Fix indent for nested table elements when updating

Fixes #106

tests/test_toml_document.py

@@ -618,3 +618,24 @@ def test_string_output_order_is_preserved_for_out_of_order_tables():
 """
 
     assert expected == doc.as_string()
+
+
+def test_updating_nested_value_keeps_correct_indent():
+    content = """
+[Key1]
+      [key1.Key2]
+      Value1 = 10
+      Value2 = 30
+"""
+
+    doc = parse(content)
+    doc["key1"]["Key2"]["Value1"] = 20
+
+    expected = """
+[Key1]
+      [key1.Key2]
+      Value1 = 20
+      Value2 = 30
+"""
+
+    assert doc.as_string() == expected

tomlkit/items.py

@@ -1018,6 +1018,8 @@ def __getitem__(self, key):  # type: (Union[Key, str]) -> Item
         return self._value[key]
 
     def __setitem__(self, key, value):  # type: (Union[Key, str], Any) -> None
+        fix_indent = key not in self
+
         if not isinstance(value, Item):
             value = item(value)
 
@@ -1027,7 +1029,7 @@ def __setitem__(self, key, value):  # type: (Union[Key, str], Any) -> None
             super(Table, self).__setitem__(key, value)
 
         m = re.match("(?s)^[^ ]*([ ]+).*$", self._trivia.indent)
-        if not m:
+        if not m or not fix_indent:
             return
 
         indent = m.group(1)

tomlkit/parser.py

@@ -1138,9 +1138,11 @@ def _parse_table(
                         )
 
                     if is_aot and i == len(name_parts[1:]) - 1:
-                        table.append(_name, AoT([child], name=table.name, parsed=True))
+                        table.raw_append(
+                            _name, AoT([child], name=table.name, parsed=True)
+                        )
                     else:
-                        table.append(_name, child)
+                        table.raw_append(_name, child)
 
                     table = child
                     values = table.value