Relationship Loading Techniques — SQLAlchemy Documentation
The above wildcard will apply to all relationships not just on Order besides anonymous aliases as targets, the results of which are routed into collections and . from pugliablog.info import relationship, backref . to find all the employees whose name starts with "C", we can use startswith() to achieve our goal: In order to handle a many-to-many relationship between Department and. Module pugliablog.infosion in _literal_as_text Do I need to convert that relation into a hybrid_property, or do When you're inside of @ expression, everything you're doing is towards the goal of producing a.
At the mapping level, this looks like: This is achieved using the joinedload loader option: For an attribute that is guaranteed to have an element, such as a many-to-one reference to a related object where the referencing foreign key is NOT NULL, the query can be made more efficient by using an inner join; this is available at the mapping level via the relationship.
Older versions of SQLAlchemy would convert right-nested joins into subuqeries in all cases.
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Such as above, if the User object we loaded referred to three Address objects, the result of the SQL statement would have had three rows; yet the Query returns only one User object.
As additional rows are received for a User object just loaded in a previous row, the additional columns that refer to new Address objects are directed into additional results within the User. Joined eager loading used for scalar references is however compatible with Query.
It is critical to understand the distinction that while Query. A particular query might start out using all lazy loads. After using it in context, it might be revealed that particular attributes or collections are always accessed, and that it would be more efficient to change the loader strategy for these. The strategy can be changed with no other modifications to the query, the results will remain identical, but fewer SQL statements would be emitted. In theory and pretty much in practicenothing you can do to the Query would make it load a different set of primary or related objects based on a change in loader strategy.
The correct way to load the User records and order by email address is to use Query. We can add joinedload back in, so that there are two joins - one is that which we are ordering on, the other is used anonymously to load the contents of the User. In this case, the two joins most probably appear redundant - which they are. But to see why joinedload does what it does, consider if we were filtering on a particular Address: One will match exactly one row, that of the join of User and Address where Address.
By changing the usage of joinedload to another style of loading, we can change how the collection is loaded completely independently of SQL used to retrieve the actual User rows we want. Below we change joinedload into subqueryload: The operation of subquery eager loading is to emit a second SELECT statement for each relationship to be loaded, across all result objects at once.
This SELECT statement refers to the original SELECT statement, wrapped inside of a subquery, so that we retrieve the same list of primary keys for the primary object being returned, then link that to the sum of all the collection members to load them at once: Secondly, it allows for many collections to be eagerly loaded without producing a single query that has many JOINs in it, which can be even less efficient; each relationship is loaded in a fully separate query.
Configuring Relationships — SQLAlchemy Documentation
Finally, because the additional query only needs to load the collection items and not the lead object, it can use an inner JOIN in all cases for greater query efficiency. Disadvantages of subqueryload include that the complexity of the original query is transferred to the relationship queries, which when combined with the use of a subquery, can on some backends in some cases notably MySQL produce significantly slow queries.
The newer style of loading provided by selectinload solves these limitations of subqueryload. Without it, there is a chance that the inner query could return the wrong rows: Additionally, select IN loading applies itself to subsets of the load result at a time, so unlike joined and subquery eager loading, is compatible with batching of results using Query.
Overall, especially as of the 1. The only scenario in which selectin eager loading is not feasible is when the model is using composite primary keys, and the backend database does not support tuples with IN, which includes SQLite, Oracle and SQL Server. New in version 1. This style of loading emits a SELECT that refers to the primary key values of the parent object inside of an IN clause, in order to load related associations: Because the relatonship between User and Address provides that the primary key values for User can be derived from Address.
Changed in version 1. Things to know about this kind of loading include: In comparison, joined and subquery eager loading always refer to multiple JOINs up to the original parent. As such, taking advantage of a new feature with ColumnOperators. The strategy will only query for at most parent primary key values at a time, as the primary keys are rendered into a large IN expression in the SQL statement. Current database drivers that support simultaneous cursors include SQLite, Postgresql.
This syntax is not supported on every database; currently it is known to be only supported by modern Postgresql and MySQL versions. Therefore selectin loading is not platform-agnostic for composite primary keys. There is no special logic in SQLAlchemy to check ahead of time which platforms support this syntax or not; if run against a non-supporting platform such as SQLitethe database will return an error immediately.
As always, developers should spend time looking at the statements and results generated by their applications in development to check that things are working efficiently. What Kind of Loading to Use?
SQLAlchemy 1.3 Documentation
Lets take two examples, a relationship which references a collection, and a relationship that references a scalar many-to-one reference.
One to Many Collection When using the default lazy loading, if you load objects, and then access a collection on each of them, a total of SQL statements will be emitted, although each statement will typically be a simple SELECT without any joins. When using joined loading, the load of objects and their collections will emit only one SQL statement. See the SQLAlchemy object relational tutorial and mapper configuration documentation for an overview of how this module is used.
This corresponds to a parent-child or associative table relationship. The constructed class is an instance of RelationshipProperty. A typical relationshipused in a classical mapping: This can be used to resolve order-of-declaration and other dependency issues, such as if Child is declared below Parent in the same file: These string arguments are converted into callables that evaluate the string as Python code, using the Declarative class-registry as a namespace.
This allows the lookup of related classes to be automatic via their string name, and removes the need to import related classes at all into the local module space: See also Configuring Relationships - further detail on relationship configuration when using Declarative.
In less common circumstances, the argument may also be specified as an Alias construct, or even a Join construct. When using Declarative, it may also be a string argument noting the name of a Table that is present in the MetaData collection associated with the parent-mapped Table. The secondary keyword argument is typically applied in the case where the intermediary Table is not otherwise expressed in any direct class mapping.
Self-Referential Many-to-Many Relationship - Specifics on using many-to-many in a self-referential case. Association Object - an alternative to secondary when composing association table relationships, allowing additional attributes to be specified on the association table.
New in version 0. This flag is available for applications that make use of attributes.
The other property will be created automatically when the mappers are configured. Can also be passed as a backref object to control the configuration of the new relationship. See also Linking Relationships with Backref - Introductory documentation and examples.
Set to False if the join condition of the relationship has unusual features that might not respond well to statement caching. Changed in version 1.
New in version 1. This defaults to False, which means the default cascade should be used - this default cascade is "save-update, merge". The available cascades are save-update, merge, expunge, delete, delete-orphan, and refresh-expire.