Foregin Key PostgreSQL, indexing?

i posted this but did not get a response.

i would like to add that i am using postgresql. if i use the primary
key i can do look as below but can not do with another type of key.
do i need to index these?

i also read this but it didn’t lead to the solution.
http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/eb70d8c45a6a3837/864be680bb9977e6?lnk

model user
id (automatic primary)
fname
lname
user_rrid (foreign key) - string “123456”
model email
id (automatic primary)
e_addresss
user_rrid (foreign key) - string “123456”
e_type
i would like to find a user by fname
@user = User.find_by_fname(“John”)
then use…
@user.email.e_address
@user.email.e_type
i would also like to go the other way. search email based on
e_address
and find the user.
can you please show how the user.rb and email.rb files should be set
up. i have looked thru the docs and googled and everything i try does
not work.
i have tried using foreign_key but it does not work.
thanks.

http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/eb70d8c45a6a3837/864be680bb9977e6?lnk=gst&q=foreign+key#

On Tue, 2008-04-01 at 20:51 -0700, rashantha wrote:

model user
@user = User.find_by_fname(“John”)
thanks.


doesn’t sound like a foreign key or index issue at all.

model user
has_one :email

create_table “users”, :force => true do |t|
t.column “fname”, :string, :limit => 25
t.column “email_id”, :string, :limit => 25, :null => false
end

model email
belongs_to :user

create_table “emails”, :force => true do |t|
t.column “e_address”, :string, :limit => 32
t.column “e_type”, :string, :limit => 25
t.column “user_id”, :string, :limit => 25, :null => false
end

Craig

On Apr 1, 9:03 pm, Craig W. [email protected] wrote:


doesn’t sound like a foreign key or index issue at all.

model user
has_one :email

create_table “users”, :force => true do |t|
t.column “fname”, :string, :limit => 25
t.column “email_id”, :string, :limit => 25, :null => false
end

here is the problem i don’t want to use the automatic primary key
(email_id) or (user_id)
i have created user_rrid and email_rrid to have a value of string

does this have to be changed to an integer?

can’t you use this as a foreign key?

On Wed, 2008-04-02 at 07:10 -0700, rashantha wrote:

create_table “users”, :force => true do |t|
t.column “fname”, :string, :limit => 25
t.column “email_id”, :string, :limit => 25, :null => false
end

here is the problem i don’t want to use the automatic primary key
(email_id) or (user_id)
i have created user_rrid and email_rrid to have a value of string

does this have to be changed to an integer?


no - a key can be anything unique

can’t you use this as a foreign key?


I would imagine that you can. I’ll let someone else who’s used foreign
key’s answer from here on out since I have never needed to use them.

Craig

On Tue, 2008-04-01 at 21:03 -0700, Craig W. wrote:

i would like to find a user by fname
i have tried using foreign_key but it does not work.
end

model email
belongs_to :user

create_table “emails”, :force => true do |t|
t.column “e_address”, :string, :limit => 32
t.column “e_type”, :string, :limit => 25
t.column “user_id”, :string, :limit => 25, :null => false
end


sorry…correction

both email_id and user_id fields should be :integer, not :string

Craig

Woops, rails does handle it correctly. The problem was you have to
specify foreign key names in lower case or all things turn to custard.

Mark

On Apr 3, 438 am, James B. [email protected]

It seems to me that you are confusing several distinct issues.

  1. 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

  2. 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.

  3. 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.

  4. 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

In Rails (ActiveRecord), it’s better to use integers as primary and
foreign keys. Rails doesn’t seem to like things much if this isn’t the
case.

Julian.

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