refactor methaclass functions into helper files, add docstrings

ormar/models/helpers/models.py

@@ -0,0 +1,39 @@
+from typing import Dict, List, Optional, TYPE_CHECKING, Type
+
+from ormar import ModelDefinitionError
+from ormar.fields.foreign_key import ForeignKeyField
+
+if TYPE_CHECKING:  # pragma no cover
+    from ormar import Model
+
+
+def validate_related_names_in_relations(
+    model_fields: Dict, new_model: Type["Model"]
+) -> None:
+    """
+    Performs a validation of relation_names in relation fields.
+    If multiple fields are leading to the same related model
+    only one can have empty related_name param
+    (populated by default as model.name.lower()+'s').
+    Also related_names have to be unique for given related model.
+
+    :raises: ModelDefinitionError if validation of related_names fail
+    :param model_fields: dictionary of declared ormar model fields
+    :type model_fields: Dict[str, ormar.Field]
+    :param new_model:
+    :type new_model: Model class
+    """
+    already_registered: Dict[str, List[Optional[str]]] = dict()
+    for field in model_fields.values():
+        if issubclass(field, ForeignKeyField):
+            previous_related_names = already_registered.setdefault(field.to, [])
+            if field.related_name in previous_related_names:
+                raise ModelDefinitionError(
+                    f"Multiple fields declared on {new_model.get_name(lower=False)} "
+                    f"model leading to {field.to.get_name(lower=False)} model without "
+                    f"related_name property set. \nThere can be only one relation with "
+                    f"default/empty name: '{new_model.get_name() + 's'}'"
+                    f"\nTip: provide different related_name for FK and/or M2M fields"
+                )
+            else:
+                previous_related_names.append(field.related_name)