for now revert type changes to avoid mypy errors, add validation for through models, clean docs etc

tests/test_through_relations_fail.py

@@ -0,0 +1,51 @@
+# type: ignore
+
+import databases
+import pytest
+import sqlalchemy
+
+import ormar
+from ormar import ModelDefinitionError
+from tests.settings import DATABASE_URL
+
+database = databases.Database(DATABASE_URL, force_rollback=True)
+metadata = sqlalchemy.MetaData()
+
+
+def test_through_with_relation_fails():
+    class BaseMeta(ormar.ModelMeta):
+        database = database
+        metadata = metadata
+
+    class Category(ormar.Model):
+        class Meta(BaseMeta):
+            tablename = "categories"
+
+        id = ormar.Integer(primary_key=True)
+        name = ormar.String(max_length=40)
+
+    class Blog(ormar.Model):
+        class Meta(BaseMeta):
+            pass
+
+        id: int = ormar.Integer(primary_key=True)
+        title: str = ormar.String(max_length=200)
+
+    class PostCategory(ormar.Model):
+        class Meta(BaseMeta):
+            tablename = "posts_x_categories"
+
+        id: int = ormar.Integer(primary_key=True)
+        sort_order: int = ormar.Integer(nullable=True)
+        param_name: str = ormar.String(default="Name", max_length=200)
+        blog = ormar.ForeignKey(Blog)
+
+    with pytest.raises(ModelDefinitionError):
+
+        class Post(ormar.Model):
+            class Meta(BaseMeta):
+                pass
+
+            id: int = ormar.Integer(primary_key=True)
+            title: str = ormar.String(max_length=200)
+            categories = ormar.ManyToMany(Category, through=PostCategory)