detect Rails DB connection and use it

lib/queue_classic.rb

@@ -57,7 +57,13 @@ def self.has_connection?
   end
 
   def self.default_conn_adapter
-    @conn_adapter ||= ConnAdapter.new
+    return @conn_adapter if @conn_adapter
+    if Object.const_defined?("ActiveRecord") && ActiveRecord::Base.respond_to?("connection")
+      @conn_adapter = ConnAdapter.new(ActiveRecord::Base.connection.raw_connection)
+    else
+      @conn_adapter = ConnAdapter.new
+    end
+    @conn_adapter
   end
 
   def self.default_conn_adapter=(conn)

readme.md

@@ -148,18 +148,6 @@ rails generate queue_classic:install
 rake db:migrate
 ```
 
-By default, queue_classic will use the QC_DATABASE_URL falling back on DATABASE_URL. The URL must be in the following format: `postgres://username:password@localhost/database_name`.  If you use Heroku's PostgreSQL service, this will already be set. If you don't want to set this variable, you can set the connection in an initializer. **QueueClassic will maintain its own connection to the database.** This may double the number of connections to your database.
-
-You can share your active record connection with queue_classic –**however this is not thread safe.**
-
-```ruby
-require 'queue_classic'
-QC.default_conn_adapter = QC::ConnAdapter.new(
-	ActiveRecord::Base.connection.raw_connection)
-```
-
-**Note on using ActiveRecord migrations:** If you use the migration, and you wish to use commands that reset the database from the stored schema (e.g. `rake db:reset`), your application must be configured with `config.active_record.schema_format = :sql` in `config/application.rb`.  If you don't do this, the PL/pgSQL function that queue_classic creates will be lost when you reset the database.
-
 ### Rake Task Setup
 
 Alternatively, you can use the Rake task to prepare your database.
@@ -172,6 +160,19 @@ $ bundle exec rake qc:create
 $ bundle exec rake qc:drop
 ```
 
+### Database connection
+
+#### Ruby on Rails
+
+Starting with with queue_classic 3.1, Rails is automatically detected and its connection is used.
+
+**Note on using ActiveRecord migrations:** If you use the migration, and you wish to use commands that reset the database from the stored schema (e.g. `rake db:reset`), your application must be configured with `config.active_record.schema_format = :sql` in `config/application.rb`.  If you don't do this, the PL/pgSQL function that queue_classic creates will be lost when you reset the database.
+
+
+#### Other Ruby apps
+
+By default, queue_classic will use the QC_DATABASE_URL falling back on DATABASE_URL. The URL must be in the following format: `postgres://username:password@localhost/database_name`.  If you use Heroku's PostgreSQL service, this will already be set. If you don't want to set this variable, you can set the connection in an initializer. **QueueClassic will maintain its own connection to the database.** This may double the number of connections to your database.
+
 ## Upgrade from V2 to V3
 If you are upgrading from a previous version of queue_classic, you might need some new database columns and/or functions. Luckily enough for you, it is easy to do so.
 

test/lib/queue_classic_rails_connection_test.rb

@@ -0,0 +1,28 @@
+require File.expand_path("../../helper.rb", __FILE__)
+
+class QueueClassicRailsConnectionTest < QCTest
+  def before_setup
+    Object.send :const_set, :ActiveRecord, Module.new
+    ActiveRecord.const_set :Base, Module.new
+
+    QC.default_conn_adapter = nil
+  end
+
+  def after_teardown
+    ensure
+      ActiveRecord.send :remove_const, :Base
+      Object.send :remove_const, :ActiveRecord
+  end
+
+  def test_uses_active_record_connection_if_exists
+    connection = Minitest::Mock.new
+    connection.expect(:raw_connection, QC::ConnAdapter.new.connection)
+
+    ActiveRecord::Base.define_singleton_method(:connection) do
+      connection
+    end
+
+    QC.default_conn_adapter
+    assert connection.verify
+  end
+end