Enhance error handling in DBConnection and improve DBConnectionError message formatting

Author: luisggc

sqlcompare/db/connection.py

@@ -196,12 +196,22 @@ def _run(self, sql: str, params: dict[str, Any] | None = None) -> Result:
             res = self.conn.execute(text(sql), params or {})
             return res
         except SQLAlchemyError as e:
+            # Extract just the database error message without the full SQL statement
+            error_msg = str(e)
+            # SQLAlchemy often includes the SQL in square brackets at the end
+            # Format: "error message [SQL: long query here]"
+            if "[SQL:" in error_msg:
+                # Extract just the part before [SQL:
+                db_error = error_msg.split("[SQL:")[0].strip()
+            else:
+                db_error = error_msg
+
             raise DBConnectionError(
-                "SQL execution failed.",
+                db_error,
                 conn_id=self.conn_id,
                 sql=sql,
                 original=e,
-            ) from e
+            ) from None  # Suppress the original exception chain to avoid showing SQL twice
         finally:
             self.last_elapsed_ms = int((time.perf_counter() - t0) * 1000)
 

sqlcompare/db/exceptions.py

@@ -11,3 +11,17 @@ def __init__(
         self.conn_id = conn_id
         self.sql = sql
         self.original = original
+
+    def __str__(self) -> str:
+        # The main error message is already set in __init__, just add SQL context if helpful
+        msg = super().__str__()
+
+        # Optionally show a brief SQL snippet for context
+        if self.sql and len(self.sql) > 200:
+            # Only show snippet if SQL is long (short SQL is fine to show in full)
+            sql_lines = self.sql.strip().split('\n')
+            first_line = sql_lines[0] if sql_lines else self.sql
+            sql_snippet = first_line[:100] + "..." if len(first_line) > 100 else first_line
+            msg += f"\n\nSQL operation: {sql_snippet}"
+
+        return msg