Fix ActiveRecord :connection incompatibility with serialization libraries

lib/ddtrace/contrib/active_record/events/sql.rb

@@ -24,7 +24,7 @@ def span_name
           end
 
           def process(span, event, _id, payload)
-            config = Utils.connection_config(payload[:connection])
+            config = Utils.connection_config(payload[:connection], payload[:connection_id])
             settings = Datadog.configuration[:active_record, config]
             adapter_name = Datadog::Utils::Database.normalize_vendor(config[:adapter])
             service_name = if settings.service_name != Datadog::Utils::Database::VENDOR_DEFAULT

lib/ddtrace/contrib/active_record/patcher.rb

@@ -1,5 +1,4 @@
 require 'ddtrace/contrib/patcher'
-require 'ddtrace/contrib/active_record/patches/abstract_adapter'
 require 'ddtrace/contrib/active_record/events'
 
 module Datadog
@@ -18,7 +17,6 @@ def patched?
         def patch
           do_once(:active_record) do
             begin
-              ::ActiveRecord::ConnectionAdapters::AbstractAdapter.send(:include, Patches::AbstractAdapter)
               Events.subscribe!
             rescue StandardError => e
               Datadog::Tracer.log.error("Unable to apply Active Record integration: #{e}")

lib/ddtrace/contrib/active_record/utils.rb

@@ -21,14 +21,37 @@ def self.adapter_port
           connection_config[:port]
         end
 
-        def self.connection_config(connection = nil)
-          connection.nil? ? default_connection_config : connection_config_from_connection(connection)
-        end
+        # In newer Rails versions, the `payload` contains both the `connection` and its `object_id` named `connection_id`.
+        #
+        # So, if rails is recent we'll have a direct access to the connection.
+        # Else, we'll find it thanks to the passed `connection_id`.
+        #
+        # See this PR for more details: https://github.com/rails/rails/pull/34602
+        #
+        def self.connection_config(connection = nil, connection_id = nil)
+          return default_connection_config if connection.nil? && connection_id.nil?
+
+          conn = if !connection.nil?
+                   connection
+                 # Rails 3.0 - 3.2
+                 elsif Gem.loaded_specs['activerecord'].version < Gem::Version.new('4.0')
+                   ::ActiveRecord::Base
+                     .connection_handler
+                     .connection_pools
+                     .values
+                     .flat_map(&:connections)
+                     .find { |c| c.object_id == connection_id }
+                 # Rails 4.2+
+                 else
+                   ::ActiveRecord::Base
+                     .connection_handler
+                     .connection_pool_list
+                     .flat_map(&:connections)
+                     .find { |c| c.object_id == connection_id }
+                 end
 
-        # Typical of ActiveSupport::Notifications `sql.active_record`
-        def self.connection_config_from_connection(connection)
-          if connection.instance_variable_defined?(:@config)
-            connection.instance_variable_get(:@config)
+          if conn.instance_variable_defined?(:@config)
+            conn.instance_variable_get(:@config)
           else
             EMPTY_CONFIG
           end

spec/ddtrace/contrib/sinatra/activerecord_spec.rb

@@ -70,7 +70,9 @@ def migrate_db
       ActiveRecord::Base.establish_connection(
         adapter: 'sqlite3',
         database: ':memory:'
-      )
+      ).tap do |conn|
+        conn.connection.execute('SELECT 42')
+      end
     end
 
     let(:sinatra_span) { spans.first }