Rails says, "SQL Server does not exist or access denied."

Using SQL Server 2005 with Rails 1.2.6, I got the following (less than helpful) error message.

    OLE error code:80004005 in Microsoft OLE DB Provider for SQL Server

      [DBNETLIB][ConnectionOpen (Connect()).]SQL Server does not exist or access denied.

    HRESULT error code:0x80020009

      Exception occurred.

database.yml:

development:

  adapter: sqlserver

  database: some_database

  host: my_machine.local

  username: some_username

  password: some_password

The Rails driver does this (modified to work in irb):

require 'DBI'

require 'c:\ruby\lib\ruby\site_ruby\1.8\DBD\ADO\ado.rb'

ado = DBI::DBD::ADO::Driver.new

ado.connect( dbname, user, auth, attr )

dbname is misleading here for two reasons. First, Rails passes in a connection string. Second, the ADO driver uses it as an odbc name instead of as a host name, etc.  The contents of dbname looked like this:

Provider=SQLOLEDB;Data Source=my_machine.local;Initial Catalog=some_database;User Id=some_username;Password=some_password;

THE SOLUTION

In my case, the local SQL Server installation uses a named instance, as opposed to a default instance. I copied the 'server name' field from the login dialog in Sql Server Management Studio, and that worked.