improved error reporting on bad assignment

fixes #670

src/inmanta/ast/statements/assign.py

@@ -124,6 +124,9 @@ def resume(self,
                queue: QueueScheduler,
                target: ResultVariable) -> None:
         instance = self.instance.execute(requires, resolver, queue)
+        if not isinstance(instance, Instance):
+            raise TypingException(self, "The object at %s is not an Entity but a %s with value %s" %
+                                  (self.instance, type(instance), instance))
         var = instance.get_attribute(self.attribute_name)
         if self.list_only and not var.is_multi():
             raise TypingException(self, "Can not use += on relations with multiplicity 1")

tests/test_compiler_errors.py

@@ -15,6 +15,7 @@
 
     Contact: code@inmanta.com
 """
+from conftest import snippetcompiler
 
 
 def test_plugin_excn(snippetcompiler):
@@ -382,3 +383,16 @@ def test_610_multi_add(snippetcompiler):
         """,
         "The object __config__::A (instantiated at {dir}/main.cf:13) is not complete:"
         " attribute b ({dir}/main.cf:11:11) requires 2 values but only 1 are set")
+
+
+def test_670_assign_on_relation(snippetcompiler):
+    snippetcompiler.setup_for_error_re(
+        """
+        h = std::Host(name="test", os=std::linux)
+        f = std::ConfigFile(host=h, path="a", content="")
+
+        h.files.path = "1"
+
+        """,
+        "The object at h.files is not an Entity but a <class 'list'> with value \[std::ConfigFile [0-9a-fA-F]+\]"
+        " \(reported in h.files.path = '1' \({dir}/main.cf:5\)\)")