How to Mock Database with GoMock? - https://medium.com/@eftal/how-to-mock-database-with-gomock-9bd0a92ffc10

order_test.go

package order

import (
	"context"
	"eftal/medium/mocks"
	"errors"
	"fmt"
	"testing"

	"github.com/golang/mock/gomock"
	"github.com/jackc/pgx/v4"
)

func TestMakeAnOrder(t *testing.T) {
	ctx := context.Background()
	req := &orderRequest{
		TableNumber: "B-1",
		Products: map[string]int{
			"Pizza":          5,
			"Chefs' Special": 3,
			"Coke":           2,
		},
	}

	ctrl := gomock.NewController(t)
	defer ctrl.Finish()

	pool := mocks.NewMockFakePoolInterface(ctrl)
	dummyError := fmt.Errorf("db.BeginTx failed with err: connection failed")
	pool.EXPECT().BeginTx(ctx, pgx.TxOptions{}).Return(nil, dummyError)

	ordersvc := &OrderService{db: pool}
	_, err := ordersvc.Make(ctx, req)
	if !errors.Is(err, dummyError) {
		t.Fatalf("erros doesn't matched!")
	}
}