[4.2.x] Backport null adapter

README.md

@@ -2,7 +2,11 @@
 
 [![Build Status](https://travis-ci.com/localytics/odbc_adapter.svg?token=kQUiABmGkzyHdJdMnCnv&branch=master)](https://travis-ci.com/localytics/odbc_adapter)
 
-An ActiveRecord ODBC adapter.
+An ActiveRecord ODBC adapter. Master branch is working off of edge Rails. Previous work has been done to make it compatible with Rails 3.2 and 4.2; for those versions use the 3.2.x or 4.2.x gem releases.
+
+This adapter will work for basic queries for most DBMSs out of the box, without support for migrations. Full support is built-in for MySQL 5 and PostgreSQL 9 databases. You can register your own adapter to get more support for your DBMS using the `ODBCAdapter.register` function.
+
+A lot of this work is based on [OpenLink's ActiveRecord adapter](http://odbc-rails.rubyforge.org/) which works for earlier versions of Rails.
 
 ## Installation
 

lib/odbc_adapter/adapters/null_odbc_adapter.rb

@@ -0,0 +1,31 @@
+module ODBCAdapter
+  module Adapters
+    # A default adapter used for databases that are no explicitly listed in the
+    # registry. This allows for minimal support for DBMSs for which we don't
+    # have an explicit adapter.
+    class NullODBCAdapter < ActiveRecord::ConnectionAdapters::ODBCAdapter
+      class BindSubstitution < Arel::Visitors::ToSql
+        include Arel::Visitors::BindVisitor
+      end
+
+      # Using a BindVisitor so that the SQL string gets substituted before it is
+      # sent to the DBMS (to attempt to get as much coverage as possible for
+      # DBMSs we don't support).
+      def arel_visitor
+        BindSubstitution.new(self)
+      end
+
+      # Explicitly turning off prepared_statements in the null adapter because
+      # there isn't really a standard on which substitution character to use.
+      def prepared_statements
+        false
+      end
+
+      # Turning off support for migrations because there is no information to
+      # go off of for what syntax the DBMS will expect.
+      def supports_migrations?
+        false
+      end
+    end
+  end
+end

lib/odbc_adapter/dbms.rb

@@ -42,8 +42,7 @@ def adapter
               adapter if reported =~ pattern
             end
 
-          raise ArgumentError, "ODBCAdapter: Unsupported database (#{reported})" if found.nil?
-          found.last
+          (found && found.last) || :Null
         end
     end
   end