12/27/2023 0 Comments On delete cascade postgresSQLAlchemy knows this, and decided there is no need to get them from the database. In my test script I created the customers and orders before the DELETE-statement meaning that the order objects are in the session. You would expect three SQL-statements, the first one selecting all orders of the customer. # delete customer orders when customer deletedĬustomer_id = sa.Column(sa.Integer, sa.ForeignKey('customer.id'), nullable=False, index=True)Īssuming a customer with two orders, the following SQL-statements are generated by SQLAlchemy: DELETE FROM "order" WHERE "order".id = ?ĭELETE FROM customer WHERE customer.id = ? Name = sa.Column(sa.String(100), nullable=False) Id = sa.Column(sa.Integer, primary_key=True) We make the Cascade Delete work by adding the following line to the relationship in the Customer: cascade='save-update, merge, delete' The reason is that we do not want orders without a customer floating around. Note that in the ForeignKey Order.customer_id I added the attribute 'nullable=False'. A customer can refer to orders using Customer.orders and an order can refer to a customer using Order.customer. A customer can have zero or more orders and an order is part of only one customer. Customer-Order is a One-To-Many relationship. When we delete a customer, we also want to delete all orders of this customer. This means we do not want 'all' but probably: cascade='save-update, merge, delete' Example: Customer, Order (One-To-Many) There is a warning in the documentation that 'refresh-expire' changes the default behavior and also 'expunge' changes the default behavior. This means we can also write: cascade='all'īut wait, 'all' includes also 'refresh-expire' and 'expunge'. This is very confusing because 'delete' is already part of 'all'. In the example the Cascade Delete line is: cascade='all, delete' The SQLAlchemy documentation then gives an example of Cascade Delete: When the parent is deleted, its children also must be deleted. To keep default behavior, you must add 'save-update' and 'merge' to the 'cascade'-option. This implies that if you were not using the 'cascade'-option and want to add to a 'cascade'-option to a relationship, you are changing (!) the default behavior. The 'cascade'-option has a default value. SQLAlchemy Cascade DeletesĬascade Deletes in SQLAlchemy can be specified in a relationship using the 'cascade'-option. The only way to make sure that we are doing the right thing is to read the documentation, read the documentation again, and create some examples that confirm that we understood. Huh? What? And then after some time: WTF! Help! But I want only recipes!īut there are no recipes. The 'all' symbol is a synonym for 'save-update, merge', 'refresh-expire', 'expunge', 'delete', and using it in conjunction with 'delete-orphan' indicates that the child object should follow along with its parent in all cases, and be deleted once it is no longer associated with that parent. The typical alternative setting for this parameter is either all or more commonly 'all, delete-orphan'. The following scripts create both tables along with their records.The default value of the relationship 'cascade'-option is 'save-update, merge'. Here, an Employee is the parent table, and Payment is the child table. Both tables are related through a foreign key with on delete cascade operation. First, we are going to create two tables named Employee and Payment. Let us understand how we can use the ON DELETE CASCADE clause in the MySQL table. If the ON DELETE CASCADE is defined for one FOREIGN KEY clause only, then cascading operations will throw an error. Next, we define an ON DELETE CASCADE clause for one FOREIGN KEY that must be set for the other to succeed in the cascading operations. Suppose we have created two tables with a FOREIGN KEY in a foreign key relationship, making both tables a parent and child. It is a kind of referential action related to the foreign key. ON DELETE CASCADE clause in MySQL is used to automatically remove the matching records from the child table when we delete the rows from the parent table.
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