Using Rails, how can I set my primary key to not be an integer-typed column?

Question

I'm using Rails migrations to manage a database schema, and I'm creating a simple table where I'd like to use a non-integer value as the primary key (in particular, a string). To abstract away from my problem, let's say there's a table employees where employees are identified by an alphanumeric string, e.g. "134SNW".

I've tried creating the table in a migration like this:

create_table :employees, {:primary_key => :emp_id} do |t|
    t.string :emp_id
    t.string :first_name
    t.string :last_name
end

What this gives me is what seems like it completely ignored the line t.string :emp_id and went ahead and made it an integer column. Is there some other way to have rails generate the PRIMARY_KEY constraint (I'm using PostgreSQL) for me, without having to write the SQL in an execute call?

NOTE: I know it's not best to use string columns as primary keys, so please no answers just saying to add an integer primary key. I may add one anyway, but this question is still valid.

Answer 1

you have to use the option :id => false

create_table :employees, :id => false, :primary_key => :emp_id do |t|
    t.string :emp_id
    t.string :first_name
    t.string :last_name
end

Answer 2

Hey,

It looks like it is possible to do using this approach:

create_table :widgets, :id => false do |t|
  t.string :widget_id, :limit => 20, :primary => true

  # other column definitions
end

class Widget < ActiveRecord::Base
  set_primary_key "widget_id"
end

That will make the column widget_id the primary key for the Widget class, then it is up to you to populate the field when objects are created. You should be able to do so using the before create callback.

So something along the lines of

class Widget < ActiveRecord::Base
  set_primary_key "widget_id"

  before_create :init_widget_id

  private
  def init_widget_id
    self.widget_id = generate_widget_id
    # generate_widget_id represents whatever logic you are using to generate a unique id
  end
end

Answer 3

I have one way of handling this. The executed SQL is ANSI SQL so it will likely work on most ANSI SQL compliant relational databases. I have tested that this works for MySQL.

Migration:

create_table :users, :id => false do |t|
    t.string :oid, :limit => 10, :null => false
    ...
end
execute "ALTER TABLE users ADD PRIMARY KEY (oid);"

In your model do this:

class User < ActiveRecord::Base
    set_primary_key :oid
    ...
end

Answer 4

Unfortunately, I've determined it's not possible to do it without using execute.

Why it doesn't work

By examining the ActiveRecord source, we can find the code for create_table:

In schema_definitions.rb:

def create_table(table_name, options={})
  ...
  table_definition.primary_key(options[:primary_key] || Base.get_primary_key(table_name.to_s.singularize)) unless options[:id] == false
  ...
end

So we can see that when you try to specify a primary key in the create_table options, it creates a primary key with that specified name (or, if none is specified, id). It does this by calling the same method you can use inside a table definition block: primary_key.

In schema_statements.rb:

def primary_key(name)
  column(name, :primary_key)
end

This just creates a column with the specified name of type :primary_key. This is set to the following in the standard database adapters:

PostgreSQL: "serial primary key"
MySQL: "int(11) DEFAULT NULL auto_increment PRIMARY KEY"
SQLite: "INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL"

The workaround

Since we're stuck with these as the primary key types, we have to use execute to create a primary key that is not an integer (PostgreSQL's serial is an integer using a sequence):

create_table :employees, {:id => false} do |t|
  t.string :emp_id
  t.string :first_name
  t.string :last_name
end
execute "ALTER TABLE employees ADD PRIMARY KEY (emp_id);"

And as Sean McCleary mentioned, your ActiveRecord model should set the primary key using set_primary_key:

class Employee < ActiveRecord::Base
  set_primary_key :emp_id
  ...
end

Answer 5

I am on Rails 2.3.5 and my following way works with SQLite3

create_table :widgets, { :primary_key => :widget_id } do |t|
  t.string :widget_id

  # other column definitions
end

There is no need for :id => false.