next available record id

Question

@user = User.new 

@user.id returns nil but i need to know it before i save. Is it possible ?

Answer 1

YES you can!

I had the same question and investigated the docs. The ability to solve this question is very related to your database type in fact.

Oracle and Postgresql do have useful functions to easily solve this. For MySQL(oracle) or SkySQL(open-source) it seems more complicated (but still possible). I would recommend you avoid using these (MySQL/SkySQL) databases if you need advanced database tools.

First you must try to avoid this situation as much as possible in your application design, as it is dangerous to play with IDs before they get saved.

There may be situation where you don't have any other choice: For instance when two tables are referencing themselves and for security reason you don't allow DELETE or UPDATE on these tables.

When this is the case, you can use the (PostgreSQL, Oracle) database nextval function to generate the next ID number without actually inserting a new record.

Use it in conjunction with the find_by_sql rails method.

To do this with postgreSQL and Rails for instance, choose one of your rails models and add a class method (not an instance method!). This is possible with the "self" word at the beginning of the method name. self tells Ruby that this method is usable only by the class, not by its instance variables (the objects created with 'new').

My Rails model:

class MyToy < ActiveRecord::Base

...

  def self.my_next_id_sequence
    self.find_by_sql "SELECT nextval('my_toys_id_seq') AS my_next_id"
  end
end

When you generate a table with a Rails migration, by default Rails automatically creates a column called id and sets it as the primary key's table. To ensure that you don't get any "duplicate primary key error", Rails automatically creates a sequence inside the database and applies it to the id column. For each new record (row) you insert in your table, the database will calculate by itself what will be the next id for your new record.

Rails names this sequence automatically with the table name append with "_id_seq". The PostgreSQL nextval function must be applied to this sequence as explained here.

Now about find_by_sql, as explained here, it will create an array containing new objects instances of your class. Each of those objects will contain all the columns the SQL statement generates. Those columns will appear in each new object instance under the form of attributes. Even if those attributes don't exist in your class model !

As you wisely realized, our nextval function will only return a single value. So find_by_sql will create an array containing a single object instance with a single attribute. To make it easy to read the value of this very attribute, we will name the resulting SQL column with "my_next_id", so our attribute will have the same name.

So that's it. We can use our new method:

my_resulting_array = MyToy.my_next_id_sequence
my_toy_object = my_resulting_array[0]
my_next_id_value = my_toy_object.my_next_id

And use it to solve our dead lock situation :

my_dog = DogModel.create(:name => 'Dogy', :toy_id => my_next_id_value)

a_dog_toy = MyToy.new(:my_dog_id => my_dog.id)
a_dog_toy.id = my_next_id_value
a_dog_toy.save

Be aware that if you don't use your my_next_id_value this id number will be lost forever. (I mean, it won't be used by any record in the future).

The database doesn't wait on you to use it. If somewhere at any time, your application needs to insert a new record in your my_table_example (maybe at the same time as we are playing with my_next_id_sequence), the database will always assign an id number to this new record immediately following the one you generated with my_next_id_sequence, considering that your my_next_id_value is reserved. This may lead to situations where the records in your my_table_example don't appear to be sorted by the time they were created.

Answer 2

No, you can't get the ID before saving. The ID number comes from the database but the database won't assign the ID until you call save. All this is assuming that you're using ActiveRecord of course.

Answer 3

I had a similar situation. I called the sequence using find_by_sql on my model which returns the model array. I got the id from the first object of the arry. something like below.

Class User < ActiveRecord::Base
        set_primary_key 'user_id'
        alias user_id= id=
    def self.get_sequence_id
        self.find_by_sql "select TEST_USER_ID_SEQ.nextval as contact_id from dual"
    end
end

and on the class on which you reference the user model,

@users = User.get_sequence_id
user = users[0]

Answer 4

Normally the ID is filled from a database sequence automatically.

In rails you can use the after_create event, which gives you access to the object just after it has been saved (and thus it has the ID). This would cover most cases.

When using Oracle i had the case where I wanted to create the ID ourselves (and not use a sequence), and in this post i provide the details how i did that. In short the code:

# a small patch as proposed by the author of OracleEnhancedAdapter: http://blog.rayapps.com/2008/05/13/activerecord-oracle-enhanced-adapter/#comment-240
# if a ActiveRecord model has a sequence with name "autogenerated", the id will not be filled in from any sequence
ActiveRecord::ConnectionAdapters::OracleEnhancedAdapter.class_eval do
  alias_method :orig_next_sequence_value, :next_sequence_value

  def next_sequence_value(sequence_name)
    if sequence_name == 'autogenerated'
      # we assume id must have gotten a good value before insert!
      id
    else
      orig_next_sequence_value(sequence_name)
    end
  end
end

while this solution is specific to Oracle-enhanced, i am assuming the other databases will have a similar method that you could redefine.

So, while it is definitely not advised and you want to be absolutely sure why you would not want to use an id generated by a sequence, if it is needed it is most definitely possible. It is why I love ruby and Ruby on Rails! :)