Longer example

model.go

type PayrollModel interface {
  GetPayrollForCustomerID(context.Context, CustomerID) (*Payroll, error)
  InsertOrUpdatePayroll(context.Context, UpsertPayrollInput) error
}

type payrollModelImpl struct {
  dbx db.SQLExecutor
}

func (impl *payrollModelImpl) GetPayrollForCustomerID(ctx context.Context, CustomerID) (*Payroll, error) {
  return impl.dbx.Select(...)
}

func (impl *payrollModelImpl) InsertOrUpdatePayroll(ctx context.Context, input UpsertPayrollInput) error {
  if input.SomeField.IsIncluded() {
    // ...
  }
  impl.dbx.Upsert(...)
  return nil
}

func TestWithMock(t *testing.T) {
  // Setup gomock controller to verify expected calls, etc.
  ctrl := gomock.NewController(t)
  defer ctrl.Finish()
  // Create a mock implementation to record expected calls and return values.
  modelImpl := NewMockPayrollModel(ctrl)
  modelImpl.EXPECT().GetPayrollForCustomerID(context.Background(), "some id").Return("test payroll", nil)
  payrollService := payroll_service.NewPayrollService(modelImpl)
  payrollService.DoRealStuffWithPayroll(...)
  // The defer statement above will FAIL if expected calls are missing.
}

func RealCode() {
  dbx := db.ConnectToRealDatabase()
  payrollService := payroll_service.NewPayrollService(newPayrollModel(dbx))
  payrollService.DoRealStuffWithPayroll(...)
}