How to add sequences to a migration and use them in a model?

问题

I want to have a "Customer" Model with a normal primary key and another column to store a custom "Customer Number". In addition, I want the db to handle default Customer Numbers. I think, defining a sequence is the best way to do that. I use PostgreSQL. Have a look at my migration:

class CreateAccountsCustomers < ActiveRecord::Migration
  def up

    say "Creating sequenze for customer number starting at 1002"
    execute 'CREATE SEQUENCE customer_no_seq START 1002;'

    create_table :accounts_customers do |t|
      t.string :type
      t.integer :customer_no, :unique => true
      t.integer :salutation, :limit => 1
      t.string :cp_name_1
      t.string :cp_name_2
      t.string :cp_name_3
      t.string :cp_name_4
      t.string :name_first, :limit => 55
      t.string :name_last, :limit => 55
      t.timestamps
    end

    say "Adding NEXTVAL('customer_no_seq') to column cust_id"
    execute "ALTER TABLE accounts_customers ALTER COLUMN customer_no SET DEFAULT NEXTVAL('customer_no_seq');"

  end

  def down
    drop_table :accounts_customers
    execute 'DROP SEQUENCE IF EXISTS customer_no_seq;'
  end

end

If you know a better "rails-like" approach to add sequences, would be awesome to let me know.

Now, if I do something like

cust = Accounts::Customer.new
cust.save

the field customer_no is not pre filled with the next value of the sequence (should be 1002).

Do you know a good way to integrate sequences? Or is there a good plugin? Cheers to all answers!

回答1:

I have no suggestions for a more 'rails way' of handling custom sequences, but I can tell you why the customer_no field appears not to be being populated after a save.

When ActiveRecord saves a new record, the SQL statement will only return the ID of the new record, not all of its fields, you can see where this happens in the current rails source here https://github.com/rails/rails/blob/cf013a62686b5156336d57d57cb12e9e17b5d462/activerecord/lib/active_record/persistence.rb#L313

In order to see the value you will need to reload the object...

cust = Accounts::Customer.new
cust.save
cust.reload

If you always want to do this, consider adding an after_create hook in to your model class...

class Accounts::Customer < ActiveRecord::Base
  after_create :reload
end

回答2:

I believe that roboles answer is not correct.

I tried to implement this on my application (exactly the same env: RoR+PostgreSQL), and I found out that when save is issued on RoR with the object having empty attributes, it tries to perform an INSERT on the database mentioning that all VALUES shall be set to NULL. The problem is the way PostgreSQL handles NULLs: in this case, the new row will be created but with all values empty, i.e. the DEFAULT will be ignored. If save only wrote on the INSERT statement attributes filled on RoR, this would work fine.

In other words, and focusing only on the type and customer_no attribute mentioned above, this is the way PostgreSQL behaves:

SITUATION 1:

INSERT INTO accounts_customers (type, customer_no) VALUES (NULL, NULL);

(this is how Rails' save works)

Result: a new row with empty type and empty customer_no

SITUATION 2:

INSERT INTO accounts_customers (type) VALUES (NULL);

Result: a new row with empty type and customer_no filled with the sequence's NEXTVAL

I have a thread going on about this, check it out at:

Ruby on Rails+PostgreSQL: usage of custom sequences

回答3:

I faced a similar problem, but I also put :null => false on the field hopping that it will be auto-populated with nextval.

Well, in my case AR was still trying to insert NULL if no attribute was supplied in the request, and this resulted in an exception for not-null constraint violation.

Here's my workaround. I just deleted this attribute key from @attributes and @changed_attributes and in this case postgres correctly put the expected sequence nextval.

I've put this in the model:

before_save do
  if (@attributes["customer_no"].nil? || @attributes["customer_no"].to_i == 0)
    @attributes.delete("customer_no")
    @changed_attributes.delete("customer_no")
  end
end

Rails 3.2 / Postgres 9.1