Allow to extend service using shorthand notation. Closes #1989

Signed-off-by: Karol Duleba <mr.fuxi@gmail.com>

compose/config/config.py

@@ -275,15 +275,19 @@ class ServiceLoader(object):
         self.service_dict['environment'] = env
 
     def validate_and_construct_extends(self):
+        extends = self.service_dict['extends']
+        if not isinstance(extends, dict):
+            extends = {'service': extends}
+
         validate_extends_file_path(
             self.service_name,
-            self.service_dict['extends'],
+            extends,
             self.filename
         )
         self.extended_config_path = self.get_extended_config_path(
-            self.service_dict['extends']
+            extends
         )
-        self.extended_service_name = self.service_dict['extends']['service']
+        self.extended_service_name = extends['service']
 
         full_extended_config = pre_process_config(
             load_yaml(self.extended_config_path)

compose/config/fields_schema.json

@@ -57,14 +57,21 @@
         },
 
         "extends": {
-          "type": "object",
+          "oneOf": [
+            {
+              "type": "string"
+            },
+            {
+              "type": "object",
 
-          "properties": {
-            "service": {"type": "string"},
-            "file": {"type": "string"}
-          },
-          "required": ["service"],
-          "additionalProperties": false
+              "properties": {
+                "service": {"type": "string"},
+                "file": {"type": "string"}
+              },
+              "required": ["service"],
+              "additionalProperties": false
+            }
+          ]
         },
 
         "extra_hosts": {"$ref": "#/definitions/list_or_dict"},