How to mock functions in golang

Question

I have written a simple package, which basically consists of a lot of getter functions. Each file in this package corresponds to a service, so for instance the product file, contains functions relating to the product service/db, order file to order service etc. Each function takes as parameters a db resource to the underlying db, and parameters for the sql, eg. productid, name, orderid. Each of the functions returns a struct (eg. Order, product) or an error:

// product.go

package lib

type Product struct {
   ID int
   Name string
   Price float
}

func GetProductById(DB *sql.DB, ID int) (p Product, err error) {
   q := "SELECT * FROM product WHERE id = " + ID
   ...
}

func GetProductByName(DB *sql.DB, name string) (p Product, err error) {
   ...
}

// order.go

package lib

type Order struct {
   ID int
   Date string
   Items []items
}

func GetOrderById(DB *sql.DB, ID int) (o Order, err error) {
   ...
}

The problem is that I'm not able to mock these functions from my main package. What I really like to do, is to rewrite the package, so I somehow can pass to function to a type instead. But I'm not sure how to do this. Especially not when the functions take different input parameters and return different structs. Is there a way to do this?

Answer 1

In Go you can NOT mock a function, or a method declared on a concrete type.

For example:

func F()

func (T) M()

F and M are not mockable in Go.

---

However you can mock function values, whether they are variables, fields on a struct, or parameters passed to other functions.

For example:

var Fn = func() { ... }

type S struct {
    Fn func()
}

func F(Fn func())

Fn in all three cases is mockable.

---

The other thing that you can mock in Go, and the prefered option most of the time, is an interface.

For example:

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

// the real implementater of the interface
type ProductStore struct{}

func (ProductStore) GetProductById(DB *sql.DB, ID int) (p Product, err error) {
    q := "SELECT * FROM product WHERE id = " + ID
    // ...
}

// the mock implementer
type ProductRepositoryMock struct {}

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

Now any piece of code that depends on ProductRepository can be passed a value of type ProductStore when you're in "normal mode" and a value of type ProductRepositoryMock when you're testing.

---

Another option using interfaces, which allows you to keep your functions mostly unchaged is to define an interface that mimics the methods of *sql.DB then use that interface type as the type to be passed to your functions, implement a mock version of that interface and use that during testing.

For example:

type DBIface interface {
    Query(query string, args ...interface{}) (*sql.Rows, error)
    // ...
    // It's enough to implement only those methods that
    // the functions that depend on DBIface actually use.
    // If none of your functions ever calls SetConnMaxLifetime
    // you don't need to declare that method on the DBIface type.
}

type DBMock struct {}

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

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

DB parameter to GetProductByName is now mockable.