id    name
-------------
1     post
2     like
3     follow
4     favorite
5     tag
6     share

id             (int)
user_id        (int)
action_id      (tinyint)
target_id      (int)
object_id      (tinyint)
date_created   (datetime)

id    name
-------------
1     media
2     scene
3     brand
4     event
5     user

id            (int)
type          (int)
thumbnail     (varchar)
source        (varchar)
description   (varchar)

id        (int)
user_id   (int)
name      (varchar)
city      (int)
address   (varchar)
starts    (time)
ends      (time)
about     (varchar)

id                (int)
username          (varchar)
profile_picture   (varchar)
location          (int)

id             (int)
user_id        (int)
action_id      (tinyint)
object_id      (tinyint)
date_created   (datetime)

activity_id    (int)
target_id      (int)

id            (int)
type          (int)
thumbnail     (varchar)
source        (varchar)
description   (varchar)

id        (int)
user_id   (int)
name      (varchar)
city      (int)
address   (varchar)
starts    (time)
ends      (time)
about     (varchar)

SELECT
 activity.id,
 activity.user_id,
 activity.action_id,
 action.name,
 activity.object_id,
 object.name,
 media.id as media_id,
 media.type,
 media.thumbnail,
 media.source,
 media.description,
 event.id as event_id,
 event.name,
 ...
FROM
 activity
 LEFT JOIN action ON (action.id = activity.action_id)
 INNER JOIN mediaToActivity ON (mediaToActivity.activity_id = activity.id)
 LEFT JOIN media ON (media.id = mediaToActivity.target_id)
 INNER JOIN eventToActivity ON (eventToActivity.activity_id = activity.id)
 LEFT JOIN event ON (event.id = eventToActivity.target_id)

create table activity (
  id            int,
  user_id       int,
  action_id     int,
  target_id     int,
  object_id     int,
  date_created  datetime
);
create table action (
  id int,
  name varchar(80)
);
create table object (
  id int,
  name varchar(80)
);
create table media (
  id int,
  type int,
  thumbnail varchar(255),
  source varchar(255),
  description varchar(255)
);
create table event (
  id       int,
  user_id   int,
  name      varchar(255),
  city      int,
  address   varchar(255),
  starts    time,
  ends      time,
  about     varchar(255)
);

-- setup
insert into action values (1, "post");
insert into object values (1, "media");
insert into object values (2, "event");

-- new event
insert into event values (1, null, "breakfast", null, "123 main st", null, null, "we will eat");
insert into activity values (1, null, 1, 1, 2,  null);

-- new media
insert into media values (1, null, null, null, "new media");
insert into activity values (2, null, 1, 1, 1,  null);

SELECT *
FROM
 activity
 left join event on (event.id = activity.target_id and activity.object_id = 2)
 left join media on (media.id = activity.target_id and activity.object_id = 1);