fix isnull typo and formatting

docs/api/models/mixins/merge-model-mixin.md

@@ -19,7 +19,7 @@ in the end all parent (main) models should be unique.
 
 ```python
  | @classmethod
- | merge_instances_list(cls, result_rows: Sequence["Model"]) -> Sequence["Model"]
+ | merge_instances_list(cls, result_rows: List["Model"]) -> List["Model"]
 ```
 
 Merges a list of models into list of unique models.
@@ -41,7 +41,7 @@ populated, each instance is one row in db and some models can duplicate
 
 ```python
  | @classmethod
- | merge_two_instances(cls, one: "Model", other: "Model") -> "Model"
+ | merge_two_instances(cls, one: "Model", other: "Model", relation_map: Dict = None) -> "Model"
 ```
 
 Merges current (other) Model and previous one (one) and returns the current
@@ -51,6 +51,7 @@ If needed it's calling itself recurrently and merges also children models.
 
 **Arguments**:
 
+- `relation_map (Dict)`: map of models relations to follow
 - `one (Model)`: previous model instance
 - `other (Model)`: current model instance
 
@@ -58,3 +59,30 @@ If needed it's calling itself recurrently and merges also children models.
 
 `(Model)`: current Model instance with data merged from previous one.
 
+<a name="models.mixins.merge_mixin.MergeModelMixin._merge_items_lists"></a>
+#### \_merge\_items\_lists
+
+```python
+ | @classmethod
+ | _merge_items_lists(cls, field_name: str, current_field: List, other_value: List, relation_map: Optional[Dict]) -> List
+```
+
+Takes two list of nested models and process them going deeper
+according with the map.
+
+If model from one's list is in other -> they are merged with relations
+to follow passed from map.
+
+If one's model is not in other it's simply appended to the list.
+
+**Arguments**:
+
+- `field_name (str)`: name of the current relation field
+- `current_field (List[Model])`: list of nested models from one model
+- `other_value (List[Model])`: list of nested models from other model
+- `relation_map (Dict)`: map of relations to follow
+
+**Returns**:
+
+`(List[Model])`: merged list of models
+