flake8 & mypy

core/dbt/contracts/graph/manifest.py

@@ -381,6 +381,14 @@ class NameSearcher(Generic[N]):
                 return model
         return None
 
+    def search_many(self, haystack: Iterable[N]) -> List[N]:
+        """Find multiple entries in the given iterable by name."""
+        found = []
+        for model in haystack:
+            if self._matches(model):
+                found.append(model)
+        return found
+
 
 D = TypeVar('D')
 
@@ -515,6 +523,16 @@ class Manifest:
             }
         }
 
+    def find_nodes_by_name(
+        self, name: str, package: Optional[str] = None
+    ) -> List[ManifestNode]:
+        searcher: NameSearcher = NameSearcher(
+            name, package, NodeType.refable()
+        )
+
+        result = searcher.search_many(self.nodes.values())
+        return result
+
     def find_disabled_by_name(
         self, name: str, package: Optional[str] = None
     ) -> Optional[ManifestNode]:

core/dbt/exceptions.py

@@ -761,6 +761,21 @@ def raise_duplicate_macro_name(node_1, node_2, namespace) -> NoReturn:
     )
 
 
+def raise_ambiguous_ref(node_1, node_2):
+    duped_name = node_1.name
+    get_func = f'{{{{ ref("{duped_name}") }}}}'
+
+    raise_compiler_error(
+        f'The reference {get_func} is ambiguous because there are two nodes '
+        f'with the name "{duped_name}". To fix this, either change the name '
+        'of one of these nodes, or use the two-argument version of ref() '
+        'to specify the package that contains the intended node. Found:\n'
+        f'- {node_1.unique_id} ({node_1.original_file_path})\n'
+        f'- {node_2.unique_id} ({node_2.original_file_path})\n\n'
+        f'Example: {{{{ ref("{node_1.package_name}", "{node_1.name}") }}}}'
+    )
+
+
 def raise_duplicate_resource_name(node_1, node_2):
     duped_name = node_1.name
 

core/dbt/parser/manifest.py

@@ -522,11 +522,22 @@ def _check_resource_uniqueness(
         relation = relation_cls.create_from(config=config, node=node)
         full_node_name = str(relation)
 
-        existing_node = names_resources.get(name)
-        if existing_node is not None:
-            dbt.exceptions.raise_duplicate_resource_name(
-                existing_node, node
-            )
+        # Check refs - is there an unqualified ref to a
+        # model that exists in two different packages?
+        for ref in node.refs:
+            if len(ref) == 1:
+                matched = manifest.find_nodes_by_name(ref[0])
+            else:
+                # two-arg refs are namespaced, so no need
+                # to check for duplicates. Nodes must still
+                # have unique names within a package
+                continue
+
+            if len(matched) > 1:
+                dbt.exceptions.raise_ambiguous_ref(
+                    matched[0],
+                    matched[1]
+                )
 
         existing_alias = alias_resources.get(full_node_name)
         if existing_alias is not None: