Fixed #19822 -- Added validation for uniqueness on USERNAME_FIELD on custom User models.

Thanks to Claude Peroz for the draft patch.

1 files changed, 5 insertions, 1 deletions

diff --git a/django/core/management/validation.py b/django/core/management/validation.py
index f49a3c2232..587d3a0ad7 100644
--- a/django/core/management/validation.py
+++ b/django/core/management/validation.py
@@ -49,12 +49,16 @@ def get_validation_errors(outfile, app=None):
             # No need to perform any other validation checks on a swapped model.
             continue
 
-        # This is the current User model. Check known validation problems with User models
+        # If this is the current User model, check known validation problems with User models
         if settings.AUTH_USER_MODEL == '%s.%s' % (opts.app_label, opts.object_name):
             # Check that the USERNAME FIELD isn't included in REQUIRED_FIELDS.
             if cls.USERNAME_FIELD in cls.REQUIRED_FIELDS:
                 e.add(opts, 'The field named as the USERNAME_FIELD should not be included in REQUIRED_FIELDS on a swappable User model.')
 
+            # Check that the username field is unique
+            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.')
+
         # 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':