How to use gomock (or similar) to mock/verify calls to the DB?

Go here, using gorm to or/map to the DB (PSQL).

I have the following code:

package dbstuff

import (
    "errors"

  "github.com/google/uuid"
  "github.com/jinzhu/gorm"
    _ "github.com/jinzhu/gorm/dialects/postgres"
)

type OrderPersister struct {
        db *gorm.DB
}

func (p *OrderPersister) GetOrder(id uuid.UUID) (*Order, error) {
        ret := &Order{}

        err := p.db.Table("orders").Where("order_id = ?", id).Scan(ret).Error
        return ret, err
}

I am trying to write a unit test for it like so:

package dbstuff

import (
    "testing"
  "errors"

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

func TestErrInternalServerError(t *testing.T) {

  // given
  id := uuid.New()
  op := OrderPersister{}

  // when
  order, err := op.GetOrder(id)

  // then
  assert.NotNil(t, order)
  assert.NotNil(t, err)

}

When I run this I get invalid memory address or nil pointer dereference errors because I'm not instantiating a setting a *gorm.DB on my OrderPersister instance. Is there an easy way to mock/stub this out so that my test will confirm we attempted to query the orders table and return the or/mapped results?

I will use testify package to writing unit tests for your code. Instead of using concrete type *gorm.DB, declaring DB interface for OrderPersister struct. Due to we CAN'T mock concrete type and its methods in Go. We need to create a abstract layer - the interface.

63622995/db/db.go:

package db

type OrmDBWithError struct {
    OrmDB
    Error error
}

type OrmDB interface {
    Table(name string) OrmDB
    Where(query interface{}, args ...interface{}) OrmDB
    Scan(dest interface{}) *OrmDBWithError
}

63622995/main.go:

package main

import (
    "github.com/google/uuid"
    _ "github.com/jinzhu/gorm/dialects/postgres"
    "github.com/mrdulin/golang/src/stackoverflow/63622995/db"
)

type Order struct {
    order_id string
}

type OrderPersister struct {
    DB db.OrmDB
    //DB *gorm.DB
}

func (p *OrderPersister) GetOrder(id uuid.UUID) (*Order, error) {
    ret := &Order{}

    err := p.DB.Table("orders").Where("order_id = ?", id).Scan(ret).Error
    return ret, err
}

Created mock object for db which implements the OrmDB interface. Then, you can create and pass this mock DB object to OrderPersister struct.

63622995/mocks/db.go:

package mocks

import (
    "github.com/mrdulin/golang/src/stackoverflow/63622995/db"
    "github.com/stretchr/testify/mock"
)

type MockedOrmDB struct {
    mock.Mock
}

func (s *MockedOrmDB) Table(name string) db.OrmDB {
    args := s.Called(name)
    return args.Get(0).(db.OrmDB)
}

func (s *MockedOrmDB) Where(query interface{}, args ...interface{}) db.OrmDB {
    arguments := s.Called(query, args)
    return arguments.Get(0).(db.OrmDB)
}

func (s *MockedOrmDB) Scan(dest interface{}) *db.OrmDBWithError {
    args := s.Called(dest)
    return args.Get(0).(*db.OrmDBWithError)
}

63622995/main_test.go:

package main

import (
    "testing"

    "github.com/google/uuid"
    "github.com/mrdulin/golang/src/stackoverflow/63622995/db"
    "github.com/mrdulin/golang/src/stackoverflow/63622995/mocks"
    "github.com/stretchr/testify/assert"
    "github.com/stretchr/testify/mock"
)

func TestOrderPersister_GetOrder(t *testing.T) {
    assert := assert.New(t)
    t.Run("should get order", func(t *testing.T) {
        testDb := new(mocks.MockedOrmDB)
        id := uuid.New()
        testDb.
            On("Table", "orders").
            Return(testDb).
            On("Where", "order_id = ?", mock.Anything).
            Return(testDb).
            On("Scan", mock.Anything).Run(func(args mock.Arguments) {
            ret := args.Get(0).(*Order)
            ret.order_id = "123"
        }).
            Return(&db.OrmDBWithError{Error: nil})
        op := OrderPersister{DB: testDb}
        got, err := op.GetOrder(id)
        testDb.AssertExpectations(t)
        assert.Nil(err)
        assert.Equal(Order{order_id: "123"}, *got)
    })

    t.Run("should return error", func(t *testing.T) {
        testDb := new(mocks.MockedOrmDB)
        id := uuid.New()
        testDb.
            On("Table", "orders").
            Return(testDb).
            On("Where", "order_id = ?", mock.Anything).
            Return(testDb).
            On("Scan", mock.Anything).
            Return(&db.OrmDBWithError{Error: errors.New("network")})
        op := OrderPersister{DB: testDb}
        got, err := op.GetOrder(id)
        testDb.AssertExpectations(t)
        assert.Equal(Order{}, *got)
        assert.Equal(err.Error(), "network")
    })
}

unit test results:

=== RUN   TestOrderPersister_GetOrder
=== RUN   TestOrderPersister_GetOrder/should_get_order
    TestOrderPersister_GetOrder/should_get_order: main_test.go:32: PASS:    Table(string)
    TestOrderPersister_GetOrder/should_get_order: main_test.go:32: PASS:    Where(string,string)
    TestOrderPersister_GetOrder/should_get_order: main_test.go:32: PASS:    Scan(string)
=== RUN   TestOrderPersister_GetOrder/should_return_error
    TestOrderPersister_GetOrder/should_return_error: main_test.go:49: PASS: Table(string)
    TestOrderPersister_GetOrder/should_return_error: main_test.go:49: PASS: Where(string,string)
    TestOrderPersister_GetOrder/should_return_error: main_test.go:49: PASS: Scan(string)
--- PASS: TestOrderPersister_GetOrder (0.00s)
    --- PASS: TestOrderPersister_GetOrder/should_get_order (0.00s)
    --- PASS: TestOrderPersister_GetOrder/should_return_error (0.00s)
PASS

Process finished with exit code 0

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