Just create a local variable whenever we need the record, rather than
doing an extra find for every test on the setup method.

Ensure the child can actually be touched. The `Bulb` model has no
timestamp column, thus trying to touch it would return `nil`.

This solves order depndent problems in `base_test.rb`. The leaked column
information crashed test cases using the `Weird` class later on:

```
  2) Error:
BasicsTest#test_group_weirds_by_from:
SyntaxError: /Users/senny/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:387: invalid hex escape
... attribute_before_type_cast("\x{A4CA}\x{A4DE}\x{A4A8}", *arg...
...                               ^
/Users/senny/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:387: invalid hex escape
...te_before_type_cast("\x{A4CA}\x{A4DE}\x{A4A8}", *args)
...                               ^
/Users/senny/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:387: invalid hex escape
...e_type_cast("\x{A4CA}\x{A4DE}\x{A4A8}", *args)
...                               ^
    /Users/senny/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:385:in `module_eval'
    /Users/senny/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:385:in `define_proxy_call'
    /Users/senny/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:294:in `block in define_attribute_method'
    /Users/senny/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:285:in `each'
    /Users/senny/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:285:in `define_attribute_method'
    /Users/senny/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:252:in `block in define_attribute_methods'
    /Users/senny/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:252:in `each'
    /Users/senny/Projects/rails/activemodel/lib/active_model/attribute_methods.rb:252:in `define_attribute_methods'
    /Users/senny/Projects/rails/activerecord/lib/active_record/attribute_methods.rb:72:in `block in define_attribute_methods'
    /Users/senny/.rbenv/versions/2.0.0-p353/lib/ruby/2.0.0/mutex_m.rb:73:in `synchronize'
    /Users/senny/.rbenv/versions/2.0.0-p353/lib/ruby/2.0.0/mutex_m.rb:73:in `mu_synchronize'
    /Users/senny/Projects/rails/activerecord/lib/active_record/attribute_methods.rb:69:in `define_attribute_methods'
    /Users/senny/Projects/rails/activerecord/lib/active_record/attribute_methods.rb:174:in `method_missing'
    /Users/senny/Projects/rails/activerecord/lib/active_record/attribute_assignment.rb:45:in `public_send'
    /Users/senny/Projects/rails/activerecord/lib/active_record/attribute_assignment.rb:45:in `_assign_attribute'
    /Users/senny/Projects/rails/activerecord/lib/active_record/attribute_assignment.rb:32:in `block in assign_attributes'
    /Users/senny/Projects/rails/activerecord/lib/active_record/attribute_assignment.rb:26:in `each'
    /Users/senny/Projects/rails/activerecord/lib/active_record/attribute_assignment.rb:26:in `assign_attributes'
    /Users/senny/Projects/rails/activerecord/lib/active_record/core.rb:458:in `init_attributes'
    /Users/senny/Projects/rails/activerecord/lib/active_record/core.rb:191:in `initialize'
    /Users/senny/Projects/rails/activerecord/lib/active_record/inheritance.rb:30:in `new'
    /Users/senny/Projects/rails/activerecord/lib/active_record/inheritance.rb:30:in `new'
    /Users/senny/Projects/rails/activerecord/lib/active_record/persistence.rb:33:in `create'
    test/cases/base_test.rb:646:in `test_group_weirds_by_from'
```

This makes the tests inside `migration_test.rb` order independent.
The assignments to `Reminder.table_name_prefix` and `Reminder.table_name_suffix`
previously leaked and had impact on following test cases.

This patch isolates the assignments on a throw-away subclass.

This solves order dependent issues where the last test
leaked the query cache state. This resulted in the following error:

```
QueryCacheExpiryTest#test_cache_is_expired_by_habtm_delete [test/cases/query_cache_test.rb:275]:
not all expectations were satisfied
unsatisfied expectations:
- expected exactly twice, invoked once: #<ActiveRecord::ConnectionAdapters::SQLite3Adapter:0x7f93e0c65838>.clear_query_cache(any_parameters)
```

This fixes test order issues.

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

https://github.com/rails/rails/blob/e64a83cdbd3ce836f3339c600c1006b20c23d8fd/activerecord/test/cases/relation/merging_test.rb#L80-91

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

Conflicts:
	activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb

This fixes broken `rake test_sqlite3_mem` suite for Active Record.
The problem is that that the old database with the schema is lost
when establishing a new connection. Upon reconnting we are left
with a blank database and tests down the line start failing.

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

- We have to restore DATABASE_URL to its previous state irrespective of
  previous value is nil or not

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.