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|