How Can I Effectively Mock Functions in Golang?

Mocking Functions in Golang

In Golang, mocking functions declared on concrete types is not directly supported. However, there are several strategies to achieve similar functionality.

Function Values

Function values, including variables, struct fields, or parameters, can be mocked. Consider the following:

var Fn = func() { ... }

type S struct {
    Fn func()
}

func F(Fn func())

Fn in all these instances is mockable.

Interfaces

Mocking interfaces is a preferred option. Create an interface that represents the methods of your target functions:

type ProductRepository interface {
    GetProductById(DB *sql.DB, ID int) (p Product, err error)
}

Implement a real and a mock version of this interface:

// Real implementation
type ProductStore struct{}

func (ProductStore) GetProductById(DB *sql.DB, ID int) (p Product, err error) {
    // ...
}

// Mock implementation
type ProductRepositoryMock struct {}

func (ProductRepositoryMock) GetProductById(DB *sql.DB, ID int) (p Product, err error) {
    // ...
}

Code that depends on ProductRepository can now be passed the real implementation for production use and the mock implementation for testing.

Interface Mimicking

Alternatively, define an interface that mimics the methods of *sql.DB, then use that interface type as the parameter type for your functions:

type DBIface interface {
    Query(query string, args ...interface{}) (*sql.Rows, error)
    // ...
}

type DBMock struct {}

func (DBMock) Query(query string, args ...interface{}) (*sql.Rows, error) {
    // ...
}

func GetProductByName(DB DBIface, name string) (p Product, err error) {
    // ...
}

This makes the DB parameter mockable.

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