Rails database replica setup

While reading the official documentation, I did not understand some parts of the set up. Let's me explain here.

My setup:

• 1 main database

• 1 replica

What I did not understand:

• Where to put the connects_to setting

• Proper/clean configuration in database.yml

The database.yml

Notes:

• All environment needs primary and primary_replica otherwise Rails will raise an exception (at boot).

• The *_replica needs replica: true

• I did not put primary and primary_replica inside default because any change in a sub-object will erase all properties (I think)

• In that example, only production has a proper replica with different settings

default: &default
  adapter: trilogy
  encoding: utf8mb4
  pool: 5
  database: <%= ENV.fetch('DB_NAME') { 'db_name' } %>
  username: <%= ENV.fetch('DB_USER') { 'user' } %>
  password: <%= ENV.fetch('DB_PASS') { 'password' } %>
  host: <%= ENV.fetch('DB_HOST') { 'db.example.com' } %>
  port: <%= ENV.fetch('DB_PORT') { '3306' } %>

development:
  primary:
    <<: *default
  primary_replica:
    <<: *default
    replica: true

test:
  primary:
    <<: *default
  primary_replica:
    <<: *default
    replica: true

staging:
  primary:
    <<: *default
  primary_replica:
    <<: *default
    replica: true

production:
  primary:
    <<: *default
  primary_replica:
    <<: *default
    username: <%= ENV.fetch('DB_USER_REPLICA') { 'user_replica' } %>
    password: <%= ENV.fetch('DB_PASS_REPLICA') { 'password_replica' } %>
    host: <%= ENV.fetch('DB_HOST_REPLICA') { 'db-replica.example.com' } %>
    replica: true

Model set-up: connects_to

I thought I could use connects_to directly on a model (eg: User) but no, it should be on an abstract_class or ActiveRecord::Base (what the exception says) but I didn't understand it. I thought I should put abstract_class on the User but no, it should the parent class ApplicationRecord or anything other parent class (eg: AdminApplicationRecord for admin user).

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
  connects_to database: {
    writing: :primary,
    reading: :primary_replica
  }
end

If it helped, please share it.

Have fun, bye.