TIL 2 September 2025

Rails picks the replica by request method, not by the query

We had this in ApplicationRecord and I was curious how it actually worked:

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

I assumed Rails looked at each query and sent reads to the replica and writes to the primary. It does not. The automatic switching comes from the DatabaseSelector middleware, and it decides based on the HTTP request method, not the query:

• GET and HEAD use the reading (replica) connection.
• everything else (POST, PUT, PATCH, DELETE) uses the writing (primary) connection.

So if a GET action runs a write, you get an ActiveRecord::ReadOnlyError, even for a tiny update. Usually that means the write does not belong in a GET in the first place, but it caught me off guard the first time.

Docs: ActiveRecord::Middleware::DatabaseSelector.