Update discriminator to have the correct mapping (#881)

See
https://swagger.io/docs/specification/v3_0/data-models/inheritance-and-polymorphism/#discriminator

When specifying discriminators, mapping must be specified unless the
value of the discriminator is the subtype itself (which in our case is
not.)

The changes in the YAML are self-explanatory.

docs/openapi_generator/strong_typing/classdef.py

@@ -122,10 +122,16 @@ class JsonSchemaAnyOf(JsonSchemaNode):
     anyOf: List["JsonSchemaAny"]
 
 
+@dataclass
+class Discriminator:
+    propertyName: str
+    mapping: Dict[str, str]
+
+
 @dataclass
 class JsonSchemaOneOf(JsonSchemaNode):
     oneOf: List["JsonSchemaAny"]
-    discriminator: Optional[str]
+    discriminator: Optional[Discriminator]
 
 
 JsonSchemaAny = Union[

docs/openapi_generator/strong_typing/schema.py

@@ -456,8 +456,19 @@ class JsonSchemaGenerator:
                 ]
             }
             if discriminator:
+                # for each union type, we need to read the value of the discriminator
+                mapping = {}
+                for union_type in typing.get_args(typ):
+                    props = self.type_to_schema(union_type, force_expand=True)[
+                        "properties"
+                    ]
+                    mapping[props[discriminator]["default"]] = self.type_to_schema(
+                        union_type
+                    )["$ref"]
+
                 ret["discriminator"] = {
                     "propertyName": discriminator,
+                    "mapping": mapping,
                 }
             return ret
         elif origin_type is Literal: