create table dialogs(
    id integer primary key autoincrement not null,
    participant_1 integer not null,
    participant_2 integer not null,
    created_at timestamp not null,
    message_count integer not null default 0,
    unique (participant_1, participant_2)
);

create table dialog_messages(
    dialog_id integer not null,
    id integer not null,  -- unique per dialog, sequential in one dialog
    author_id integer not null,
    content string not null,
    created_at timestamp not null,
    primary key (dialog_id, id)
);