TinyTds::Error: Cannot insert the value NULL into column 'ID' - ruby-on-rails

Question

My Ruby on Rails system is moving from Oracle to Microsoft SQL Server 2012. The back end database has already been converted by a third part from Oracle to Microsoft SQL Server. I have no control over the schema structure. This cannot be changed.
Using activerecord-sqlserver-adapter, tiny_tds and Freetds I can connect to the new database. Most DB requests work well.
However, many of the tables have ID primary keys that are auto incremented by SQL Server, in other words, PK IDENTITY(1,1) columns. The same code that worked with Oracle connections fails under SQL Server with the following error:
TinyTds::Error: Cannot insert the value NULL into column 'ID'
I know why it is doing this as the ID column is indeed a primary_key, and IDENTITY(1,1) column and includes the NOT NULL restriction. That is fine.
Inserting into the table works fine if using raw SQL execute statements where of course I exclude the ID column from the INSERT statement.
However I have spent days googling and I cannot find a way of telling Ruby on Rails not to try and save the ID column.
So I have an instance of a class, #book, that contains
Library::Book(id: integer, title: string, isbn: string ...)
When I do a #book.save! it generates the error above.
#book = Library::Book.new( .. )
:
:
#book.save!
TinyTds::Error: Cannot insert the value NULL into column 'ID'
Rather that resort to bare metal SQL, how do I do things more Railsy and tell it I want to save the record but not try and save the ID field as it is auto incremented? So effectively I am trying to save
Library::Book(title: string, isbn: string ...) if a new insert entry
or
Library::Book(id: integer, title: string, isbn: string ...) if trying to update an entry.
Due to imposed restrictions I am using:
Ruby 2.3.3p222
Rails 4.0.13
activerecord (4.0.13)
activerecord-sqlserver-adapter (4.0.4)
tiny_tds (1.0.2)
Freetds 1.00.27

Answer 1

You can use ActiveRecord::Relation#find_or_initialize_by. This assumes that you have enough attributes known at the time to uniquely identify the record.
That will solve the last part of your question. But it sounds like the id column is not set to auto-increment. You need to set that so when Rails sends a null id the DB will set it properly.
Update: To make the column auto-increment you will need to run a migration. You can then do:
Alter table books modify column id int(11) auto_increment;
Update: If we cannot modify the database we can do:
class Book
private
def create_or_update(*args, &block)
self.id ||= Book.maximum(:id) + 1
super
rescue ActiveRecord::RecordNotUnique
self.id = nil
retry
end
end
It's pretty janky, and I would highly recommend updating the column if possible.