Fixed #14028 - Added validation for clashing db_columns.

Thanks akaariai for the suggestion.

django/core/management/validation.py

@@ -63,6 +63,9 @@ def get_validation_errors(outfile, app=None):
             if not opts.get_field(cls.USERNAME_FIELD).unique:
                 e.add(opts, 'The USERNAME_FIELD must be unique. Add unique=True to the field parameters.')
 
+        # Store a list of column names which have already been used by other fields.
+        used_column_names = []
+
         # Model isn't swapped; do field-specific validation.
         for f in opts.local_fields:
             if f.name == 'id' and not f.primary_key and opts.pk.name == 'id':
@@ -75,6 +78,17 @@ def get_validation_errors(outfile, app=None):
                 # consider NULL and '' to be equal (and thus set up
                 # character-based fields a little differently).
                 e.add(opts, '"%s": Primary key fields cannot have null=True.' % f.name)
+
+            # Column name validation.
+            # Determine which column name this field wants to use.
+            _, column_name = f.get_attname_column()
+
+            # Ensure the column name is not already in use.
+            if column_name and column_name in used_column_names:
+                e.add(opts, "Field '%s' has column name '%s' that is already used." % (f.name, column_name))
+            else:
+                used_column_names.append(column_name)
+
             if isinstance(f, models.CharField):
                 try:
                     max_length = int(f.max_length)

tests/invalid_models/invalid_models/models.py

@@ -364,6 +364,56 @@ class BadIndexTogether1(models.Model):
         ]
 
 
+class DuplicateColumnNameModel1(models.Model):
+    """
+    A field (bar) attempts to use a column name which is already auto-assigned
+    earlier in the class. This should raise a validation error.
+    """
+    foo = models.IntegerField()
+    bar = models.IntegerField(db_column='foo')
+
+    class Meta:
+        db_table = 'foobar'
+
+
+class DuplicateColumnNameModel2(models.Model):
+    """
+    A field (foo) attempts to use a column name which is already auto-assigned
+    later in the class. This should raise a validation error.
+    """
+    foo = models.IntegerField(db_column='bar')
+    bar = models.IntegerField()
+
+    class Meta:
+        db_table = 'foobar'
+
+
+class DuplicateColumnNameModel3(models.Model):
+    """Two fields attempt to use each others' names.
+
+    This is not a desirable scenario but valid nonetheless.
+
+    It should not raise a validation error.
+    """
+    foo = models.IntegerField(db_column='bar')
+    bar = models.IntegerField(db_column='foo')
+
+    class Meta:
+        db_table = 'foobar3'
+
+
+class DuplicateColumnNameModel4(models.Model):
+    """Two fields attempt to use the same db_column value.
+
+    This should raise a validation error.
+    """
+    foo = models.IntegerField(db_column='baz')
+    bar = models.IntegerField(db_column='baz')
+
+    class Meta:
+        db_table = 'foobar'
+
+
 model_errors = """invalid_models.fielderrors: "charfield": CharFields require a "max_length" attribute that is a positive integer.
 invalid_models.fielderrors: "charfield2": CharFields require a "max_length" attribute that is a positive integer.
 invalid_models.fielderrors: "charfield3": CharFields require a "max_length" attribute that is a positive integer.
@@ -477,6 +527,9 @@ invalid_models.hardreferencemodel: 'm2m_4' defines a relation with the model 'in
 invalid_models.badswappablevalue: TEST_SWAPPED_MODEL_BAD_VALUE is not of the form 'app_label.app_name'.
 invalid_models.badswappablemodel: Model has been swapped out for 'not_an_app.Target' which has not been installed or is abstract.
 invalid_models.badindextogether1: "index_together" refers to field_that_does_not_exist, a field that doesn't exist.
+invalid_models.duplicatecolumnnamemodel1: Field 'bar' has column name 'foo' that is already used.
+invalid_models.duplicatecolumnnamemodel2: Field 'bar' has column name 'bar' that is already used.
+invalid_models.duplicatecolumnnamemodel4: Field 'bar' has column name 'baz' that is already used.
 """
 
 if not connection.features.interprets_empty_strings_as_nulls: