Context #24542.

Since 8ebe1f2f, it has lost stripping date part for a time value. But I
confirmed it is still needed even if MariaDB 10.2.6 GA.

MariaDB 10.2.6, `prepared_statements: true`:

```
% ARCONN=mysql2 be ruby -w -Itest test/cases/time_precision_test.rb -n test_formatting_time_according_to_precision
Using mysql2
Run options: -n test_formatting_time_according_to_precision --seed 37614

F

Failure:
TimePrecisionTest#test_formatting_time_according_to_precision [test/cases/time_precision_test.rb:53]:
Failed assertion, no message given.

bin/rails test test/cases/time_precision_test.rb:46

Finished in 0.040279s, 24.8268 runs/s, 24.8268 assertions/s.

1 runs, 1 assertions, 1 failures, 0 errors, 0 skips
```

Dates are able to be natively handled by the mysql2 gem. libmysql (and
the wire protocol) represent each portion of the date as an integer,
which is significantly faster to encode and decode. By passing the Ruby
date objects through directly, we can save a good bit of time and
memory.

Since 5c71000d, it has lost to be able to unscope `default_scope` in STI
associations. This change will use `.empty_scope?` instead of
`.values.empty?` to regard as an empty scope if only have
`type_condition`.

This method needs to be protected.

Without this fix, `JoinDependency` doesn't use a custom table alias:

```
% ARCONN=sqlite3 be ruby -w -Itest test/cases/relations_test.rb -n test_using_a_custom_table_with_joins_affects_the_wheres
Using sqlite3
Run options: -n test_using_a_custom_table_with_joins_affects_the_wheres --seed 14531

E

Error:RelationTest#test_using_a_custom_table_with_joins_affects_the_wheres:
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: posts.author_id: SELECT  "omg_posts".* FROM "posts" "omg_posts" INNER JOIN "authors" ON "authors"."id" = "posts"."author_id" WHERE "omg_posts"."title" = ? LIMIT ?
```

Without this fix, SELECT clause doesn't use a custom table alias name:

```
% ARCONN=sqlite3 be ruby -w -Itest test/cases/relations_test.rb -n test_using_a_custom_table_affects_the_wheres
Using sqlite3
Run options: -n test_using_a_custom_table_affects_the_wheres --seed 31818

E

Error:
RelationTest#test_using_a_custom_table_affects_the_wheres:
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such table: posts: SELECT  "posts".* FROM "posts" "omg_posts" WHERE "omg_posts"."title" = ? LIMIT ?
```

`self.class.delete` is delegated to `all` and `all` is affected by
scoping. It should use `unscoped` to not be affected by that.

Between 4.2 and 5.0 the behavior of how multiparameter attributes
interact with `_before_type_cast` changed. In 4.2 it returns the
post-type-cast value. After 5.0, it returns the hash that gets sent to
the type. This behavior is correct, but will cause an issue if you then
tried to render that value in an input like `text_field` or
`hidden_field`.

In this case, we want those fields to use the post-type-cast form,
instead of the `_before_type_cast` (the main reason it uses
`_before_type_cast` at all is to avoid losing data when casting a
non-numeric string to integer).

I've opted to modify `came_from_user?` rather than introduce a new
method for this as I want to avoid complicating that contract further,
and technically the multiparameter hash didn't come from assignment, it
was constructed internally by AR.

Close #27888.

This change was introduced by #18109. The intent of that change was to
specifically apply `unscoped`, not to allow all changes to
`current_scope` to affect the join. The idea of allowing `current_scope`
to affect joins is interesting and potentially more consistent, but has
sever problems associated with it. The fact that we're specifically
stripping out joins indicates one such problem (and potentially leads to
invalid queries).

Ultimately it's difficult to reason about what `Posts.joins(:users)`
actually means if it's affected by `User.current_scope`, and it's
difficult to specifically control what does or doesn't get added. If we
were starting from scratch, I don't think I'd have `joins` be affected
by `default_scope` either, but that's too big of a breaking change to
make at this point.

With this change, we no longer apply `current_scope` when bringing in
joins, with the singular exception of the motivating use case which
introduced this bug, which is providing a way to *opt-out* of having the
default scope apply to joins.

Fixes #29338.

`arel_engine` is only used in `raise_record_not_found_exception!` to use
`engine.connection` (and `connection.visitor`) in `arel.where_sql`.

https://github.com/rails/arel/blob/v8.0.0/lib/arel/select_manager.rb#L183

But `klass.connection` will work as expected even if not using
`arel_engine` (described by `test_connection`). So `arel_engine` is no
longer needed.

[ci skip]

I investigated where `scope_for_create` is reused in tests with the
following code:

```diff
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -590,6 +590,10 @@ def where_values_hash(relation_table_name = table_name)
     end

     def scope_for_create
+      if defined?(@scope_for_create) && @scope_for_create
+        puts caller
+        puts "defined"
+      end
       @scope_for_create ||= where_values_hash.merge!(create_with_value.stringify_keys)
     end
```

It was hit only `test_scope_for_create_is_cached`. This means that
`scope_for_create` will not be reused in normal use cases. So we can
remove caching `scope_for_create` to respect changing `where_clause` and
`create_with_value`.

Using `@last` and `@order_clause` was removed at 8bb52746 and 90d1524a.
`@join_dependency` was added at b9599502 but it is unused in the first
place.

This is related with #27680.

Since `where_values_hash` keys constructed by `where` are string, so we
need `stringify_keys` to `create_with_value` before merging it.

If `table.table_alias` is not nil, it is enough to use `table` simply.

connection_active? will sometimes return true when the connection is actually dead/disconnected (see #3392 for a discussion of why this is).  When this happens, a query is run on the dead connection which causes various postgres connection errors to be raised.  This fix catches any such errors and ignores them.

Closes #29760

Post.where(id: 1).or(Post.where(id: 2)).where(foo: 3).unscope(where: :foo).where_clause.binds.map(&:value)
Would return [2, 3] instead of the expected [1,2]

Abstract boolean serialization has been using 't' and 'f', with MySQL
overriding that to use 1 and 0.

This has the advantage that SQLite natively recognizes 1 and 0 as true
and false, but does not natively recognize 't' and 'f'.

This change in serialization requires a migration of stored boolean data
for SQLite databases, so it's implemented behind a configuration flag
whose default false value is deprecated. The flag itself can be
deprecated in a future version of Rails.  While loaded models will give
the correct result for boolean columns without migrating old data,
where() clauses will interact incorrectly with old data.

While working in this area, also change the abstract adapter to use
`"TRUE"` and `"FALSE"` as quoted values and `true` and `false` for
unquoted.  These are supported by PostreSQL, and MySQL remains
overriden.

By making the Rails minitest behave like a standard minitest plugin
we're much more likely to not break when people use other minitest
plugins. Like minitest-focus and pride.

To do this, we need to behave like minitest: require files up front
and then perform the plugin behavior via the at_exit hook.
This also saves us a fair bit of wrangling with test file loading.

Finally, since the environment and warnings options have to be applied
as early as possible, and since minitest loads plugins at_exit, they
have to be moved to the test command.

* Don't expect the root method.

It's likely this worked because we eagerly loaded the Rails minitest plugin
and that somehow defined a root method on `Rails`.

* Assign a backtrace to failed exceptions.

Otherwise Minitest pukes when attempting to filter the backtrace (which
Rails' backtrace cleaner then removes).

Means the exception message test has to be revised too.

This is likely caused by the rails minitest plugin now being loaded for
these tests and assigning a default backtrace cleaner.

The uuid validation regex was allowing uuids to have a single leading
curly brace or single trailing curly brace. Saving with such a uuid
would cause Postgres to generate an exception even though the record
seemed valid. With this change, the regex requires both a leading *and*
a trailing curly brace or neither to be valid.

Currently preload query cannot be prepared statements even if
`prepared_statements: true` due to array handler in predicate builder
doesn't support making bind params. This makes preload query to
preparable by don't passing array value if possible.

Since MariaDB 10.2, `CURRENT_TIMESTAMP` is shown as a function
(`current_timestamp()`). Fix matching column default to address that
case.

Fixes #29698.

Since MariaDB 10.2.5, `information_schema` supports Virtual Columns.

Fixes #29670.

This name more accurately describes what the method does, and also
disambiguates it from `_write_attribute`, which ignores aliases.

We can also make the method private, since it's not public API and only
called from one place - `update_columns` - without an explicit receiver.

Currently `version` method always returns `5.5.5` because the
`full_version` is `5.5.5-10.x.y-MariaDB...` since MariaDB 10.x.
It should be ignored if the prefix is `5.5.5-`.

The `find_each`, `find_in_batches` and `in_batches` APIs usually operate
on large numbers of records, where it's preferable not to load them all
into memory at once.

If the query cache is enabled, it will hold onto the query results until
the end of the execution context (request/job), which means the memory
used is still proportional to the total number of records. These queries
are typically not repeated, so the query cache isn't desirable here.

The AbstractAdapter will translate all StandardErrors generated during the course of a query into ActiveRecord::StatementInvalids. Unfortunately, it'll also mangle non-database-related errors generated in ActiveSupport::Notification callbacks after the query has successfully completed. This should prevent it from translating errors from ActiveSupport::Notifications.

We already have database agnostic `Type::Json` since #29220.

`attributes` (`values` in `LazyAttributeHash` in `AttributeSet`) has
string keys, not symbols.

Fixes #29665.

Fixes #28324.