How do you structure integration tests in Go that require real databases or external services?

Integration tests verify that your code works with real infrastructure. Go's testing tools make this clean: build tags separate unit from integration tests, TestMain handles setup/teardown, and testcontainers-go spins up real dependencies in Docker.

// integration_test.go
//go:build integration

package repository_test

import (
    "context"
    "testing"
    "github.com/testcontainers/testcontainers-go"
    "github.com/testcontainers/testcontainers-go/modules/postgres"
)

var testDB *sql.DB

// TestMain: shared setup/teardown for the whole package
func TestMain(m *testing.M) {
    ctx := context.Background()

    // Start a real Postgres container
    pgContainer, err := postgres.RunContainer(ctx,
        testcontainers.WithImage("postgres:15"),
        postgres.WithDatabase("testdb"),
        postgres.WithUsername("test"),
        postgres.WithPassword("test"),
    )
    if err != nil { log.Fatalf("container start: %v", err) }
    defer pgContainer.Terminate(ctx)

    dsn, _ := pgContainer.ConnectionString(ctx, "sslmode=disable")
    testDB, err = sql.Open("postgres", dsn)
    if err != nil { log.Fatalf("open db: %v", err) }

    // Run migrations
    if err := runMigrations(testDB); err != nil {
        log.Fatalf("migrate: %v", err)
    }

    os.Exit(m.Run()) // run all tests in the package
}

// Individual integration test using shared testDB
func TestUserRepository_Save(t *testing.T) {
    repo := postgres.NewUserRepository(testDB)
    ctx := context.Background()

    t.Cleanup(func() {
        testDB.ExecContext(ctx, "DELETE FROM users WHERE email = $1",
            "test@example.com")
    })

    user := &User{Name: "Test", Email: "test@example.com"}
    if err := repo.Save(ctx, user); err != nil {
        t.Fatalf("Save: %v", err)
    }
    if user.ID == 0 { t.Error("expected ID to be set after save") }

    got, err := repo.FindByID(ctx, user.ID)
    if err != nil { t.Fatalf("FindByID: %v", err) }
    if got.Email != user.Email {
        t.Errorf("got email %q, want %q", got.Email, user.Email)
    }
}

// Run integration tests:
// go test -tags integration ./...

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