new in rails 6 I'm interested in

add support for setting Optimizer Hints on databases

allow to pass pg_hint_plan for PostgreSQL:

Topic.optimizer_hints("SeqScan(topics)", "Parallel(topics 8)")
# SELECT /*+ SeqScan(topics) Parallel(topics 8) */ "topics".* FROM "topics"

for pg_hint_plan: https://postgrespro.com/docs/enterprise/9.6/pg-hint-plan

Add negative scopes for all enum values

no need gem to add scopes for enum

class Post < ActiveRecord::Base
  enum status: %i[ drafted active trashed ]
end

Post.drafted # => where(status: :drafted)
Post.not_drafted # => where.not(status: :drafted)

if you don't want the scopes, add _scopes: false

automatically switch primary/replica database

for example:
A request will be sent to the replica if:

The request is a read request (GET or HEAD)
AND It's been 5 seconds since the last write to the database

A request will be sent to the primary if:

It's not a GET/HEAD request (ie is a POST, PATCH, etc)
Has been less than 5 seconds since the last write to the database

block writes to a database

ActiveRecord::Base.connected_to(role: :writing) do
  Dog.connection.while_blocking_writes do
    Dog.create! # will raise because we're blocking writes
  end
end

ActiveRecord::Base.connected_to(role: :reading) do
  Dog.connection.while_blocking_writes do
    Dog.first # will not raise because we're not writing
  end
end

switching connect to Multi db

class AnimalsBase < ApplicationRecord
  connects_to database: { writing: :animals, reading: :animals_replica }
end

specify the role

ActiveRecord::Base.connected_to(role: :reading) do
  Dog.first # finds dog from replica connected to AnimalsBase
  Book.first # doesn't have a reading connection, will raise an error
end

specify the database

ActiveRecord::Base.connected_to(database: :slow_replica) do
  SlowReplicaModel.first # if the db config has a slow_replica configuration this will be used to do the lookup, otherwise this will throw an exception
end

support ruby 2.6 endless ranges in where

relation.where(column: (1..))
same as
relation.where(column: 1..Float::INFINITY)

override the implicit order column

when calling first or last without order, ActiveRecord will sort records by primary key, if your primary key is not integer, eg: uuid. you probably need this

class Project < ActiveRecord::Base
  self.implicit_order_column = "created_at"
end

create_table if if_not_exists

create_table :posts, if_not_exists: true do |t|
  t.string :title
end

rake db tasks

# support for multiple databases 
rails db:schema:cache:dump
rails db:schema:cache:clear

mask sensitive data in #inspect

class Account < ActiveRecord::Base
  self.filter_attributes = [:credit_card_number]
end

set config.filter_parameters also mask the column

Rails.application.config.filter_parameters += [:credit_card_number]
Account.last.insepct # => #<Account id: 123, credit_card_number: [FILTERED]

also see ActiveSupport::ParameterFilter

customize format of ActiveModel::Errors#full_message

eg: we want #{message} not #{attribute} #{message}

en:
  activemodel:
    errors:
      models:
        user:
          format: '%{message}'

user.errors.full_messages
=> ["Email email_required", "Email invalid_email"]

index_with mapper

if we want map something to hash, we need

POST_ATTRIBUTES.map { |attr_name| [ attr_name, public_send(attr_name) ] }.to_h

now with index_with

POST_ATTRIBUTES.index_with { |attr_name| public_send(attr_name) }

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