ormar/docs/api/query-set/prefetch-query.md

queryset.prefetch_query

sort_models

sort_models(models: List["Model"], orders_by: Dict) -> List["Model"]

Since prefetch query gets all related models by ids the sorting needs to happen in python. Since by default models are already sorted by id here we resort only if order_by parameters was set.

Arguments:

• models (List[tests.test_prefetch_related.Division]): list of models already fetched from db
• orders_by (Dict[str, str]): order by dictionary

Returns:

(List[tests.test_prefetch_related.Division]): sorted list of models

set_children_on_model

set_children_on_model(model: "Model", related: str, children: Dict, model_id: int, models: Dict, orders_by: Dict) -> None

Extract ids of child models by given relation id key value.

Based on those ids the actual children model instances are fetched from already fetched data.

If needed the child models are resorted according to passed orders_by dict.

Also relation is registered as each child is set as parent related field name value.

Arguments:

• model (Model): parent model instance
• related (str): name of the related field
• children (Dict[int, set]): dictionary of children ids/ related field value
• model_id (int): id of the model on which children should be set
• models (Dict): dictionary of child models instances
• orders_by (Dict): order_by dictionary

PrefetchQuery Objects

class PrefetchQuery()

Query used to fetch related models in subsequent queries. Each model is fetched only ones by the name of the relation. That means that for each prefetch_related entry next query is issued to database.

prefetch_related

 | async prefetch_related(models: Sequence["Model"], rows: List) -> Sequence["Model"]

Main entry point for prefetch_query.

Receives list of already initialized parent models with all children from select_related already populated. Receives also list of row sql result rows as it's quicker to extract ids that way instead of calling each model.

Returns list with related models already prefetched and set.

Arguments:

• models (List[Model]): list of already instantiated models from main query
• rows (List[sqlalchemy.engine.result.RowProxy]): row sql result of the main query before the prefetch

Returns:

(List[Model]): list of models with children prefetched

_extract_ids_from_raw_data

 | _extract_ids_from_raw_data(parent_model: Type["Model"], column_name: str) -> Set

Iterates over raw rows and extract id values of relation columns by using prefixed column name.

Arguments:

• parent_model (Type[Model]): ormar model class
• column_name (str): name of the relation column which is a key column

Returns:

(set): set of ids of related model that should be extracted

_extract_ids_from_preloaded_models

 | _extract_ids_from_preloaded_models(parent_model: Type["Model"], column_name: str) -> Set

Extracts relation ids from already populated models if they were included in the original query before.

Arguments:

• parent_model (Type["Model"]): model from which related ids should be extracted
• column_name (str): name of the relation column which is a key column

Returns:

(set): set of ids of related model that should be extracted

_extract_required_ids

 | _extract_required_ids(parent_model: Type["Model"], reverse: bool, related: str) -> Set

Delegates extraction of the fields to either get ids from raw sql response or from already populated models.

Arguments:

• parent_model (Type["Model"]): model from which related ids should be extracted
• reverse (bool): flag if the relation is reverse
• related (str): name of the field with relation

Returns:

(set): set of ids of related model that should be extracted

_get_filter_for_prefetch

 | _get_filter_for_prefetch(parent_model: Type["Model"], target_model: Type["Model"], reverse: bool, related: str) -> List

Populates where clause with condition to return only models within the set of extracted ids.

If there are no ids for relation the empty list is returned.

Arguments:

• parent_model (Type["Model"]): model from which related ids should be extracted
• target_model (Type["Model"]): model to which relation leads to
• reverse (bool): flag if the relation is reverse
• related (str): name of the field with relation

Returns:

(List[sqlalchemy.sql.elements.TextClause]):

_populate_nested_related

 | _populate_nested_related(model: "Model", prefetch_dict: Dict, orders_by: Dict) -> "Model"

Populates all related models children of parent model that are included in prefetch query.

Arguments:

• model (Model): ormar model instance
• prefetch_dict (Dict): dictionary of models to prefetch
• orders_by (Dict): dictionary of order bys

Returns:

(Model): model with children populated

_prefetch_related_models

 | async _prefetch_related_models(models: Sequence["Model"], rows: List) -> Sequence["Model"]

Main method of the query.

Translates select nad prefetch list into dictionaries to avoid querying the same related models multiple times.

Keeps the list of already extracted models.

Extracts the related models from the database and later populate all children on each of the parent models from list.

Arguments:

• models (List[Model]): list of parent models from main query
• rows (List[sqlalchemy.engine.result.RowProxy]): raw response from sql query

Returns:

(List[Model]): list of models with prefetch children populated

_extract_related_models

 | async _extract_related_models(related: str, target_model: Type["Model"], prefetch_dict: Dict, select_dict: Dict, excludable: "ExcludableItems", orders_by: Dict) -> None

Constructs queries with required ids and extracts data with fields that should be included/excluded.

Runs the queries against the database and populated dictionaries with ids and with actual extracted children models.

Calls itself recurrently to extract deeper nested relations of related model.

Arguments:

• related (str): name of the relation
• target_model (Type[Model]): model to which relation leads to
• prefetch_dict (Dict): prefetch related list converted into dictionary
• select_dict (Dict): select related list converted into dictionary
• fields (Union[Set[Any], Dict[Any, Any], None]): fields to include
• exclude_fields (Union[Set[Any], Dict[Any, Any], None]): fields to exclude
• orders_by (Dict): dictionary of order bys clauses

Returns:

(None): None

_run_prefetch_query

 | async _run_prefetch_query(target_field: "BaseField", excludable: "ExcludableItems", filter_clauses: List, related_field_name: str) -> Tuple[str, str, List]

Actually runs the queries against the database and populates the raw response for given related model.

Returns table prefix as it's later needed to eventually initialize the children models.

Arguments:

• target_field ("BaseField"): ormar field with relation definition
• filter_clauses (List[sqlalchemy.sql.elements.TextClause]): list of clauses, actually one clause with ids of relation

Returns:

(Tuple[str, List]): table prefix and raw rows from sql response

_get_select_related_if_apply

 | @staticmethod
 | _get_select_related_if_apply(related: str, select_dict: Dict) -> Dict

Extract nested related of select_related dictionary to extract models nested deeper on related model and already loaded in select related query.

Arguments:

• related (str): name of the relation
• select_dict (Dict): dictionary of select related models in main query

Returns:

(Dict): dictionary with nested related of select related

_update_already_loaded_rows

 | _update_already_loaded_rows(target_field: "BaseField", prefetch_dict: Dict, orders_by: Dict) -> None

Updates models that are already loaded, usually children of children.

Arguments:

• target_field ("BaseField"): ormar field with relation definition
• prefetch_dict (Dict): dictionaries of related models to prefetch
• orders_by (Dict): dictionary of order by clauses by model

_populate_rows

 | _populate_rows(rows: List, target_field: "ForeignKeyField", parent_model: Type["Model"], table_prefix: str, exclude_prefix: str, excludable: "ExcludableItems", prefetch_dict: Dict, orders_by: Dict) -> None

Instantiates children models extracted from given relation.

Populates them with their own nested children if they are included in prefetch query.

Sets the initialized models and ids of them under corresponding keys in already_extracted dictionary. Later those instances will be fetched by ids and set on the parent model after sorting if needed.

Arguments:

• excludable (ExcludableItems): structure of fields to include and exclude
• rows (List[sqlalchemy.engine.result.RowProxy]): raw sql response from the prefetch query
• target_field ("BaseField"): field with relation definition from parent model
• parent_model (Type[Model]): model with relation definition
• table_prefix (str): prefix of the target table from current relation
• prefetch_dict (Dict): dictionaries of related models to prefetch
• orders_by (Dict): dictionary of order by clauses by model