NMemory Overview

public class MyDatabase : Database
{
	public MyDatabase()
	{
		var members = this.Tables.Create<Member, int>(x => x.Id);
		var groups = base.Tables.Create<Group, int>(g => g.Id);

		this.Members = members;
		this.Groups = groups;

		RelationOptions options = new RelationOptions(
			cascadedDeletion: true);

		var peopleGroupIdIndex = members.CreateIndex(
			new RedBlackTreeIndexFactory(),
			p => p.GroupId);

		this.Tables.CreateRelation(
			groups.PrimaryKeyIndex,
			peopleGroupIdIndex,
			x =>  x ?? -1,
			x => x,
			options);
	}

	public ITable<Member> Members { get; private set; }

	public ITable<Group> Groups { get; private set; }
}

MyDatabase myDatabase = new MyDatabase();

myDatabase.Groups.Insert(new Group() { Id = 1});

myDatabase.Groups.Insert(new Group() { Id = 2});

myDatabase.Members.Insert(new Member() { Id = 1, GroupId = 1, Name = "It's Member_1" });

myDatabase.Members.Insert(new Member() { Id = 2, Name = "It's Member_2" });