fix: provide helpful error when table heading is not configured

When using tables from non-activated schemas, operations that access
the heading now raise a clear DataJointError instead of confusing
"NoneType has no attribute" errors.

Example:
    schema = dj.Schema()  # Not activated
    @Schema
    class MyTable(dj.Manual): ...

    MyTable().heading  # Now raises: "Table `MyTable` is not properly
                       # configured. Ensure the schema is activated..."

Closes #1039

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: dimitri-yatsenko

src/datajoint/table.py

@@ -120,6 +120,22 @@ def table_name(self):
     def class_name(self):
         return self.__class__.__name__
 
+    @property
+    def heading(self):
+        """
+        Return the table's heading, or raise a helpful error if not configured.
+
+        Overrides QueryExpression.heading to provide a clear error message
+        when the table is not properly associated with an activated schema.
+        """
+        if self._heading is None:
+            raise DataJointError(
+                f"Table `{self.__class__.__name__}` is not properly configured. "
+                "Ensure the schema is activated before using the table. "
+                "Example: schema.activate('database_name') or schema = dj.Schema('database_name')"
+            )
+        return self._heading
+
     @property
     def definition(self):
         raise NotImplementedError("Subclasses of Table must implement the `definition` property")

tests/integration/test_schema.py

@@ -110,6 +110,32 @@ class UndecoratedClass(dj.Manual):
         print(a.full_table_name)
 
 
+def test_non_activated_schema_heading_error():
+    """
+    Tables from non-activated schemas should raise informative errors.
+    Regression test for issue #1039.
+    """
+    # Create schema without activating (no database name)
+    schema = dj.Schema()
+
+    @schema
+    class TableA(dj.Manual):
+        definition = """
+        id : int
+        ---
+        value : float
+        """
+
+    # Accessing heading should raise a helpful error
+    instance = TableA()
+    with pytest.raises(dj.DataJointError, match="not properly configured"):
+        _ = instance.heading
+
+    # Operations that use heading should also raise helpful errors
+    with pytest.raises(dj.DataJointError, match="not properly configured"):
+        _ = instance.primary_key  # Uses heading.primary_key
+
+
 def test_reject_decorated_part(schema_any):
     """
     Decorating a dj.Part table should raise an informative exception.