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On delete cascade postgres8/23/2023 The overall subject of this behavior is knownĪs “cascade backrefs”, and represents a change in behavior that was standardized Which a child object that is associated with a Session isĪssigned, will not result in an automatic addition of that parent object to the This effect an object that’s not associated with a Session, upon However, the same operation in reverse will not have Session, will be automatically added to that same Relationship() objects which refer to each other.Īn object that’s not associated with a Session, when assigned toĪn attribute or collection on a parent object that is associated with a The relationship.back_populates or relationship.backref The save-update cascade takes place uni-directionally in the context ofĪ bi-directional relationship, i.e. Behavior of save-update cascade with bi-directional relationships ¶ Session.add() to register an entire structure of objects within The save-update cascade is on by default, and is typically takenįor granted it simplifies code by allowing a single call to but it still gets added to the new session, > address1 in sess2 # because it's still "pending" for flush True remove ( address1 ) # address1 no longer associated with user1 > sess2 = Session () > sess2. close () # user1, address1 no longer associated with sess1 > user1. ORA-14705: unique or primary keys referenced by enabled foreign keys in table "TEST".> user1 = sess1. If we clear down the rows in T3, the TRUNCATE CASCADE still fails, but the DELETE CASCADE works as expected. ORA-14705: unique or primary keys referenced by enabled foreign keys in table "TEST"."T3" ORA-02292: integrity constraint (TEST.T3_T2_FK) violated - child record found To see this, recreate the T3 table as follows and populate the tables again.Ĭonstraint t3_t2_fk foreign key (t2_id) references t2 (id)Īs expected, both the DELETE CASCADE and TRUNCATE CASCADE fail. It's worth noting this difference between the DELETE CASCADE and the TRUNCATE CASCADE commands.Īs you would expect, a ROLLBACK has no effect on a TRUNCATE.īoth the DELETE CASCADE and the TRUNCATE CASCADE fail if any of the relationships in the hierarchy are not defined with the ON DELETE CASCADE clause. Notice all rows have been removed from the dependent tables, even those that were not related to rows in their parent table. CASCADE command succeeds and recursively truncates all the dependent tables. ORA-02266: unique/primary keys in table referenced by enabled foreign keys Rollback the previous deletion to return the data to its original state.Ī normal TRUNCATE command fails with the following error. Notice the rows with null values in the foreign key columns are not deleted, as strictly speaking they were not orphaned by the initial deletion. The presence of the ON DELETE CASCADE relationships allows us to delete from any of the tables, with any dependent child records deleted automatically. Select (select count(*) from t1) as t1_count, Insert into t3 values (2, null, 't3 TWO') Insert into t2 values (2, null, 't2 TWO') Notice that T2 and T3 both have a row that relates back to their respective parent and a row with a null value in the foreign key column. We'll take advantage of this and insert some data for each table. Since the foreign key columns are optional, rows in the dependent tables can have a null value and not be part of the relationship. Create the test tables using ON DELETE CASCADEĬonstraint t2_t1_fk foreign key (t1_id) references t1 (id) on delete cascadeĬonstraint t3_t2_fk foreign key (t2_id) references t2 (id) on delete cascade All of the foreign key columns are optional. All the referential integrity constraints are defined using the ON DELETE CASCADE clause. The following code creates three tables in a grandparent-parent-child relationship. The functionality requires all referential integrity constraints involved to be defined using the ON DELETE CASCADE clause. Oracle 12c introduced the CASCADE clause of the TRUNCATE TABLE command, allowing you to recursively truncate down the tables in a hierarchy. CASCADE in Oracle Database 12c Release 1 (12.1) Home » Articles » 12c » Here TRUNCATE TABLE.
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