Added pydantic serialization to ULID (#21)

* Added serializer to the model.

* Fixed serializer to work only on Json.

* Added serialization tests.

* Changed to to string schema.

Author: Avihais12344

tests/test_ulid.py

@@ -1,3 +1,4 @@
+import json
 import time
 import uuid
 from collections.abc import Callable
@@ -177,3 +178,13 @@ class Model(BaseModel):
     for value in [b"not-enough", "not-enough"]:
         with pytest.raises(ValidationError):
             Model(ulid=value)
+
+    model_dict = model.model_dump()
+    ulid_from_dict = model_dict["ulid"]
+    assert ulid_from_dict == ulid
+    assert isinstance(ulid_from_dict, ULID)
+    assert Model(**model_dict) == model
+
+    model_json = model.model_dump_json()
+    assert isinstance(json.loads(model_json)["ulid"], str)
+    assert Model.model_validate_json(model_json) == model

ulid/__init__.py

@@ -261,6 +261,9 @@ def __get_pydantic_core_schema__(cls, source: Any, handler: GetCoreSchemaHandler
                     core_schema.no_info_plain_validator_function(ULID),
                 ]
             ),
+            serialization=core_schema.to_string_ser_schema(
+                when_used="json-unless-none",
+            ),
         )
 
     @classmethod