Archive for February, 2009

Foreign Key constraints in SPDO

February 25th, 2009 by Sjan Evardsson

While I have long advocated for doing referential integrity in the application space I have also always made use of things like foreign key constraints in PostgreSQL. I know that MySQL has methods for enforcing foreign key constraints in InnoDB tables, but I typically don’t use InnoDB. And until recently I never thought about using SQLite for anything really serious (you can take this to mean I am thinking about using SQLite for something serious). So I began to look into triggers for SQLite to mimic foreign key constraints. This got me thinking about using triggers in MySQL to do the same thing for MyISAM tables. A quick search turned up the answer. I fully expected foreign key constraints in MySQL 6. Of course this thinking comes from the phrase on MySQL documentation for 4.1, 5.0, 5.1, and (sadly) now 6.0:

For storage engines other than InnoDB, MySQL Server parses the FOREIGN KEY syntax in CREATE TABLE statements, but does not use or store it. In the future, the implementation will be extended to store this information in the table specification file so that it may be retrieved by mysqldump and ODBC. At a later stage, foreign key constraints will be implemented for MyISAM tables as well.

Wishful thinking aside, I did get a request to add foreign key constraints to the SPDO implementation, so I think I will get started on adding the following:

  • A SPDOException class to throw back useful information (such as foreign key constraint violation)
  • A method to create tables
  • A method to add foreign key constraints to tables created by SPDO

While I don’t want to get too convoluted with the structure of SPDO, I do want a well-encapsulated and easy-to-use PDO. In that vein the table creation and addition of foreign key constraints may come about via a Table class that provides a few simple methods to do all these things. I tend to determine my data structure before I begin development and build all my tables first, however, if I am careful about how the Table class is constructed I should be able to build all the tables in whatever RDBMS I happen to be using, with the proper triggers and constraints. entirely programmatically.

Portable code

February 1st, 2009 by Sjan Evardsson

When developing applications against a single RDBMS there is no need to worry about portability. However, when you are developing with the idea that the application may be deployed against any of a range of RDBMSes then you need to start thinking about how you formulate your queries, table structures, etc. While it is common practice to have separate classes for working with different databases, there are some things you can do to make that even easier.

ENUMs are EVIL

Developers who spend a lot of time developing against MySQL get into the habit of creating ENUM fields. (I will admit a certain amount of guilt here, too). This is not only not portable, but it doesn’t work quite the way you would expect. Even though the values in the ENUM are meant to be the only values you can insert into that field, MySQL does not enforce this quite the way you might think it should.

As an example:

CREATE TABLE enumevil (
    id INTEGER NOT NULL AUTO_INCREMENT PRIMARY KEY,
    evil ENUM('foo','bar','baz') NOT NULL DEFAULT 'foo'
);
Query OK, 0 rows affected (0.01 sec)
INSERT INTO enumevil (evil) VALUES ('bip');
Query OK, 1 row affected, 1 warning (0.02 sec)
SELECT * FROM enumevil;
+----+------+
| id | evil |
+----+------+
|  1 |      | 
+----+------+
1 row IN SET (0.00 sec)

Rather than spitting back an error, it quietly sets the value of the ENUM field to the empty string. That is not in the list of allowed values nor is it the default. So you are left with checking the values in code before inserting (which you should do anyway – see the next section on referential integrity.)

Instead of using an ENUM field consider this:

CREATE TABLE noevil_enum_choice (
    choice VARCHAR(3) NOT NULL PRIMARY KEY
);
INSERT INTO noevil_enum_choice VALUES ('foo');
INSERT INTO noevil_enum_choice VALUES ('bar');
INSERT INTO noevil_enum_choice VALUES ('baz');
CREATE TABLE noevil (
    id INTEGER NOT NULL AUTO_INCREMENT PRIMARY KEY,
    choice VARCHAR(3) NOT NULL REFERENCES noevil_enum_choice.choice
);

Although MySQL does not enforce referential integrity, this is now something can be easily converted to PostgreSQL as:

CREATE TABLE noevil_enum_choice (
    choice VARCHAR(3) NOT NULL PRIMARY KEY
);
INSERT INTO noevil_enum_choice VALUES ('foo');
INSERT INTO noevil_enum_choice VALUES ('bar');
INSERT INTO noevil_enum_choice VALUES ('baz');
CREATE TABLE noevil (
    id SERIAL PRIMARY KEY,
    choice VARCHAR(3) NOT NULL REFERENCES noevil_enum_choice(choice)
);

And PostgreSQL does enforce referential integrity. This also makes it easier to expand your choice list by just inserting a new row.

Referential Integrity

On the flip-side of the coin, developers who spend a good deal of time developing against PostgreSQL come to rely on the referential integrity built in to that RDBMS. While there is nothing wrong with that, your code should enforce this as well, if you want to be able to move from one RDBMS to another. In the example above we could rely on PostgreSQL’s REFERENCES statement to kick back an error whenever a bad value (like ‘bip’) is inserted into the choice field of the noevil table. However, as soon as you move your application to MySQL or sqlite it will happily insert anything you like into that field (with MySQL truncating it to three characters).

This is why it is important for your applications to take into consideration their own referential integrity. Here’s some python to illustrate:

import string
from spdo import *
 
class noevil(object):
    __init__(self, choice):
        self.db = SPDO('test')
        self.choices = []
        mydict = self.db.select("SELECT * FROM noevil_enum_choice")
        for c in mydict:
            self.choices.append(c['choice'])
        if choice not in choices:
            ex[] = ["Invalid value for choice: ", choice, " Valid options are: "]
            for c in self.choices:
                ex.append[c]
                ex.append[" "]
            raise Exception(string.join(ex, ''))
        # continue with normal processing here

This is by no means the entirety of the topic, or even more than merest tip of the iceberg, but it is a good place to start.