![]() What if there was the use case that we'd like to load Author from theĭatabase, and for each author, we'd like to know how many bestsellers they In case we don't want to load related Authors on every Book fetch, this canĪlso be done on-demand using when writing theĭatabase query. In : book = session.query(Book).get( 1 )Ģ017 - 12 - 17 09: 51 : 04, 034 INFO SELECT books.id AS books_id, books.title AS books_title, thor_id AS books_author_id, books.is_bestseller AS books_is_bestseller, authors_1.id AS authors_1_id,įROM books LEFT OUTER JOIN authors AS authors_1 ON authors_1.id = thor_idĢ017 - 12 - 17 09: 51 : 04, 034 INFO ( 1 ,) class Book ( Base ):Īuthor = relationship( 'Author', backref = 'books', lazy = 'joined' )ĭefining the author relationship this way tells SQLAlchemy that every time aīook is loaded, the related Author should be loaded as well via a LEFT OUTER JOIN. Modifying the author relationship definition as follows. ![]() We can ask SQLAlchemy to load related objects using a JOIN statement, Large (assume 100 objects), then this involves one additional query for eachįortunately, SQLAlchemy lets you specify what kind of loading strategy to use,Īll of which are extensively documented here. Has to execute extra queries to load them. If related objects are lazy loaded, the ORM It can be seen that SQLAlchemy is lazy-loading the author. If we now load a book from the database and check its author, In : book = session.query(Book).get( 1 )Ģ017 - 12 - 17 09: 23 : 39, 833 INFO BEGIN (implicit)Ģ017 - 12 - 17 09: 23 : 39, 833 INFO SELECT books.id AS books_id, books.title AS books_title, thor_id AS books_author_id, books.is_bestseller AS books_is_bestsellerĢ017 - 12 - 17 09: 23 : 39, 833 INFO ( 1 ,)Ģ017 - 12 - 17 09: 23 : 43, 586 INFO SELECT authors.id AS authors_id, authors.name AS authors_nameĢ017 - 12 - 17 09: 23 : 43, 586 INFO ( 1 ,) Session_factory = sessionmaker(bind = engine) from sqlalchemy.engine import create_engineĮngine = create_engine( 'sqlite:///:memory:', echo = True ) To see this in action, the database engine needs to first be configured to echoĪll the queries being executed. The default is set to lazy, which means fetchingĪ Book would not immediately result in fetching the Author. SQLAlchemy defines a bunch of strategies about how such related objects shouldīe loaded from the database. Since we've defined a backref, accessing the books propertyįor an Author object would return a list of Book objects that this author This means thatĪccessing the author property on a Book object would return the associatedĪuthor object. The two models are linked via the author relationship. Is_bestseller = Column(Boolean, default = False )Īuthor = relationship( 'Author', backref = 'books' ) Title = Column(String( 255 ), nullable = False )Īuthor_id = Column(BigInteger, ForeignKey( 'authors.id' ), nullable = False ) Name = Column(String( 255 ), nullable = False ) Id = Column(BigInteger, primary_key = True ) from sqlalchemy import BigInteger, Boolean, Column, ForeignKey, Stringįrom import declarative_base This most definitely involves having a books table, and anĪuthors table. DescriptionĬonsider an application which serves as an online books catalog, something like That makes all the database-interactions in Python applications much easier. ![]() Instead, it adds an abstraction layer on top ![]() On top of that, it does notĪttempt to hide the database layer. Instance where it didn't solve a use case I had. With SQLAlchemy for a bit more than 5 years now and I can't recall a single In this blog post, we'll be using SQLAlchemy, which is an excellent database On the other hand, using an ORMĭoesn't mean you can forget about how the underlying database actually works.Ī personal rule of thumb is - if an ORM is not focusing on hiding the database On the one hand they takeĬare of a lot of common database-related issues. Most web application frameworks these days provide some sort of an ORM to handleĭatabase queries. Into the picture, and how we can work around 2 variants of these problems when In this blog post, I'llĭescribe what this problem exactly is, how ORMs (Object Relational Mappers) come Into when working on a database-backed web application. The "N + 1" selects problem is one of the most common problems one might run Handling the “N + 1 selects” problem in SQLAlchemy Handling the “N + 1 selects” problem in SQLAlchemy | Siddhant Goel
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |