refactor: move database_prefix deprecation warning to Schema.activate()

Move the DeprecationWarning from Pydantic model validator (fires at
config load time with unhelpful stacklevel) to Schema.activate() where
database_prefix is consumed. The warning now points to the user's
dj.Schema(...) call — exactly where they need to make a change.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kushalbakshi

src/datajoint/schemas.py

@@ -172,6 +172,13 @@ def activate(
             self.connection = connection
         if self.connection is None:
             self.connection = _get_singleton_connection()
+        if self.connection._config.get("database.database_prefix"):
+            warnings.warn(
+                "database_prefix is deprecated and will be removed in DataJoint 2.3. "
+                "Use database.name to select a PostgreSQL database instead.",
+                DeprecationWarning,
+                stacklevel=2,
+            )
         self.database = schema_name
         if create_schema is not None:
             self.create_schema = create_schema

src/datajoint/settings.py

@@ -223,19 +223,6 @@ def set_default_port_from_backend(self) -> "DatabaseSettings":
             self.port = 5432 if self.backend == "postgresql" else 3306
         return self
 
-    @model_validator(mode="after")
-    def warn_database_prefix_deprecated(self) -> "DatabaseSettings":
-        """Emit deprecation warning when database_prefix is set."""
-        if self.database_prefix:
-            import warnings
-
-            warnings.warn(
-                "database_prefix is deprecated and will be removed in DataJoint 2.3. "
-                "Use database.name to select a PostgreSQL database instead.",
-                DeprecationWarning,
-                stacklevel=2,
-            )
-        return self
 
 
 class ConnectionSettings(BaseSettings):

tests/unit/test_settings.py

@@ -800,23 +800,8 @@ def test_database_name_override_context_manager(self):
             assert dj.config.database.name == "override_db"
         assert dj.config.database.name == original
 
-    def test_database_prefix_deprecation_warning(self, monkeypatch):
-        """Non-empty database_prefix emits DeprecationWarning."""
-        import warnings
-
-        from datajoint.settings import DatabaseSettings
-
-        monkeypatch.setenv("DJ_DATABASE_PREFIX", "test_")
-        with warnings.catch_warnings(record=True) as w:
-            warnings.simplefilter("always")
-            DatabaseSettings()
-            deprecation_warnings = [
-                x for x in w if issubclass(x.category, DeprecationWarning) and "database_prefix" in str(x.message)
-            ]
-            assert len(deprecation_warnings) >= 1
-
     def test_database_prefix_empty_no_warning(self):
-        """Empty database_prefix does not emit DeprecationWarning."""
+        """Empty database_prefix does not emit DeprecationWarning at config load."""
         import warnings
 
         from datajoint.settings import DatabaseSettings