Add unique index to svc_inst_op on svc_inst_id

* Add unique index on column service_instance_id to prevent double entries for the same
  service_instance
* Remove duplicate service_instance_id entries based on max(updated_at) (=keep the newest entry)
  to prepare for adding a uniqueness constraint
* Drop foreign_key constraint which references service_instance_id (needed for mysql) and
  old index before creating the unique index

Compared to service bindings, service keys and route bindings there is a UNIQUE index
defined for the "resource_id" field; for service instances the UNIQUE keyword is missing.

db/migrations/20220818142407_add_unique_index_to_service_instance_operations_service_instance_id.rb

@@ -0,0 +1,35 @@
+Sequel.migration do
+  up do
+    # Remove duplicate service_instance_operations.service_instance_id to prepare for adding a uniqueness constraint
+    max_updated_at_group = self[:service_instance_operations].
+                           select(Sequel.function(:max, :updated_at)).
+                           group_by(:service_instance_id).
+                           having { count.function.* >= 1 }
+    service_instance_id_group = self[:service_instance_operations].
+                                select(:service_instance_id).
+                                group_by(:service_instance_id).
+                                having { count.function.* >= 1 }
+    self[:service_instance_operations].exclude(id: self[:service_instance_operations].select(:id).
+      where { Sequel.&(Sequel.|({ service_instance_id: service_instance_id_group }, { service_instance_id: nil }),
+                       { updated_at: max_updated_at_group })
+      }).each do |row|
+      self[:service_instance_operations].where(id: row[:id]).delete
+    end
+    # for mysql the foreign_key constraint which references service_instance_id has to be removed before you can
+    # delete the old index from service_instance_id
+    alter_table :service_instance_operations do
+      drop_constraint :fk_svc_inst_op_svc_instance_id, type: :foreign_key
+      drop_index :service_instance_id, name: :svc_instance_id_index
+      add_index :service_instance_id, name: :svc_inst_op_svc_instance_id_unique_index, unique: true
+      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
+      drop_index :service_instance_id, name: :svc_inst_op_svc_instance_id_unique_index
+      add_index :service_instance_id, name: :svc_instance_id_index
+      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_operation_spec.rb

@@ -51,5 +51,37 @@ module VCAP::CloudController
         expect(operation.state).to eq 'finished'
       end
     end
+
+    describe 'when two are created with the same id' do
+      describe 'when a ServiceInstanceOperation exists' do
+        let(:service_instance_attrs) do
+          {
+            name: 'bommel_instance',
+            space: VCAP::CloudController::Space.make
+          }
+        end
+
+        let(:service_instance) { ServiceInstance.create(service_instance_attrs) }
+        let(:operation_attributes2) do
+          {
+            service_instance_id: service_instance.id,
+            state: 'in progress',
+            description: '50% all the time',
+            type: 'create',
+            proposed_changes: {
+              name: 'pizza',
+              service_plan_guid: '1800-pizza',
+            },
+          }
+        end
+        before { ServiceInstanceOperation.create(operation_attributes2) }
+
+        it 'raises an exception when creating another ServiceInstanceOperation' do
+          expect {
+            ServiceInstanceOperation.create(operation_attributes2)
+          }.to raise_error(Sequel::UniqueConstraintViolation)
+        end
+      end
+    end
   end
 end