In MySQL how does one create global/general foreign IDs? - mysql

Question

Question:
Is there a way to make the foreign ID point to something more generic than one specific table?
Details:
Often I run into the situation where I have several tables which have nothing to do with each other, but still need a common table (in below examples engine is innodb)
CREATE TABLE IF NOT EXISTS movies
(
id INT NOT NULL auto_increment,
name VARCHAR(100) NOT NULL ,
PRIMARY KEY(id)
);
CREATE TABLE IF NOT EXISTS books
(
id INT NOT NULL auto_increment,
name VARCHAR(100) NOT NULL ,
PRIMARY KEY(id)
);
CREATE TABLE IF NOT EXISTS songs
(
id INT NOT NULL auto_increment,
name VARCHAR(100) NOT NULL ,
PRIMARY KEY(id)
);
CREATE TABLE IF NOT EXISTS news_papers
(
id INT NOT NULL auto_increment,
name VARCHAR(100) NOT NULL ,
PRIMARY KEY(id)
);
CREATE TABLE IF NOT EXISTS scrolls
(
id INT NOT NULL auto_increment,
name VARCHAR(100) NOT NULL ,
PRIMARY KEY(id)
);
CREATE TABLE IF NOT EXISTS sumarian_wheat_tablets
(
id INT NOT NULL auto_increment,
name VARCHAR(100) NOT NULL ,
PRIMARY KEY(id)
);
Now I want to keep a record of every time each is viewed like so
CREATE TABLE IF NOT EXISTS movie_history
(
id INT NOT NULL auto_increment,
foreign_id INT NOT NULL ,
view_date TIMESTAMP DEFAULT now(),
FOREIGN KEY (foreign_id) REFERENCES movies ( id ),
PRIMARY KEY(id)
);
CREATE TABLE IF NOT EXISTS book_history
(
id INT NOT NULL auto_increment,
foreign_id INT NOT NULL ,
view_date TIMESTAMP DEFAULT now(),
FOREIGN KEY (foreign_id) REFERENCES books ( id ),
PRIMARY KEY(id)
);
CREATE TABLE IF NOT EXISTS song_history
(
id INT NOT NULL auto_increment,
foreign_id INT NOT NULL ,
view_date TIMESTAMP DEFAULT now(),
FOREIGN KEY (foreign_id) REFERENCES songs ( id ),
PRIMARY KEY(id)
);
CREATE TABLE IF NOT EXISTS news_paper_history
(
id INT NOT NULL auto_increment,
foreign_id INT NOT NULL ,
view_date TIMESTAMP DEFAULT now(),
FOREIGN KEY (foreign_id) REFERENCES news_papers ( id ),
PRIMARY KEY(id)
);
CREATE TABLE IF NOT EXISTS scroll_history
(
id INT NOT NULL auto_increment,
foreign_id INT NOT NULL ,
view_date TIMESTAMP DEFAULT now(),
FOREIGN KEY (foreign_id) REFERENCES scrolls ( id ),
PRIMARY KEY(id)
);
CREATE TABLE IF NOT EXISTS sumarian_wheat_tablet_history
(
id INT NOT NULL auto_increment,
foreign_id INT NOT NULL ,
view_date TIMESTAMP DEFAULT now(),
FOREIGN KEY (foreign_id) REFERENCES sumarian_wheat_tablets ( id ),
PRIMARY KEY(id)
);
Is there a more correct way to handle such situations without making n new tables? I realize that I can make one history table and copy it over with CREATE TABLE...LIKE... but that still requires making n new tables, plus I have to go in and ALTER the foreign_id.

Answer 1

My first thought is just dump the fk reference and have one history table:
CREATE TABLE history(
base_table VARCHAR,
base_table_id INT,
view_date TIMESTAMP DEFAULT now()
);
But I assume you want the fk to maintain the integrity (question: is this really necessary, or can this be worked around?). I guess you could accomplish this by creating a table of "pks in use". For example:
create a table "keys" with columns id (autoincrement) and base_table_name
create a table "movies", where id is both pk and also a fk to "keys.id" (but not an autoincrement column)
add a "before insert" trigger to "movies" which inserts a record into "keys" returning the generated id to be used as the id for the "movie" record
create a history table with a fk to "keys"
create a "delete" trigger on "movies" which also removes the record from "keys" if you want the integrity maintained, or cascading deletes, etc
So the generated "id" is shared across many tables. There is a school of thought that suggests using a primary key unique across all relations within the database (an "enterprise key"), so it is not unprecedented. Instead of using sequences or autogenerated columns, sometimes a GUID or UUID is used.
This replaces extra history tables with triggers on each base table, which might not be a great thing, depending on your environment. I haven't done this myself, just throwing some thoughts out there, so take it for what its worth.

Answer 2

This depends on the record that you're keeping. If you just want to know hits, add one field to each table that is incremented each time your 'hit' criteria is met (ie, there is a read from a webpage). If you want to hold more information:
CREATE TABLE IF NOT EXISTS view_history
(
id INT NOT NULL,
table VARCHAR NOT NULL,
//other relevant stats to a given view, such as ip and so on.
)
The id and table form a composite key as to what table it refers to.

Answer 3

I don't think there is a way to specify more than one table on a single foreign key.
If you define a single history table, you cannot enforce referential integrity using a single foreign key. You could enforce it programmaticaly as explained here
This describes how to do it for other storage engines that do not support FKs, but could be used as a guide to implement what you need. It suggests creating triggers that will enforce same validations a foreign key would.
Other approach:
CREATE TABLE IF NOT EXISTS history
(
id INT NOT NULL auto_increment,
movie_id INT,
book_id INT,
song_id INT,
news_paper_id INT,
view_date TIMESTAMP DEFAULT now(),
FOREIGN KEY (movie_id) REFERENCES movie ( id ),
FOREIGN KEY (book_id) REFERENCES book ( id ),
FOREIGN KEY (song_id) REFERENCES song ( id ),
FOREIGN KEY (news_paper_id) REFERENCES news_paper ( id ),
PRIMARY KEY(id)
);