It seems to me that you are confusing several distinct issues.
-
Primary Keys. As distributed, Rails only really works with surrogate
integer keys, however there are plugins that accommodate so-called
‘natural’ primary keys. Surrogate key values are assigned by the DBMS
via a sequencer. Primary keys are automatically given the additional
attribute of an INDEX UNIQUE. By default, Rails defines primary keys to
have the attribute name ‘id’ but you can override this in environment.rb
or in the migration
-
Indexes are DBMS specific processes to speedily retrieve subsets of
data. These can be defined in Rails migrations and are independent of
keys. They can be defined as UNIQUE or not and a single index may span
multiple columns. Indexes are not directly accessible via FIND, the
DBMS planner decides whether or not a search by index makes sense.
-
Foreign Key lookups are constrained to the primary key of the
reference table. Thus they must have (or be cast to) the same data type
as the reference key. Rails does not employ DBMS foreign key
constraints, rather ActiveRecord uses a FIND followed by an INSERT to
enforce this requirement, a practice which can fail under high loads. By
default, Rails considers a foreign key field to possess the pattern
_id but this can be modified in environment.rb or overridden
in the model itself.
-
SQL FINDs require neither a key nor an index, only an attribute and a
value. The DBMS planner decides on the basis of the schema and the
statistical data it possesses how best to conduct the search and return
the tuples. It is often the case that with small tables a planner will
simply do a serial search of the entire table in memory rather than
bother with the indices at all.
What I believe that you should do in this case is ignore INDEXES
altogether until you have your design finalized and performance
experience indicates that an INDEX on one of your FIND attributes might
improve response time. The FIND will work with or without an index.
However, if the issue is one of constraining the use of a particular
fname to just one instance then an INDEX UNIQUE is really the only way
to go. (If UNIQUEness is the goal then I have no idea why you would
place such a restriction of fname alone, it seems to me that a composite
key on lname+fname is a far better, albeit not very good, approach).
You should also just stick with the Rails defaults of integer valued
surrogate keys. They work just fine in almost every case even if the
concept may disturb some.
Your migrations should look something like this:
class CreateUsers < ActiveRecord::Migration
def self.up
create_table :users do |t|
t.string :fname, :null => false,
:limit => 40
t.string :lname, :null => false,
:limit => 40
end
# Constrain fname in DBMS
add_index :users, :fname,
:name => :idxU_users_fname,
:unique => true
end
def self.down
remove_index :userss, :name => :idxU_users_fname
drop_table :users
end
end
class CreateEmail < ActiveRecord::Migration
def self.up
create_table :emails do |t|
t.string :e_address, :null => false
t.string :e_type, :null => false
t.integer :user_id, :null => false
end
end
def self.down
drop_table :emails
end
end
The your models look something like this:
class User < ActiveRecord::Base
has_many :emails, :dependent => :destroy
validates_presence_of :fname
validates_presence_of :lname
note that validates_uniqueness_of does not enforce
but only checks uniqueness. It is possible to encounter
a race condition where the entity name is taken between
the check and the insert.
validates_uniqueness_of :fname
end
class Email < ActiveRecord::Base
belongs_to :user
validates_presence_of :e_address
validates_presence_of :e_type
end
Your FINDs belong in the appropriate controllers, either
users_controller.rb or emails_controller.rb. There are also routes to
consider as email is clearly a nested resource of users. What you end
up with in a controller would look something like this:
…
@emails = @emails.find(:all, :conditions => [ :fname =?, @user.fname
])
…
There are a number of Rails tutorials and screencasts on this. Take a
look at railscasts.com and peepcode.com. Just bear in mind that with
SQL, and therefore in Rails, you can always FIND on any attribute
defined in a table not just those that are designated as being keys or
indices.
HTH