Add support for pydantic dataclasses (#406)

src/betterproto/plugin/parser.py

@@ -11,6 +11,7 @@ from typing import (
 from betterproto.lib.google.protobuf import (
     DescriptorProto,
     EnumDescriptorProto,
+    FieldDescriptorProto,
     FileDescriptorProto,
     ServiceDescriptorProto,
 )
@@ -30,6 +31,7 @@ from .models import (
     OneOfFieldCompiler,
     OutputTemplate,
     PluginRequestCompiler,
+    PydanticOneOfFieldCompiler,
     ServiceCompiler,
     ServiceMethodCompiler,
     is_map,
@@ -91,6 +93,11 @@ def generate_code(request: CodeGeneratorRequest) -> CodeGeneratorResponse:
             # skip outputting Google's well-known types
             request_data.output_packages[output_package_name].output = False
 
+        if "pydantic_dataclasses" in plugin_options:
+            request_data.output_packages[
+                output_package_name
+            ].pydantic_dataclasses = True
+
     # Read Messages and Enums
     # We need to read Messages before Services in so that we can
     # get the references to input/output messages for each service
@@ -145,6 +152,24 @@ def generate_code(request: CodeGeneratorRequest) -> CodeGeneratorResponse:
     return response
 
 
+def _make_one_of_field_compiler(
+    output_package: OutputTemplate,
+    source_file: "FileDescriptorProto",
+    parent: MessageCompiler,
+    proto_obj: "FieldDescriptorProto",
+    path: List[int],
+) -> FieldCompiler:
+
+    pydantic = output_package.pydantic_dataclasses
+    Cls = PydanticOneOfFieldCompiler if pydantic else OneOfFieldCompiler
+    return Cls(
+        source_file=source_file,
+        parent=parent,
+        proto_obj=proto_obj,
+        path=path,
+    )
+
+
 def read_protobuf_type(
     item: DescriptorProto,
     path: List[int],
@@ -168,11 +193,8 @@ def read_protobuf_type(
                     path=path + [2, index],
                 )
             elif is_oneof(field):
-                OneOfFieldCompiler(
-                    source_file=source_file,
-                    parent=message_data,
-                    proto_obj=field,
-                    path=path + [2, index],
+                _make_one_of_field_compiler(
+                    output_package, source_file, message_data, field, path + [2, index]
                 )
             else:
                 FieldCompiler(