Fix aggregate functions to return numeric value consistently even on custom attribute type.

Ryuta Kamizono
Support bulk insert/upsert on relation to preserve scope values.

Josef Šimánek, Ryuta Kamizono
Preserve column comment value on changing column name on MySQL.

Islam Taha
Add support for if_exists option for removing an index.

The remove_index method can take an if_exists option. If this is set to true an error won't be raised if the index doesn't exist.

Eileen M. Uchitelle
Remove ibm_db, informix, mssql, oracle, and oracle12 Arel visitors which are not used in the code base.

Ryuta Kamizono
Prevent build_association from touching a parent record if the record isn't persisted for has_one associations.

Fixes #38219.

Josh Brody
Add support for if_not_exists option for adding index.

The add_index method respects if_not_exists option. If it is set to true index won't be added.

Usage:
```
  add_index :users, :account_id, if_not_exists: true
```
The if_not_exists option passed to create_table also gets propagated to indexes created within that migration so that if table and its indexes exist then there is no attempt to create them again.

Prathamesh Sonpatki
Add ActiveRecord::Base#previously_new_record? to show if a record was new before the last save.

Tom Ward
Support descending order for find_each, find_in_batches, and in_batches.

Batch processing methods allow you to work with the records in batches, greatly reducing memory consumption, but records are always batched from oldest id to newest.

This change allows reversing the order, batching from newest to oldest. This is useful when you need to process newer batches of records first.

Pass order: :desc to yield batches in descending order. The default remains order: :asc.
```
Person.find_each(order: :desc) do |person|
  person.party_all_night!
end
```
Alexey Vasiliev
Fix insert_all with enum values.

Fixes #38716.

Joel Blum
Add support for db:rollback:name for multiple database applications.

Multiple database applications will now raise if db:rollback is call and recommend using the db:rollback:[NAME] to rollback migrations.

Eileen M. Uchitelle
Relation#pick now uses already loaded results instead of making another query.

Eugene Kenny
Deprecate using return, break or throw to exit a transaction block.

Dylan Thacker-Smith
Dump the schema or structure of a database when calling db:migrate:name.

In previous versions of Rails, rails db:migrate would dump the schema of the database. In Rails 6, that holds true (rails db:migrate dumps all databases' schemas), but rails db:migrate:name does not share that behavior.

Going forward, calls to rails db:migrate:name will dump the schema (or structure) of the database being migrated.

Kyle Thompson
Reset the ActiveRecord::Base connection after rails db:migrate:name.

When rails db:migrate has finished, it ensures the ActiveRecord::Base connection is reset to its original configuration. Going forward, rails db:migrate:name will have the same behavior.

Kyle Thompson
Disallow calling connected_to on subclasses of ActiveRecord::Base.

Behavior has not changed here but the previous API could be misleading to people who thought it would switch connections for only that class. connected_to switches the context from which we are getting connections, not the connections themselves.

Eileen M. Uchitelle, John Crepezzi
Add support for horizontal sharding to connects_to and connected_to.

Applications can now connect to multiple shards and switch between their shards in an application. Note that the shard swapping is still a manual process as this change does not include an API for automatic shard swapping.

Usage:

Given the following configuration:
```
# config/database.yml
production:
  primary:
    database: my_database
  primary_shard_one:
    database: my_database_shard_one
```
Connect to multiple shards:
```
class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

  connects_to shards: {
    default: { writing: :primary },
    shard_one: { writing: :primary_shard_one }
  }
```
Swap between shards in your controller / model code:
```
ActiveRecord::Base.connected_to(shard: :shard_one) do
  # Read from shard one
end
```
The horizontal sharding API also supports read replicas. See guides for more details.

Eileen M. Uchitelle, John Crepezzi

Deprecate spec_name in favor of name on database configurations.

The accessors for spec_name on configs_for and DatabaseConfig are deprecated. Please use name instead.

Deprecated behavior:

db_config = ActiveRecord::Base.configs_for(env_name: "development", spec_name: "primary")
db_config.spec_name

New behavior:

db_config = ActiveRecord::Base.configs_for(env_name: "development", name: "primary")
db_config.name

Eileen M. Uchitelle

Add additional database-specific rake tasks for multi-database users.

Previously, rails db:create, rails db:drop, and rails db:migrate were the only rails tasks that could operate on a single database. For example:

rails db:create
rails db:create:primary
rails db:create:animals
rails db:drop
rails db:drop:primary
rails db:drop:animals
rails db:migrate
rails db:migrate:primary
rails db:migrate:animals

With these changes, rails db:schema:dump, rails db:schema:load, rails db:structure:dump, rails db:structure:load and rails db:test:prepare can additionally operate on a single database. For example:

rails db:schema:dump
rails db:schema:dump:primary
rails db:schema:dump:animals
rails db:schema:load
rails db:schema:load:primary
rails db:schema:load:animals
rails db:structure:dump
rails db:structure:dump:primary
rails db:structure:dump:animals
rails db:structure:load
rails db:structure:load:primary
rails db:structure:load:animals
rails db:test:prepare
rails db:test:prepare:primary
rails db:test:prepare:animals

Kyle Thompson

Add support for strict_loading mode on association declarations.

Raise an error if attempting to load a record from an association that has been marked as strict_loading unless it was explicitly eager loaded.

Usage:

class Developer < ApplicationRecord
  has_many :projects, strict_loading: true
end

dev = Developer.first
dev.projects.first
# => ActiveRecord::StrictLoadingViolationError: The projects association is marked as strict_loading and cannot be lazily loaded.

Kevin Deisz

Add support for strict_loading mode to prevent lazy loading of records.

Raise an error if a parent record is marked as strict_loading and attempts to lazily load its associations. This is useful for finding places you may want to preload an association and avoid additional queries.

Usage:
```
dev = Developer.strict_loading.first
dev.audit_logs.to_a
# => ActiveRecord::StrictLoadingViolationError: Developer is marked as strict_loading and AuditLog cannot be lazily loaded.
```
Eileen M. Uchitelle, Aaron Patterson
Add support for PostgreSQL 11+ partitioned indexes when using upsert_all.

Sebastián Palma

Adds support for if_not_exists to add_column and if_exists to remove_column.

Applications can set their migrations to ignore exceptions raised when adding a column that already exists or when removing a column that does not exist.

Example Usage:

class AddColumnTitle < ActiveRecord::Migration[6.1]
  def change
    add_column :posts, :title, :string, if_not_exists: true
  end
end

class RemoveColumnTitle < ActiveRecord::Migration[6.1]
  def change
    remove_column :posts, :title, if_exists: true
  end
end

Eileen M. Uchitelle

Regexp-escape table name for MS SQL Server.

Add Regexp.escape to one method in ActiveRecord, so that table names with regular expression characters in them work as expected. Since MS SQL Server uses "[" and "]" to quote table and column names, and those characters are regular expression characters, methods like pluck and select fail in certain cases when used with the MS SQL Server adapter.

Larry Reid
Store advisory locks on their own named connection.

Previously advisory locks were taken out against a connection when a migration started. This works fine in single database applications but doesn't work well when migrations need to open new connections which results in the lock getting dropped.

In order to fix this we are storing the advisory lock on a new connection with the connection specification name AdvisoryLockBase. The caveat is that we need to maintain at least 2 connections to a database while migrations are running in order to do this.

Eileen M. Uchitelle, John Crepezzi

Allow schema cache path to be defined in the database configuration file.

For example:

development:
  adapter: postgresql
  database: blog_development
  pool: 5
  schema_cache_path: tmp/schema/main.yml

Katrina Owen

Deprecate #remove_connection in favor of #remove_connection_pool when called on the handler.

#remove_connection is deprecated in order to support returning a DatabaseConfig object instead of a Hash. Use #remove_connection_pool, #remove_connection will be removed in 6.2.

Eileen M. Uchitelle, John Crepezzi
Deprecate #default_hash and it's alias #[] on database configurations.

Applications should use configs_for. #default_hash and #[] will be removed in 6.2.

Eileen M. Uchitelle, John Crepezzi
Add scale support to ActiveRecord::Validations::NumericalityValidator.

Gannon McGibbon
Find orphans by looking for missing relations through chaining where.missing:

Before:
```
Post.left_joins(:author).where(authors: { id: nil })
```
After:
```
Post.where.missing(:author)
```
Tom Rossi
Ensure :reading connections always raise if a write is attempted.

Now Rails will raise an ActiveRecord::ReadOnlyError if any connection on the reading handler attempts to make a write. If your reading role needs to write you should name the role something other than :reading.

Eileen M. Uchitelle
Deprecate "primary" as the connection_specification_name for ActiveRecord::Base.

"primary" has been deprecated as the connection_specification_name for ActiveRecord::Base in favor of using "ActiveRecord::Base". This change affects calls to ActiveRecord::Base.connection_handler.retrieve_connection and ActiveRecord::Base.connection_handler.remove_connection. If you're calling these methods with "primary", please switch to "ActiveRecord::Base".

Eileen M. Uchitelle, John Crepezzi
Add ActiveRecord::Validations::NumericalityValidator with support for casting floats using a database columns' precision value.

Gannon McGibbon
Enforce fresh ETag header after a collection's contents change by adding ActiveRecord::Relation#cache_key_with_version. This method will be used by ActionController::ConditionalGet to ensure that when collection cache versioning is enabled, requests using ConditionalGet don't return the same ETag header after a collection is modified.

Fixes #38078.

Aaron Lipman
Skip test database when running db:create or db:drop in development with DATABASE_URL set.

Brian Buchalter

Don't allow mutations on the database configurations hash.

Freeze the configurations hash to disallow directly changing it. If applications need to change the hash, for example to create databases for parallelization, they should use the DatabaseConfig object directly.

Before:

@db_config = ActiveRecord::Base.configurations.configs_for(env_name: "test", spec_name: "primary")
@db_config.configuration_hash.merge!(idle_timeout: "0.02")

After:

@db_config = ActiveRecord::Base.configurations.configs_for(env_name: "test", spec_name: "primary")
config = @db_config.configuration_hash.merge(idle_timeout: "0.02")
db_config = ActiveRecord::DatabaseConfigurations::HashConfig.new(@db_config.env_name, @db_config.spec_name, config)

Eileen M. Uchitelle, John Crepezzi

Remove :connection_id from the sql.active_record notification.

Aaron Patterson, Rafael Mendonça França
The :name key will no longer be returned as part of DatabaseConfig#configuration_hash. Please use DatabaseConfig#owner_name instead.

Eileen M. Uchitelle, John Crepezzi
ActiveRecord's belongs_to_required_by_default flag can now be set per model.

You can now opt-out/opt-in specific models from having their associations required by default.

This change is meant to ease the process of migrating all your models to have their association required.

Edouard Chin
The connection_config method has been deprecated, please use connection_db_config instead which will return a DatabaseConfigurations::DatabaseConfig instead of a Hash.

Eileen M. Uchitelle, John Crepezzi
Retain explicit selections on the base model after applying includes and joins.

Resolves #34889.

Patrick Rebsch
The database kwarg is deprecated without replacement because it can't be used for sharding and creates an issue if it's used during a request. Applications that need to create new connections should use connects_to instead.

Eileen M. Uchitelle, John Crepezzi
Allow attributes to be fetched from Arel node groupings.

Jeff Emminger, Gannon McGibbon
A database URL can now contain a querystring value that contains an equal sign. This is needed to support passing PostgreSQL options.

Joshua Flanagan
Calling methods like establish_connection with a Hash which is invalid (eg: no adapter) will now raise an error the same way as connections defined in config/database.yml.

John Crepezzi
Specifying implicit_order_column now subsorts the records by primary key if available to ensure deterministic results.

Paweł Urbanek
where(attr => []) now loads an empty result without making a query.

John Hawthorn
Fixed the performance regression for primary_keys introduced MySQL 8.0.

Hiroyuki Ishii
Add support for belongs_to to has_many inversing.

Gannon McGibbon

Allow length configuration for has_secure_token method. The minimum length is set at 24 characters.

Before:

has_secure_token :auth_token

After:

has_secure_token :default_token             # 24 characters
has_secure_token :auth_token, length: 36    # 36 characters
has_secure_token :invalid_token, length: 12 # => ActiveRecord::SecureToken::MinimumLengthError

Bernardo de Araujo

Deprecate DatabaseConfigurations#to_h. These connection hashes are still available via ActiveRecord::Base.configurations.configs_for.

Eileen Uchitelle, John Crepezzi
Add DatabaseConfig#configuration_hash to return database configuration hashes with symbol keys, and use all symbol-key configuration hashes internally. Deprecate DatabaseConfig#config which returns a String-keyed Hash with the same values.

John Crepezzi, Eileen Uchitelle

Allow column names to be passed to remove_index positionally along with other options.

Passing other options can be necessary to make remove_index correctly reversible.

Before:

add_index    :reports, :report_id               # => works
add_index    :reports, :report_id, unique: true # => works
remove_index :reports, :report_id               # => works
remove_index :reports, :report_id, unique: true # => ArgumentError

After:

remove_index :reports, :report_id, unique: true # => works

Eugene Kenny

Allow bulk ALTER statements to drop and recreate indexes with the same name.

Eugene Kenny
insert, insert_all, upsert, and upsert_all now clear the query cache.

Eugene Kenny
Call while_preventing_writes directly from connected_to.

In some cases application authors want to use the database switching middleware and make explicit calls with connected_to. It's possible for an app to turn off writes and not turn them back on by the time we call connected_to(role: :writing).

This change allows apps to fix this by assuming if a role is writing we want to allow writes, except in the case it's explicitly turned off.

Eileen M. Uchitelle
Improve detection of ActiveRecord::StatementTimeout with mysql2 adapter in the edge case when the query is terminated during filesort.

Kir Shatrov
Stop trying to read yaml file fixtures when loading Active Record fixtures.

Gannon McGibbon
Deprecate .reorder(nil) with .first / .first! taking non-deterministic result.

To continue taking non-deterministic result, use .take / .take! instead.

Ryuta Kamizono
Ensure custom PK types are casted in through reflection queries.

Gannon McGibbon
Preserve user supplied joins order as much as possible.

Fixes #36761, #34328, #24281, #12953.

Ryuta Kamizono
Allow matches_regex and does_not_match_regexp on the MySQL Arel visitor.

James Pearson
Allow specifying fixtures to be ignored by setting ignore in YAML file's '_fixture' section.

Tongfei Gao
Make the DATABASE_URL env variable only affect the primary connection. Add new env variables for multiple databases.

John Crepezzi, Eileen Uchitelle
Add a warning for enum elements with 'not_' prefix.
```
class Foo
  enum status: [:sent, :not_sent]
end
```
Edu Depetris
Make currency symbols optional for money column type in PostgreSQL.

Joel Schneider
Add support for beginless ranges, introduced in Ruby 2.7.

Josh Goodall
Add database_exists? method to connection adapters to check if a database exists.

Guilherme Mansur
Loading the schema for a model that has no table_name raises a TableNotSpecified error.

Guilherme Mansur, Eugene Kenny
PostgreSQL: Fix GROUP BY with ORDER BY virtual count attribute.

Fixes #36022.

Ryuta Kamizono
Make ActiveRecord ConnectionPool.connections method thread-safe.

Fixes #36465.

Jeff Doering
Add support for multiple databases to rails db:abort_if_pending_migrations.

Mark Lee
Fix sqlite3 collation parsing when using decimal columns.

Martin R. Schuster
Fix invalid schema when primary key column has a comment.

Fixes #29966.

Guilherme Goettems Schneider
Fix table comment also being applied to the primary key column.

Guilherme Goettems Schneider
Allow generated create_table migrations to include or skip timestamps.

Michael Duchemin

Please check 6-0-stable for previous changes.

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