models.ManyToManyField(
+                VeryLongModelNamezzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz,
+                related_name="rn3",
+                through='m2mcomplex'
+            )
+            fk = models.ForeignKey(VeryLongModelNamezzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz, related_name="rn4")
+
+        # Models used for setting `through` in M2M field.
+        class m2msimple(models.Model):
+            id2 = models.ForeignKey(ModelWithLongField)
+
+        class m2mcomplex(models.Model):
+            id2 = models.ForeignKey(ModelWithLongField)
+
+        long_field_name = 'a' * (self.max_column_name_length + 1)
+        models.ForeignKey(
+            VeryLongModelNamezzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
+        ).contribute_to_class(m2msimple, long_field_name)
+
+        models.ForeignKey(
+            VeryLongModelNamezzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz,
+            db_column=long_field_name
+        ).contribute_to_class(m2mcomplex, long_field_name)
+
+        errors = ModelWithLongField.check()
+
+        # First error because of M2M field set on the model with long name.
+        m2m_long_name = "verylongmodelnamezzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz_id"
+        expected = [
+            Error(
+                ('Autogenerated column name too long for M2M field "%s". '
+                'Maximum length is "%s" for database "%s".'
+                % (m2m_long_name, self.max_column_name_length, self.column_limit_db_alias)),
+                hint=("Use 'through' to create a separate model for "
+                    "M2M and then set column_name using 'db_column'."),
+                obj=ModelWithLongField,
+                id='models.E019',
+            )
+        ]
+
+        # Second error because the FK specified in the `through` model
+        # `m2msimple` has auto-genererated name longer than allowed.
+        # There will be no check errors in the other M2M because it
+        # specifies db_column for the FK in `through` model even if the actual
+        # name is longer than the limits of the database.
+        expected.append(
+            Error(
+                ('Autogenerated column name too long for M2M field "%s_id". '
+                'Maximum length is "%s" for database "%s".'
+                % (long_field_name, self.max_column_name_length, self.column_limit_db_alias)),
+                hint=("Use 'through' to create a separate model for "
+                    "M2M and then set column_name using 'db_column'."),
+                obj=ModelWithLongField,
+                id='models.E019',
+            )
+        )
+
+        self.assertEqual(errors, expected)
+
+    @unittest.skipIf(max_column_name_length is None,
+                    "The database doesn't have a column name length limit.")
+    def test_local_field_long_column_name(self):
+        """
+        #13711 -- Model check for long column names
+        when database does not support long names.
+        """
+        allowed_len, db_alias = get_max_column_name_length()
+
+        class ModelWithLongField(models.Model):
+            title = models.CharField(max_length=11)
+
+        long_field_name = 'a' * (self.max_column_name_length + 1)
+        long_field_name2 = 'b' * (self.max_column_name_length + 1)
+        models.CharField(max_length=11).contribute_to_class(ModelWithLongField, long_field_name)
+        models.CharField(max_length=11, db_column='vlmn').contribute_to_class(ModelWithLongField, long_field_name2)
+
+        errors = ModelWithLongField.check()
+
+        # Error because of the field with long name added to the model
+        # without specifying db_column
+        expected = [
+            Error(
+                ('Autogenerated column name too long for field "%s". '
+                'Maximum length is "%s" for database "%s".'
+                % (long_field_name, self.max_column_name_length, self.column_limit_db_alias)),
+                hint="Set the column name manually using 'db_column'.",
+                obj=ModelWithLongField,
+                id='models.E018',
+            )
+        ]
+
+        self.assertEqual(errors, expected)
+
     def test_including_separator(self):
         class Model(models.Model):
             some__field = models.IntegerField()