fix: add current_user_expr() for backend-agnostic user retrieval

- Add current_user_expr() abstract method to BaseAdapter
- MySQL: returns "user()"
- PostgreSQL: returns "current_user"
- Update connection.get_user() to use adapter method

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/adapters/base.py

@@ -879,6 +879,19 @@ def interval_expr(self, value: int, unit: str) -> str:
         """
         ...
 
+    @abstractmethod
+    def current_user_expr(self) -> str:
+        """
+        SQL expression to get the current user.
+
+        Returns
+        -------
+        str
+            SQL expression for current user (e.g., 'user()' for MySQL,
+            'current_user' for PostgreSQL).
+        """
+        ...
+
     @abstractmethod
     def json_path_expr(self, column: str, path: str, return_type: str | None = None) -> str:
         """

src/datajoint/adapters/mysql.py

@@ -783,6 +783,10 @@ def interval_expr(self, value: int, unit: str) -> str:
         # MySQL uses singular unit names
         return f"INTERVAL {value} {unit.upper()}"
 
+    def current_user_expr(self) -> str:
+        """MySQL current user expression."""
+        return "user()"
+
     def json_path_expr(self, column: str, path: str, return_type: str | None = None) -> str:
         """
         Generate MySQL json_value() expression.

src/datajoint/adapters/postgres.py

@@ -907,6 +907,10 @@ def interval_expr(self, value: int, unit: str) -> str:
         unit_plural = unit.lower() + "s" if not unit.endswith("s") else unit.lower()
         return f"INTERVAL '{value} {unit_plural}'"
 
+    def current_user_expr(self) -> str:
+        """PostgreSQL current user expression."""
+        return "current_user"
+
     def json_path_expr(self, column: str, path: str, return_type: str | None = None) -> str:
         """
         Generate PostgreSQL jsonb_extract_path_text() expression.

src/datajoint/connection.py

@@ -451,7 +451,7 @@ def get_user(self) -> str:
         str
             User name and host as ``'user@host'``.
         """
-        return self.query("SELECT user()").fetchone()[0]
+        return self.query(f"SELECT {self.adapter.current_user_expr()}").fetchone()[0]
 
     # ---------- transaction processing
     @property