Remove unused `ActiveRecord::Base.connection_id`

This method appears to have been partially used in connection pool
caching, but it was introduced without much reasoning or any tests. One
edge case test was added later on, but it was focused on implementation
details. This method is no longer used outside of tests, and as such is
removed.

5 files changed, 9 insertions, 28 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 0bd3c2e78c..0feda36a83 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Removed the unused methods `ActiveRecord::Base.connection_id` and
+    `ActiveRecord::Base.connection_id=`
+
+    *Sean Griffin*
+
 *   Ensure hashes can be assigned to attributes created using `composed_of`.
     Fixes #25210.
 
diff --git a/activerecord/lib/active_record/connection_handling.rb b/activerecord/lib/active_record/connection_handling.rb
index 99fdef67d0..f735bc697b 100644
--- a/activerecord/lib/active_record/connection_handling.rb
+++ b/activerecord/lib/active_record/connection_handling.rb
@@ -98,14 +98,6 @@ module ActiveRecord
       @connection_specification_name
     end
 
-    def connection_id
-      ActiveRecord::RuntimeRegistry.connection_id ||= Thread.current.object_id
-    end
-
-    def connection_id=(connection_id)
-      ActiveRecord::RuntimeRegistry.connection_id = connection_id
-    end
-
     # Returns the configuration of the associated connection as a hash:
     #
     #  ActiveRecord::Base.connection_config
diff --git a/activerecord/lib/active_record/query_cache.rb b/activerecord/lib/active_record/query_cache.rb
index 07d863d15e..fa3e306b3e 100644
--- a/activerecord/lib/active_record/query_cache.rb
+++ b/activerecord/lib/active_record/query_cache.rb
@@ -26,16 +26,12 @@ module ActiveRecord
     def self.run
       connection    = ActiveRecord::Base.connection
       enabled       = connection.query_cache_enabled
-      connection_id = ActiveRecord::Base.connection_id
       connection.enable_query_cache!
 
-      [enabled, connection_id]
+      enabled
     end
 
-    def self.complete(state)
-      enabled, connection_id = state
-
-      ActiveRecord::Base.connection_id = connection_id
+    def self.complete(enabled)
       ActiveRecord::Base.connection.clear_query_cache
       ActiveRecord::Base.connection.disable_query_cache! unless enabled
     end
diff --git a/activerecord/lib/active_record/runtime_registry.rb b/activerecord/lib/active_record/runtime_registry.rb
index 56e88bc661..9e86999c1b 100644
--- a/activerecord/lib/active_record/runtime_registry.rb
+++ b/activerecord/lib/active_record/runtime_registry.rb
@@ -12,9 +12,9 @@ module ActiveRecord
   class RuntimeRegistry # :nodoc:
     extend ActiveSupport::PerThreadRegistry
 
-    attr_accessor :connection_handler, :sql_runtime, :connection_id
+    attr_accessor :connection_handler, :sql_runtime
 
-    [:connection_handler, :sql_runtime, :connection_id].each do |val|
+    [:connection_handler, :sql_runtime].each do |val|
       class_eval %{ def self.#{val}; instance.#{val}; end }, __FILE__, __LINE__
       class_eval %{ def self.#{val}=(x); instance.#{val}=x; end }, __FILE__, __LINE__
     end
diff --git a/activerecord/test/cases/query_cache_test.rb b/activerecord/test/cases/query_cache_test.rb
index d5c01315c1..03ec063671 100644
--- a/activerecord/test/cases/query_cache_test.rb
+++ b/activerecord/test/cases/query_cache_test.rb
@@ -39,18 +39,6 @@ class QueryCacheTest < ActiveRecord::TestCase
     assert ActiveRecord::Base.connection.query_cache_enabled, 'cache on'
   end
 
-  def test_exceptional_middleware_assigns_original_connection_id_on_error
-    connection_id = ActiveRecord::Base.connection_id
-
-    mw = middleware { |env|
-      ActiveRecord::Base.connection_id = self.object_id
-      raise "lol borked"
-    }
-    assert_raises(RuntimeError) { mw.call({}) }
-
-    assert_equal connection_id, ActiveRecord::Base.connection_id
-  end
-
   def test_middleware_delegates
     called = false
     mw = middleware { |env|