我是否应该始终将结构属性定义为可测试性的接口？

Question

As far as I understand the only way to mock structure dependencies in Go is to use interfaces. So my question is: In cases when my struct has methods which perform some actions (when structure is not just a model storing data), should I always define the properties as interfaces in order to properly mock & test it?

Simple example:

type UserService struct {
    userRepository UserRepository
}

func (us *UserService) MaleUsers() []User {
    all := us.userRepository.FindAll()
    maleUsers := []User{}
    for _, u := range all {
        if u.gender == "male" {
            maleUsers = append(maleUsers, u)
        }
    }
    return maleUsers
}

Imagine we have user service which has a dependency: repository. Service has method which gets all users and then filters them by some criteria. By the way filtering logic could go in a separate dependency to avoid filtering in service method (SRP).

Btw. I came from Java world. If this approach of structuring applications is not ideomatic in Go please let me know.

Answer 1

In order to mock the userRepository dependency of your UserService, you are correct in thinking the best approach would be to use an interface.

Firstly, create your interface:

type UserRepository interface {
    FindAll() []Users
}

Then build a mock:

type MockUserRepository struct{}

func (mock MockUserRepository) FindAll() []Users {
    // here you would manually build a slice of users and return it
    return []Users
}

Finally, use this mock as as a dependency in your test case:

func TestMaleUsers(t *testing.T) {
    // compose service using mock
    service := UserService {
        userRepository: MockUserRepository,
    }

    // get output of method call
    users := service.MaleUsers()

    // perform assertions on output
}

This way you have created a mock of your interface which can be used in your tests without having to perform any database calls on your repository.