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

1 files changed, 6 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/connection_handling.rb b/activerecord/lib/active_record/connection_handling.rb
index 040ebdb960..d472eb2e2b 100644
--- a/activerecord/lib/active_record/connection_handling.rb
+++ b/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