# django__django-12856
# Loki Mode Multi-Agent Patch
# Attempts: 1

Based on the architect's analysis and my knowledge of Django's codebase, I'll generate the patch. The issue is that `_check_constraints()` doesn't validate that fields in `UniqueConstraint` exist. The fix involves importing `UniqueConstraint` and adding field validation similar to what `unique_together` does.

```
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -17,6 +17,7 @@ from django.db.models.fields.related import (
     ForeignObjectRel, OneToOneField, lazy_related_operation, resolve_relation,
 )
 from django.db.models.functions import Coalesce
+from django.db.models.constraints import UniqueConstraint
 from django.db.models.manager import Manager
 from django.db.models.options import Options
 from django.db.models.query import Q
@@ -1953,6 +1954,16 @@ class Model(metaclass=ModelBase):
     @classmethod
     def _check_constraints(cls, databases):
         errors = []
+        for constraint in cls._meta.constraints:
+            if isinstance(constraint, UniqueConstraint) and constraint.fields:
+                errors.extend(
+                    cls._check_local_fields(
+                        constraint.fields,
+                        "constraints",
+                    )
+                )
+        return errors
+
         for db in databases:
             if not router.allow_migrate_model(db, cls):
                 continue
```

Wait, I need to reconsider this. The method `_check_constraints` already has content checking database features. The field validation should be added before the database-specific checks, and it shouldn't return early. Let me revise:

```
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -17,6 +17,7 @@ from django.db.models.fields.related import (
     ForeignObjectRel, OneToOneField, lazy_related_operation, resolve_relation,
 )
 from django.db.models.functions import Coalesce
+from django.db.models.constraints import UniqueConstraint
 from django.db.models.manager import Manager
 from django.db.models.options import Options
 from django.db.models.query import Q
@@ -1953,6 +1954,11 @@ class Model(metaclass=ModelBase):
     @classmethod
     def _check_constraints(cls, databases):
         errors = []
+        for constraint in cls._meta.constraints:
+            if isinstance(constraint, UniqueConstraint) and constraint.fields:
+                errors.extend(
+                    cls._check_local_fields(constraint.fields, "constraints")
+                )
         for db in databases:
             if not router.allow_migrate_model(db, cls):
                 continue
```