public enum Gender
{
    Male, Female
}

// Document class must be MessagePackObject.
[MessagePackObject]
public class Person
{
    [Key(0)]
    public virtual int Id { get; set; }
    [Key(1)]
    public virtual int Age { get; set; }
    [Key(2)]
    public virtual Gender Gender { get; set; }
    [Key(3)]
    public virtual string Name { get; set; }
}

var sampleData = new[]
{
    new Person { Id = 0, Age = 13, Gender = Gender.Male,   Name = "Dana Terry" },
    new Person { Id = 1, Age = 17, Gender = Gender.Male,   Name = "Kirk Obrien" },
    new Person { Id = 2, Age = 31, Gender = Gender.Male,   Name = "Wm Banks" },
    new Person { Id = 3, Age = 44, Gender = Gender.Male,   Name = "Karl Benson" },
    new Person { Id = 4, Age = 23, Gender = Gender.Male,   Name = "Jared Holland" },
    new Person { Id = 5, Age = 27, Gender = Gender.Female, Name = "Jeanne Phelps" },
    new Person { Id = 6, Age = 25, Gender = Gender.Female, Name = "Willie Rose" },
    new Person { Id = 7, Age = 11, Gender = Gender.Female, Name = "Shari Gutierrez" },
    new Person { Id = 8, Age = 63, Gender = Gender.Female, Name = "Lori Wilson" },
    new Person { Id = 9, Age = 34, Gender = Gender.Female, Name = "Lena Ramsey" },
};

// Find(query unique key)
{
    // Memory is like Dictionary<TKey, TValue>
    var byId = new Memory<int, Person>(sampleData, x => x.Id);

    var id5 = byId.Find(5);
    Console.WriteLine(id5.Name); // Jeanne Phelps
}

// FindMany(query index key)
{
    // Memory is also like ILookup<TKey, TElement>
    var byGender = new Memory<Gender, Person>(sampleData, x => x.Gender);

    var females = byGender.FindMany(Gender.Female);
    foreach (var item in females) Console.WriteLine(item.Id); // 5, 6, 7, 8, 9(order is not guranteed).
}

// Multi key index
{
    var byGenderAndAge = new Memory<MemoryKey<Gender, int>, Person>(sampleData, x => MemoryKey.Create(x.Gender, x.Age));

    var maleNearAge30 = byGenderAndAge.FindClosest(Gender.Male, 35, selectLower: true);
    Console.WriteLine(maleNearAge30.Name + ":" + maleNearAge30.Age); // Wm Banks:31

    var males = byGenderAndAge.UseIndex1().FindMany(Gender.Male, ascendant: false); // use index1 only(Gender)
    foreach (var item in males) Console.WriteLine(item.Age); // 44, 31, 23, 17, 13(order is guranteed).
}

// use Database
{
    // Database is a collection of memories, which can be created from DatabaseBuilder
    var databaseBuilder = new DatabaseBuilder();

    // Add key string with data + primary key(for create memory)
    databaseBuilder.Add("person", sampleData, x => x.Id);

    // build database.
    var database = databaseBuilder.Build();

    // load memory
    var byId = database.GetMemory("person", (Person x) => x.Id);

    var id9 = byId.Find(9);
    Console.WriteLine(id9.Name); // Lena Ramsey

    // create secondary index
    var byGenderAndAge = byId.SecondaryIndex("Gender.Age", x => MemoryKey.Create(x.Gender, x.Age));

    // Typed FindMany as ILookup(or Typed Find as IDictionary by ToDictionaryView)
    var byGender = byGenderAndAge.UseIndex1().ToLookupView();
    foreach (var female in byGender[Gender.Female]) Console.WriteLine(female.Age); // 11, 25, 27, 34, 63(order is ascendant)

    // dump database, the binary can save to storage or transport on network.
    var binary = database.Save();
    File.WriteAllBytes("sampledb.db", binary);
}

// Open database from saved binary
{
    var database = Database.Open(File.ReadAllBytes("sampledb.db"));

    // and read memories...
    var memory = database.GetMemory("person", (Person x) => x.Id);

    // re-build
    var databaseBuilder = database.ToBuilder();

    databaseBuilder.Replace("person", memory.FindAll().Where(x => x.Age <= 50), x => x.Id);

    database = databaseBuilder.Build();
    File.WriteAllBytes("sampledb.db", database.Save());
}

// repretents schema and query of database table.
public class PersonSchema
{
    const string Key = "Person"; // unique per application.

    // in Unity, can not use IReadOnlyDictionary so use IDictionary instead.
    public readonly IReadOnlyDictionary<int, Person> ById;
    public readonly ILookup<MemoryKey<Gender, int>, Person> ByGenderAndAge;
    public readonly ILookup<Gender, Person> ByGender;

    // build database.
    public static void Build(DatabaseBuilder builder, IEnumerable<Person> persons)
    {
        builder.Add(Key, persons, x => x.Id);
    }

    // create typedschema.
    public PersonSchema(Database database)
    {
        var memory = database.GetMemory(Key, (Person x) => x.Id);
        var secondaryIndex = memory.SecondaryIndex("ByGenderAndAge", x => MemoryKey.Create(x.Gender, x.Age));

        ById = memory.ToDictionaryView();
        ByGenderAndAge = secondaryIndex.ToLookupView();
        ByGender = secondaryIndex.UseIndex1().ToLookupView();
    }
}

// wrapper of typed schema.
public class TypedDatabase
{
    // any other schemas
    public readonly PersonSchema Person;

    // datasources or datasource factory...
    public static Database Build(IEnumerable<Person> persons)
    {
        var builder = new DatabaseBuilder();
        PersonSchema.Build(builder, persons);
        return builder.Build();
    }

    public TypedDatabase(Database database)
    {
        Person = new PersonSchema(database);
    }
}