Allow creating relationships from objects, not just IDs.

stix2/__init__.py

@@ -223,9 +223,13 @@ class Relationship(_STIXBase):
 
         if not kwargs.get('source_ref'):
             raise ValueError("Missing required field for Relationship: 'source_ref'.")
+        elif isinstance(kwargs['source_ref'], _STIXBase):
+            kwargs['source_ref'] = kwargs['source_ref'].id
 
         if not kwargs.get('target_ref'):
             raise ValueError("Missing required field for Relationship: 'target_ref'.")
+        elif isinstance(kwargs['target_ref'], _STIXBase):
+            kwargs['target_ref'] = kwargs['target_ref'].id
 
         extra_kwargs = list(set(kwargs.keys()) - set(self._properties))
         if extra_kwargs:

stix2/test/test_stix2.py

@@ -324,3 +324,15 @@ def test_invalid_kwarg_to_relationship():
     with pytest.raises(TypeError) as excinfo:
         relationship = stix2.Relationship(my_custom_property="foo", **RELATIONSHIP_KWARGS)
     assert "unexpected keyword arguments: ['my_custom_property']" in str(excinfo)
+
+
+def test_create_relationship_from_objects_rather_than_ids(indicator, malware):
+    relationship = stix2.Relationship(
+        relationship_type="indicates",
+        source_ref=indicator,
+        target_ref=malware,
+    )
+
+    assert relationship.relationship_type == 'indicates'
+    assert relationship.source_ref == INDICATOR_ID
+    assert relationship.target_ref == MALWARE_ID