[Rails] Re: The need of IDs in ActiveRecord

[Eric Anderson]

Joe Van Dyk wrote:
> What is a child table?

CREATE TABLE people (
	id INT PRIMARY KEY AUTO_INCREMENT,
	first_name VARCHAR(30) NOT NULL,
	last_name VARCHAR(40) NOT NULL
)

CREATE TABLE employees (
	person INT UNIQUE NOT NULL,
	department VARCHAR(255) NOT NULL,
	boss INT NOT NULL,
	FOREIGN KEY (person) REFERENCES people.id
	FOREIGN KEY (boss) REFERENCES people.id
)

CREATE TABLE clients (
	person INT UNIQUE NOT NULL,
	company VARCHAR(255) NOT NULL,
	FOREIGN KEY (person) REFERENCES people.id
)

In this case employee and client are child tables of people. For every 
record in employees or clients you will have a record in people, but you 
may have records in people that are not employees or clients (they could 
be vendors, or something else entirely).

So:

        people
       /     \
      /       \
     /         \
employees  clients

Eric
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