foo.go

package foo

type Foo struct {
}

type DB interface {
	Find(string) (Foo, error)
}

foo_test.go

package foo

import "testing"

type InsertableDB interface {
	DB
	Insert(string, Foo) error
}

func RunDBTests(t *testing.T, db InsertableDB) {
	// Find("asdf") -> returns nothing
	// Insert("asdf", Foo{}) -> successfully inserts
	// Find("asdf") -> returns same Foo inserted above
}

memory.go

package memorydb

import "errors"

type DB struct {
	foos map[string]Foo
}

func New() *DB {
	return &DB{
		foos: make(map[string]Foo),
	}
}

func (db *DB) Find(id string) (Foo, error) {
	if foo, ok := db.foos[id]; ok {
		return foo, nil
	}
	return Foo{}, errors.New("unknown foo")
}

memory_test.go

package memorydb

import "testing"

func (db *DB) Insert(id string, foo Foo) error {
	// insert into db.foos
	return nil
}

func TestMemoryDB(t *testing.T) {
	db := New()
	foo.RunDBTests(t, db)
}

mongo.go

package mongodb

type DB struct {
	// ...
}

func New() DB {
	return &DB{}
}

func (db *DB) Find(id string) (Foo, error) {
	// ...
	return Foo{}, nil
}

mongo_test.go

package foo

import "testing"

func (db *DB) Insert(id string, foo Foo) error {
	// insert using imdb.MongoDB internals
	return nil
}

func TestMongoDB(t *testing.T) {
	db := New()
	foo.RunDBTests(t, db)
}