This has been added 9 years ago in
a677da20, and removed 6 years ago in
38f8252e.

rather than allocating a new Relation, just make the AST directly

The `subclass_from_attrs` method is called even if the column specified by
the `inheritance_column` setting doesn't exist. This prevents setting associations
via the attributes hash if the association name clashes with the value of the setting,
typically `:type`. This worked previously in Rails 3.2.

Example:

    class Conversation < ActiveRecord::Base
      enum status: [ :active, :archived ]
    end

Before:

    Conversation::STATUS # => { "active" => 0, "archived" => 1 }

After:

    Conversation.statuses # => { "active" => 0, "archived" => 1 }

It's not public API, but is not private scoped.

Closes #13650, #13672

This is an alternate implementation to solve #13650. Currently form fields
contain the enum value (eg. "1"). This breaks because the setter `enum=`
expects the label (eg. "active").

ActiveRecord::Enum allows you to use labels in your application but store numbers.
We should make sure that all parts after AR are dealing with labels and not the
underlying mapping to a number.

This patch defines `_before_type_cast` on every enum column to return the label.
This method is later used to fetch the value to display in form fields.

I deliberately copied the implementation of the enum getter instead of delegating to it.
This allows you to overwrite the getter and for example return a `Value Object` but have it
still work for form fields.

Refs #13658

Currently Active Record can be configured via the environment variable `DATABASE_URL` or by manually injecting a hash of values which is what Rails does, reading in `database.yml` and setting Active Record appropriately. Active Record expects to be able to use `DATABASE_URL` without the use of Rails, and we cannot rip out this functionality without deprecating. This presents a problem though when both config is set, and a `DATABASE_URL` is present. Currently the `DATABASE_URL` should "win" and none of the values in `database.yml` are used. This is somewhat unexpected to me if I were to set values such as `pool` in the `production:` group of `database.yml` they are ignored.

There are many ways that active record initiates a connection today:

- Stand Alone (without rails)
  - `rake db:<tasks>`
  - ActiveRecord.establish_connection
 
- With Rails
  - `rake db:<tasks>`
  - `rails <server> | <console>`
  - `rails dbconsole`


We should make all of these behave exactly the same way. The best way to do this is to put all of this logic in one place so it is guaranteed to be used.

Here is my prosed matrix of how this behavior should work:

```
No database.yml
No DATABASE_URL
=> Error
```

```
database.yml present
No DATABASE_URL
=> Use database.yml configuration
```

```
No database.yml
DATABASE_URL present
=> use DATABASE_URL configuration
```

```
database.yml present
DATABASE_URL present
=> Merged into `url` sub key. If both specify `url` sub key, the `database.yml` `url`
   sub key "wins". If other paramaters `adapter` or `database` are specified in YAML,
   they are discarded as the `url` sub key "wins".
```

### Implementation

Current implementation uses `ActiveRecord::Base.configurations` to resolve and merge all connection information before returning. This is achieved through a utility class: `ActiveRecord::ConnectionHandling::MergeAndResolveDefaultUrlConfig`.

To understand the exact behavior of this class, it is best to review the behavior in activerecord/test/cases/connection_adapters/connection_handler_test.rb though it should match the above proposal.

This reverts commit 026d0555.

Conflicts:
	activerecord/lib/active_record/fixtures.rb

Fixes #13383

Closes #13623.

Closes #13625

  - Earlier, change_table was creating database-agnostic object.
  - After this change, it will create correct object based on current
    database adapter.
  - This will ensure that create_table and change_table will get same objects.
  - This makes update_table_definition method public and nodoc.
  - Fixes #13577 and #13503

Renaming the test accordingly to its behaviour

Adding 'Fixes' statement to changelog

Improving tests legibility & changelog

Undoing mistakenly removed empty line & further improving changelog

Previously, this would give an `ArgumentError`:

   class Issue < ActiveRecord::Base
     enum :status, [:open, :finished]
   end

   Issue.open.build # => ArgumentError: '0' is not a valid status
   Issue.open.create # => ArgumentError: '0' is not a valid status

PR #13542 muted the error, but the issue remains. This commit fixes
the issue by allowing the enum value to be written directly via the
setter:

   Issue.new.status = 0 # This now sets status to :open

Assigning a value directly via the setter like this is not part of the
documented public API, so users should not rely on this behavior.

Closes #13530.

These methods were only used for the `IdentityMap` which was removed.
They are no longer used internally and should be removed without replacement.

As they were not `:nodoc:`'ed it's better to deprecate them before removal.

* Move check from generated helper to test_help.rb, so that all
  applications can benefit
* Rather than just raising when the test schema has pending migrations,
  try to load in the schema and only raise if there are pending
  migrations afterwards
* Opt out of the check by setting
  config.active_record.maintain_test_schema = false
* Deprecate db:test:* tasks. The test helper is now fully responsible
  for maintaining the test schema, so we don't need rake tasks for this.
  This is also a speed improvement since we're no longer reloading the
  test database on every call to "rake test".

Add a mention about the automatic generation of scopes based on the
allowed values of the field on the enum documentation.

Previously, the writer methods would simply check whether the passed
argument was the symbol representing the integer value of an enum field.
Therefore, it was not possible to specify the numeric value itself but
the dynamically defined scopes generate where clauses relying on this
kind of values so a chained call to a method like `find_or_initialize_by`
would trigger an `ArgumentError`.

Reference #13530

- since `Array()` calls `to_ary` or `to_a` on a subject
- the intent is to 'wrap' subject into an array

Outer joins were being built on the root relation klass rather than the
one specified in the join dependency root