How to fix IntegrityError: (psycopg2.errors.ForeignKeyViolation) update or delete on table “users” violates foreign key constraint

Solution

test# \d parent
                              Table "public.parent"
 Column │       Type        │ Collation │ Nullable │           Default            
════════╪═══════════════════╪═══════════╪══════════╪══════════════════════════════
 id     │ integer           │           │ not null │ generated always as identity
 name   │ character varying │           │          │ 
Indexes:
    "parent_name_key" UNIQUE CONSTRAINT, btree (name)
Referenced by:
    TABLE "child" CONSTRAINT "child_pname_fkey" FOREIGN KEY (pname) REFERENCES parent(name)

test# \d child
                               Table "public.child"
 Column │       Type        │ Collation │ Nullable │           Default            
════════╪═══════════════════╪═══════════╪══════════╪══════════════════════════════
 id     │ integer           │           │ not null │ generated always as identity
 pname  │ character varying │           │          │ 
Foreign-key constraints:
    "child_pname_fkey" FOREIGN KEY (pname) REFERENCES parent(name)

test# alter table child alter constraint child_pname_fkey deferrable;
ALTER TABLE
SET CONSTRAINTS
test# begin;
BEGIN
test#* set constraints child_pname_fkey deferred;
SET CONSTRAINTS
test#* update child set pname = 'Alice' where id = 1;
UPDATE 1
test#* update parent set name = 'Alice' where id = 1;
UPDATE 1
test#* commit;
COMMIT
test# alter table child alter constraint child_pname_fkey not deferrable;
ALTER TABLE
test#