Mocked returned data

With mocked implementation of the interface, you can provide mocked data to satisfy your tests. In the example below, CountRows needs a service, which we injected with the mocked version in the unit test. For the mock to work, we should set an expectation by using .On(). With each test case, we set a different return argument depending on the requirement of the test.

    mockSvc := servicemock.NewService(t)
    mockSvc.On("GetRows", ctx).Return(tt.rows, nil)

main.go

//go:generate mockery --name=Service --dir=. --output=./mocks --outpkg=servicemock --case=snake

// ...

type Service interface {
	GetRows(ctx context.Context) ([]any, error)
}

func CountRows(ctx context.Context, svc Service) (int, error) {
	rows, err := svc.GetRows(ctx)
	if err != nil {
		return 0, err
	}
	return len(rows), nil
}

main_test.go

func TestCountRows(t *testing.T) {
	ctx := context.Background()

	tests := []struct {
		name string
		rows []any
		want int
	}{
		{name: "empty", rows: nil, want: 0},
		{name: "three", rows: []any{"a", "b", "c"}, want: 3},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			mockSvc := servicemock.NewService(t)
			mockSvc.On("GetRows", ctx).Return(tt.rows, nil)

			got, err := CountRows(ctx, mockSvc)
			require.NoError(t, err)
			require.Equal(t, tt.want, got)
		})
	}
}

Mocked service below:

// Code generated by mockery v2.53.5. DO NOT EDIT.

package servicemock

import (
	context "context"

	mock "github.com/stretchr/testify/mock"
)

// Service is an autogenerated mock type for the Service type
type Service struct {
	mock.Mock
}

// GetRows provides a mock function with given fields: ctx
func (_m *Service) GetRows(ctx context.Context) ([]interface{}, error) {
	ret := _m.Called(ctx)

	if len(ret) == 0 {
		panic("no return value specified for GetRows")
	}

	var r0 []interface{}
	var r1 error
	if rf, ok := ret.Get(0).(func(context.Context) ([]interface{}, error)); ok {
		return rf(ctx)
	}
	if rf, ok := ret.Get(0).(func(context.Context) []interface{}); ok {
		r0 = rf(ctx)
	} else {
		if ret.Get(0) != nil {
			r0 = ret.Get(0).([]interface{})
		}
	}

	if rf, ok := ret.Get(1).(func(context.Context) error); ok {
		r1 = rf(ctx)
	} else {
		r1 = ret.Error(1)
	}

	return r0, r1
}

// NewService creates a new instance of Service. It also registers a testing interface on the mock and a cleanup function to assert the mocks expectations.
// The first argument is typically a *testing.T value.
func NewService(t interface {
	mock.TestingT
	Cleanup(func())
}) *Service {
	mock := &Service{}
	mock.Mock.Test(t)

	t.Cleanup(func() { mock.AssertExpectations(t) })

	return mock
}