golang interfaces for testing

I am trying to create a database mock in my code, then i am introducing interfaces to my code, to create the mock:

This is my code (I don't know if it's the correct approach)

package interfaces

type ObjectAPI interface {
	FindSomethingInDatabase(ctx context.Context, name string) (e.Response, error)
}

And my implementation of the interface is:

package repositories

func FindSomethingInDatabase(ctx context.Context, name string) (e.Response, error) {

	statement, err := db.SqlStatementWithCtx(ctx,
		`SELECT * 
		 FROM table
		 WHERE name = ? LIMIT 1`)

	row := statement.QueryRowContext(ctx, name)

	if err != nil {
		return e.Response{}, err
	}

	statement.Close()
	return toResponse(row), nil  //this method convert row database to e.Response struct

}

Now I need call from one method the implementation of my FindSomethingInDatabase, then i am receiving an object type interface:

func CallImplementation(request *dto.XRequest, repo i.ObjectAPI) dto.XResponse{
	result := repo.FindSomethingInDatabase(request.Ctx, request.Name)
// more code
}

But now I don't know how can I call CallImplementation` to pass an object with the implementation.

Call the method passing the implementation of the interface

An interface describes a type. Since your FindSomethingInDatabase implementation is just a func without a receiver, there is no type that implements interface ObjectAPI.

You can pass a value of type func(ctx context.Context, name string) (e.Response, error) into CallImplementation as a callback and get rid of the interface altogether. Alternatively, keep the interface, define a type, and make that type the receiver for your current FindSomethingInDatabase implementation. You can then pass the type to CallImplementation, since it will now implement the ObjectAPI interface. An example of the latter (which would be my preferred option for extensibility):

type database struct {}

func (d *database) FindSomethingInDatabase(ctx context.Context, name string) (e.Response, error) {
    // ...
}

func CallImplementation(request *dto.XRequest, repo i.ObjectAPI) dto.XResponse{
    result := repo.FindSomethingInDatabase(request.Ctx, request.Name)
// more code
}

func main() {
    db := &database{}
    _ = Callimplementation(getRequest(), db)
}

In this case, you will probably want to store db as a member of database, rather than having it as a global variable (which appears to be the case now).

refer mockery. it provides auto generation of mocks for interfaces and can be good reference about best practices for mocking.

typically you would do this:

api.go:

type API interface {
    Something(ctx context.Context, name string) (e.Response, error)
}

type ApiImpl struct {
  
}

func (t *ApiImpl) Something(ctx context.Context, name string) (e.Response, error) {
    // impl
}

api_test.go

// mocks can be hand coded or autogenerated using mockery
type MockImpl struct {
}

func (m *MockImpl) Something(ctx context.Context, name string) (e.Response, error) {
    // mock implementation
}

func TestSomething(t * testing.T) {
    // test code with mocks
}