further refactor into mixins

ormar/models/mixins/relation_mixin.py

@@ -0,0 +1,68 @@
+import inspect
+from typing import List, Optional, Set, TYPE_CHECKING
+
+from ormar.fields.foreign_key import ForeignKeyField
+
+
+class RelationMixin:
+    if TYPE_CHECKING:  # pragma no cover
+        from ormar import ModelMeta
+
+        Meta: ModelMeta
+        _related_names: Optional[Set]
+        _related_fields: Optional[List]
+
+    @classmethod
+    def extract_db_own_fields(cls) -> Set:
+        related_names = cls.extract_related_names()
+        self_fields = {
+            name for name in cls.Meta.model_fields.keys() if name not in related_names
+        }
+        return self_fields
+
+    @classmethod
+    def extract_related_fields(cls) -> List:
+
+        if isinstance(cls._related_fields, List):
+            return cls._related_fields
+
+        related_fields = []
+        for name in cls.extract_related_names():
+            related_fields.append(cls.Meta.model_fields[name])
+        cls._related_fields = related_fields
+
+        return related_fields
+
+    @classmethod
+    def extract_related_names(cls) -> Set:
+
+        if isinstance(cls._related_names, Set):
+            return cls._related_names
+
+        related_names = set()
+        for name, field in cls.Meta.model_fields.items():
+            if inspect.isclass(field) and issubclass(field, ForeignKeyField):
+                related_names.add(name)
+        cls._related_names = related_names
+
+        return related_names
+
+    @classmethod
+    def _extract_db_related_names(cls) -> Set:
+        related_names = cls.extract_related_names()
+        related_names = {
+            name
+            for name in related_names
+            if cls.Meta.model_fields[name].is_valid_uni_relation()
+        }
+        return related_names
+
+    @classmethod
+    def _exclude_related_names_not_required(cls, nested: bool = False) -> Set:
+        if nested:
+            return cls.extract_related_names()
+        related_names = cls.extract_related_names()
+        related_names = {
+            name for name in related_names if cls.Meta.model_fields[name].nullable
+        }
+        return related_names