update docs, add load_all(), tests for load_all, make through field optional

docs_src/relations/docs004.py

@@ -0,0 +1,29 @@
+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 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)
+
+
+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)