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, 13 insertions, 0 deletions

diff --git a/activerecord/test/cases/connection_adapters/connection_handler_test.rb b/activerecord/test/cases/connection_adapters/connection_handler_test.rb
index 6282759a10..27589966af 100644
--- a/activerecord/test/cases/connection_adapters/connection_handler_test.rb
+++ b/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