Don't update equivalent entities in import_data

Fixes #314

spinedb_api/db_mapping_base.py

@@ -574,7 +574,8 @@ class MappedItemBase(dict):
     """A dictionary that represents a db item."""
 
     fields = {}
-    """A dictionary mapping keys to a tuple of (type, value description)"""
+    """A dictionary mapping keys to a another dict mapping "type" to a Python type,
+    "value" to a description of the value for the key, and "optional" to a bool."""
     _defaults = {}
     """A dictionary mapping keys to their default values"""
     _unique_keys = ()
@@ -597,6 +598,7 @@ class MappedItemBase(dict):
     source key.
     """
     _private_fields = set()
+    """A set with fields that should be ignored in validations."""
 
     def __init__(self, db_map, item_type, **kwargs):
         """
@@ -741,7 +743,12 @@ def _something_to_update(self, other):
         def _convert(x):
             return tuple(x) if isinstance(x, list) else x
 
-        return not all(_convert(self.get(key)) == _convert(value) for key, value in other.items())
+        return not all(
+            _convert(self.get(key)) == _convert(value)
+            for key, value in other.items()
+            if value is not None
+            or self.fields.get(key, {}).get("optional", False)  # Ignore mandatory fields that are None
+        )
 
     def first_invalid_key(self):
         """Goes through the ``_references`` class attribute and returns the key of the first reference

tests/test_import_functions.py

@@ -340,6 +340,27 @@ def test_import_existing_relationship_class_parameter(self):
         db_map.close()
 
 
+class TestImportEntity(unittest.TestCase):
+    def test_import_multi_d_entity_twice(self):
+        db_map = DatabaseMapping("sqlite://", create=True)
+        import_data(
+            db_map,
+            entity_classes=(
+                ("object_class1",),
+                ("object_class2",),
+                ("relationship_class", ("object_class1", "object_class2")),
+            ),
+            entities=(
+                ("object_class1", "object1"),
+                ("object_class2", "object2"),
+                ("relationship_class", ("object1", "object2")),
+            ),
+        )
+        count, errors = import_data(db_map, entities=(("relationship_class", ("object1", "object2")),))
+        self.assertEqual(count, 0)
+        self.assertEqual(errors, [])
+
+
 class TestImportRelationship(unittest.TestCase):
     @staticmethod
     def populate(db_map):