Add delete on cascade to fk_svc_inst_op_svc_instance_id (#2905)

We can see double entries for the same service_instance_id
in the service_instance_operations table. This leads to:
PG::ForeignKeyViolation: ERROR:  update or delete on table
"service_instances" violates foreign key constraint
"fk_svc_inst_op_svc_instance_id" on table "service_instance_operations"

For service bindings, service keys and route bindings the foreign
key constraint from operation to resource contains ON DELETE CASCADE;
for service instances this is missing.

Adding ON DELETE CASCADE to FK fk_svc_inst_op_svc_instance_id in
service_instance_operations

Remove association dependency to be consistent with service binding
operations, route binding operations and service key operations.

app/models/services/service_instance.rb

@@ -24,7 +24,6 @@ class InvalidServiceBinding < StandardError; end
     serialize_attributes :json, :tags
 
     one_to_one :service_instance_operation
-    add_association_dependencies service_instance_operation: :destroy
 
     one_to_many :service_bindings, before_add: :validate_service_binding, key: :service_instance_guid, primary_key: :guid
     one_to_many :service_keys

db/migrations/20220805145100_add_delete_on_cascade_to_fk_svc_inst_op_svc_instance_id.rb

@@ -0,0 +1,18 @@
+Sequel.migration do
+  # Add DELETE CASCADE to foreign key fk_svc_inst_op_svc_instance_id to ensure that 'service instance operations'
+  # are deleted together with the referenced 'service instance'.
+
+  up do
+    alter_table :service_instance_operations do
+      drop_constraint :fk_svc_inst_op_svc_instance_id, type: :foreign_key
+      add_foreign_key [:service_instance_id], :service_instances, key: :id, name: :fk_svc_inst_op_svc_instance_id, on_delete: :cascade
+    end
+  end
+
+  down do
+    alter_table :service_instance_operations do
+      drop_constraint :fk_svc_inst_op_svc_instance_id, type: :foreign_key
+      add_foreign_key [:service_instance_id], :service_instances, key: :id, name: :fk_svc_inst_op_svc_instance_id
+    end
+  end
+end

spec/unit/models/services/service_instance_spec.rb

@@ -183,6 +183,16 @@ module VCAP::CloudController
         expect(ServiceInstanceAnnotationModel.where(id: annotation.id)).to be_empty
       end
 
+      it 'cascade deletes the related ServiceInstanceOperation for this instance' do
+        last_operation = ServiceInstanceOperation.make
+        service_instance.service_instance_operation = last_operation
+
+        service_instance.destroy
+
+        expect(ServiceInstance.find(id: service_instance.id)).to be_nil
+        expect(ServiceInstanceOperation.find(id: last_operation.id)).to be_nil
+      end
+
       it 'creates a DELETED service usage event' do
         expect {
           service_instance.destroy