how to stub method called in another method in same struct - unit-testing

Question

Code here:
package main
import (
"fmt"
)
type Connector struct {}
func (c *Connector) Pool() (interface{}, error) {
err := c.ping()
if err != nil {
fmt.Println("error handle logic");
return nil, err
}
fmt.Println("success logic")
return 1, nil
}
func (c *Connector) ping() error {
var err error
// err = some side-effect RPC operation
if err != nil {
return err
}
return nil
}
Now, I want to test the Pool method of Connector struct. Since the ping method has some side-effect RPC operation, I need to stub it and its returned value so that I can test each code branch in the Pool method.
success test case pseudo-code:
Stub(c, "ping").Return(nil)
c.Pool()
Expect(fmt.Println).ToBeCalledWith("success logic")
failure test case pseudo-code:
Stub(c, "ping").Return(errors.New("test"))
c.Pool()
Expect(fmt.Println).ToBeCalledWith("error handle logic")

Answer 1

We need to follow Dependency inversion principle, it will make the code easier to test.
Details (concrete implementations) should depend on abstractions.
Refactored code:
connector.go:
package connector
import (
"fmt"
)
type Pinger interface {
Ping() error
}
type Connector struct {
DB Pinger
}
func (c *Connector) Pool() (interface{}, error) {
err := c.DB.Ping()
if err != nil {
fmt.Println("error handle logic")
return nil, err
}
fmt.Println("success logic")
return 1, nil
}
Now, create mocked DB which implements the Pinger interface using stretchr/testify package. Then, pass this mocked DB to Connector struct, it's some kind of dependency injection.
Then we can "mock" Ping method with different return values.
connector_test.go:
package connector_test
import (
"fmt"
"testing"
connector "github.com/mrdulin/golang/src/stackoverflow/62035606"
"github.com/stretchr/testify/mock"
"github.com/stretchr/testify/require"
)
type MockedDB struct {
mock.Mock
}
func (db *MockedDB) Ping() error {
args := db.Called()
return args.Error(0)
}
func TestConnector_Pool(t *testing.T) {
t.Run("should verifies connection to the database is still alive", func(t *testing.T) {
testDB := new(MockedDB)
c := connector.Connector{DB: testDB}
testDB.On("Ping").Return(nil)
pool, err := c.Pool()
require.Nil(t, err, nil)
require.Equal(t, 1, pool)
})
t.Run("should return error if connection to the database is not alive", func(t *testing.T) {
testDB := new(MockedDB)
c := connector.Connector{DB: testDB}
testDB.On("Ping").Return(fmt.Errorf("network"))
pool, err := c.Pool()
require.Error(t, err, "network")
require.Nil(t, pool)
})
}
unit test result:
=== RUN TestConnector_Pool
=== RUN TestConnector_Pool/should_verifies_connection_to_the_database_is_still_alive
success logic
=== RUN TestConnector_Pool/should_return_error_if_connection_to_the_database_is_not_alive
error handle logic
--- PASS: TestConnector_Pool (0.00s)
--- PASS: TestConnector_Pool/should_verifies_connection_to_the_database_is_still_alive (0.00s)
--- PASS: TestConnector_Pool/should_return_error_if_connection_to_the_database_is_not_alive (0.00s)
PASS
coverage: 100.0% of statements
ok github.com/mrdulin/golang/src/stackoverflow/62035606 0.364s
Coverage report: