Fix #320 add __cache__ property into PydanticMixin (#625)

* add __cache__ property into PydanticMixin

* add new test for exclude fields on relations

* fix the return type from __cache__  property

* run pre-commit lints/checks

Co-authored-by: collerek <collerek@gmail.com>

ormar/models/mixins/pydantic_mixin.py

@@ -22,6 +22,9 @@ from ormar.queryset.utils import translate_list_to_dict
 
 
 class PydanticMixin(RelationMixin):
+
+    __cache__: Dict[str, Type[pydantic.BaseModel]] = {}
+
     if TYPE_CHECKING:  # pragma: no cover
         __fields__: Dict[str, ModelField]
         _skip_ellipsis: Callable
@@ -68,6 +71,11 @@ class PydanticMixin(RelationMixin):
         fields_to_process.sort(
             key=lambda x: list(cls.Meta.model_fields.keys()).index(x)
         )
+
+        cache_key = f"{cls.__name__}_{str(include)}_{str(exclude)}"
+        if cache_key in cls.__cache__:
+            return cls.__cache__[cache_key]
+
         for name in fields_to_process:
             field = cls._determine_pydantic_field_type(
                 name=name,
@@ -85,6 +93,7 @@ class PydanticMixin(RelationMixin):
         )
         model = cast(Type[pydantic.BaseModel], model)
         cls._copy_field_validators(model=model)
+        cls.__cache__[cache_key] = model
         return model
 
     @classmethod