Fix "parent" AttributeError when calling --collapse-root-models (#1432)

Fixes #1419

datamodel_code_generator/parser/base.py

@@ -868,7 +868,9 @@ def __collapse_root_models(
                     data_type.remove_reference()
 
                     root_type_model.reference.children = [
-                        c for c in root_type_model.reference.children if c.parent
+                        c
+                        for c in root_type_model.reference.children
+                        if getattr(c, 'parent', None)
                     ]
 
                     if not root_type_model.reference.children:

tests/data/expected/main/main_collapse_root_models_with_references_to_flat_types/output.py

@@ -0,0 +1,13 @@
+# generated by datamodel-codegen:
+#   filename:  flat_type.jsonschema
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Optional
+
+from pydantic import BaseModel
+
+
+class FooModel(BaseModel):
+    foo: Optional[str] = None

tests/data/openapi/flat_type.jsonschema

@@ -0,0 +1,16 @@
+{
+  "title": "Foo",
+  "$schema": "http://json-schema.org/schema#",
+  "description": "",
+  "type": "object",
+  "properties": {
+    "foo": {
+      "$ref": "#/definitions/foo"
+    }
+  },
+  "definitions": {
+    "foo": {
+      "type": "string"
+    }
+  }
+}

tests/test_main.py

@@ -4339,6 +4339,31 @@ def test_main_collapse_root_models_field_constraints():
         )
 
 
+@freeze_time('2019-07-26')
+def test_main_collapse_root_models_with_references_to_flat_types():
+    with TemporaryDirectory() as output_dir:
+        output_file: Path = Path(output_dir) / 'output.py'
+        return_code: Exit = main(
+            [
+                '--input',
+                str(OPEN_API_DATA_PATH / 'flat_type.jsonschema'),
+                '--output',
+                str(output_file),
+                '--collapse-root-models',
+            ]
+        )
+
+        assert return_code == Exit.OK
+        assert (
+            output_file.read_text()
+            == (
+                EXPECTED_MAIN_PATH
+                / 'main_collapse_root_models_with_references_to_flat_types'
+                / 'output.py'
+            ).read_text()
+        )
+
+
 @freeze_time('2019-07-26')
 def test_main_openapi_max_items_enum():
     with TemporaryDirectory() as output_dir: