Treat ActiveRecord::Base and ApplicationRecord as "primary"

When someone has a multi-db application their `ApplicationRecord` will
look like:

```ruby
class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

  connects_to database: { writing: :primary, reading: :replica }
end
```

This will cause us to open 2 connections to ActiveRecord::Base's
database when we actually only want 1. This is because Rails sees
`ApplicationRecord` and thinks it's a new connection, not the existing
`ActiveRecord::Base` connection because the
`connection_specification_name` is different.

This PR changes `ApplicationRecord` classes to consider themselves the
same as the "primary" connection.

Fixes #36382

activerecord/lib/active_record/connection_handling.rb

@@ -173,7 +173,7 @@ module ActiveRecord
       raise "Anonymous class is not allowed." unless name
 
       config_or_env ||= DEFAULT_ENV.call.to_sym
-      pool_name = self == Base ? "primary" : name
+      pool_name = primary_class? ? "primary" : name
       self.connection_specification_name = pool_name
 
       resolver = ConnectionAdapters::ConnectionSpecification::Resolver.new(Base.configurations)
@@ -204,11 +204,15 @@ module ActiveRecord
     # Return the specification name from the current class or its parent.
     def connection_specification_name
       if !defined?(@connection_specification_name) || @connection_specification_name.nil?
-        return self == Base ? "primary" : superclass.connection_specification_name
+        return primary_class? ? "primary" : superclass.connection_specification_name
       end
       @connection_specification_name
     end
 
+    def primary_class? # :nodoc:
+      self == Base || defined?(ApplicationRecord) && self == ApplicationRecord
+    end
+
     # Returns the configuration of the associated connection as a hash:
     #
     #  ActiveRecord::Base.connection_config

activerecord/test/cases/connection_adapters/connection_handler_test.rb

@@ -367,11 +367,24 @@ module ActiveRecord
           assert_same klass2.connection, ActiveRecord::Base.connection
         end
 
+        class ApplicationRecord < ActiveRecord::Base
+          self.abstract_class = true
+        end
+
+        class MyClass < ApplicationRecord
+        end
+
         def test_connection_specification_name_should_fallback_to_parent
           klassA = Class.new(Base)
           klassB = Class.new(klassA)
+          klassC = Class.new(MyClass)
 
           assert_equal klassB.connection_specification_name, klassA.connection_specification_name
+          assert_equal klassC.connection_specification_name, klassA.connection_specification_name
+
+          assert_equal "primary", klassA.connection_specification_name
+          assert_equal "primary", klassC.connection_specification_name
+
           klassA.connection_specification_name = "readonly"
           assert_equal "readonly", klassB.connection_specification_name
         end

railties/test/application/loading_test.rb

@@ -116,7 +116,7 @@ class LoadingTest < ActiveSupport::TestCase
     RUBY
 
     app_file "app/models/post.rb", <<-MODEL
-      class Post < ActiveRecord::Base
+      class Post < ApplicationRecord
       end
     MODEL
 
@@ -133,9 +133,9 @@ class LoadingTest < ActiveSupport::TestCase
     require "#{rails_root}/config/environment"
     setup_ar!
 
-    assert_equal [ActiveStorage::Blob, ActiveStorage::Attachment, ActiveRecord::SchemaMigration, ActiveRecord::InternalMetadata].collect(&:to_s).sort, ActiveRecord::Base.descendants.collect(&:to_s).sort
+    assert_equal [ActiveStorage::Blob, ActiveStorage::Attachment, ActiveRecord::SchemaMigration, ActiveRecord::InternalMetadata, ApplicationRecord].collect(&:to_s).sort, ActiveRecord::Base.descendants.collect(&:to_s).sort
     get "/load"
-    assert_equal [ActiveStorage::Blob, ActiveStorage::Attachment, ActiveRecord::SchemaMigration, ActiveRecord::InternalMetadata, Post].collect(&:to_s).sort, ActiveRecord::Base.descendants.collect(&:to_s).sort
+    assert_equal [ActiveStorage::Blob, ActiveStorage::Attachment, ActiveRecord::SchemaMigration, ActiveRecord::InternalMetadata, ApplicationRecord, Post].collect(&:to_s).sort, ActiveRecord::Base.descendants.collect(&:to_s).sort
     get "/unload"
     assert_equal [ActiveStorage::Blob, ActiveStorage::Attachment, ActiveRecord::SchemaMigration, ActiveRecord::InternalMetadata].collect(&:to_s).sort, ActiveRecord::Base.descendants.collect(&:to_s).sort
   end