This method can be used to see all of the fields on a model which have
been read. This can be useful during development mode to quickly find
out which fields need to be selected. For performance critical pages, if
you are not using all of the fields of a database, an easy performance
win is only selecting the fields which you need. By calling this method
at the end of a controller action, it's easy to determine which fields
need to be selected.

While writing this, I also noticed a place for an easy performance win
internally which I had been wanting to introduce. You cannot mutate a
field which you have not read. Therefore, we can skip the calculation of
in place changes if we have never read from the field. This can
significantly speed up methods like `#changed?` if any of the fields
have an expensive mutable type (like `serialize`)

```
Calculating -------------------------------------
 #changed? with serialized column (before)
                       391.000  i/100ms
 #changed? with serialized column (after)
                         1.514k i/100ms
-------------------------------------------------
 #changed? with serialized column (before)
                          4.243k (± 3.7%) i/s -     21.505k
 #changed? with serialized column (after)
                         16.789k (± 3.2%) i/s -     84.784k
```

With the old implementation, the bind values were created, and then we
search the attributes for `Relation` objects, and merge them. This
completely ignores the order that the actual `where` clause will use. If
all non-relation where parameters are before the relations, it will
work. However, if we query on both a relation and a value, with the
value coming second, it breaks. The order of the hash should not affect
the final query (especially since hashes being ordered is an
implementation detail)

Looking through the blame, this logic used to be when we actually
created the bind tuple. My guess is that `nil` couldn't be handled there
at that time. It can, now.

If set to `if_exists: true`, it generates a statement like:

    DROP TABLE IF EXISTS posts

This syntax is supported in the popular SQL servers, that is (at least)
SQLite, PostgreSQL, MySQL, Oracle and MS SQL Sever.

Closes #16366.

`.` is regexp meta character. It should be escape for `assert_match`
correctly.

When an attribute is assigned, we determine if it was already marked as
changed so we can determine if we need to clear the changes, or mark it
as changed. Since this only affects the `attributes_changed_by_setter`
hash, in-place changes are irrelevant to this process. Since calculating
in-place changes can be expensive, we can just skip it here.

I also added a test for the only edge case I could think of that would
be affected by this change.

 - This is based on https://github.com/rails/rails/issues/18400 but
   tackling same issue with update_attribute method instead of update method.

#assert was used when it should be assert_equal.

after_commit callbacks run after committing a transaction whose parent
is not `joinable?`: un-nested transactions, transactions within test
cases, and transactions in `console --sandbox`.

The types that are affected by `time_zone_aware_attributes` (which is on
by default) have been made configurable, in case this is a breaking
change for existing applications.

While we don't want to change the form input when validations fail,
blindly using `_before_type_cast` will cause the input to display the
wrong data for any type which does additional work on database values.

This behavior exists only to support fixtures, so we should handle it
there. Leaving it in `#quote` can cause very subtle bugs to slip
through, by things appearing to work when they should be blowing up
loudly, such as #18385.

I'm planning on deprecating the column argument to mirror the
deprecation in [arel].

[arel]: https://github.com/rails/arel/commit/6160bfbda1d1781c3b08a33ec4955f170e95be11

There's very little value in logging "<NULL binary data>" instead of
just "nil". I'd like to remove the column from the equation entirely,
and this case is preventing us from doing so.

This sets a precident for how we handle `attribute` calls, which aren't
backed by a database column. We should not take this as a conscious
decision on how to handle them, and this can change when we make
`attribute` public if we have better ideas in the future.

As the composed attributes API gets fleshed out, I expect the
`persistable_attributes` method to change to
`@attributes.select(&:persistable).keys`, or some more performant
variant there-of. This can probably go away completely once we fully
move dirty checking into the attribute objects once it gets moved up to
Active Model.

Fixes #18407

Update Secure Token Doc [ci skip]

remove require securerandom, core_ext/securerandom already do that ref 7e006057

This is cropping up all over the place. After a brief dive, I'm really
not sure why we have `arel.bind_values` at all. A cursory grep didn't
reveal where they're actually being assigned (it's definitely in AR, not
in Arel). I'd like to dig further into it, as I'm fairly certain we
don't actually need it, we just need a way for the predicate builder to
communicate merged binds upstream.

Fixes #18414

The reason this bug occured is that we never actually check to see if
this column has changed from it's default, since it was never assigned
and is not mutable.

It appears I was wrong in b301c402, with
my statement of "there is no longer a case where a given value would
differ from the default, but would not already be marked as changed."

However, I chose not to revert the deletion of
`initialize_internals_callback` from that commit, as I think a solution
closer to where the problem lies is less likely to get erroneously
removed. I'm not super happy with this solution, but it mirrors what is
being done in `_update_record`, and a fix for one should work for the
other.

I toyed with the idea of changing the definition of `changed?` on the
type to `changed_in_place?`. If we type cast the raw value, it'll break
a test about updating not modifying the lock column if nothing else was
changed. We could have the definition check if `raw_old_value` is `nil`,
but this feels fragile and less intention revealing. It would, however,
have the benefit of cleaning up old data that incorrectly persisted as
`nil`.

Fixes #18422

Previosly count and other AR calculations would convert
column_name_for_operation to sql on a default Arel::Table.engine
(AR::Base) connection. That could lead to trouble if current
model has a connection to a different adapter or Base connection
is inaccessible.

 - Fixtures with non-string labels such as integers should be accessed
   using integer label as key. For eg. pirates(1) or pirates(42).
 - But this results in NotFound error because the label is converted into string before
   looking up into the fixtures hash.
 - After this commit, the label is converted into string only if its a
   symbol.
 - This issue was fount out while adding a test case for
   https://github.com/rails/rails/commit/7b910917.

If you have a table without a primary key, and an `after_commit` callback on that table (ie `has_transactional_callbacks?` returns true), then trying to rollback a transaction involving that record would result in “ActiveModel::MissingAttributeError: can't write unknown attribute ``”

Addresses https://github.com/rails/rails/commit/ed56e596a0467390011bc9d56d462539776adac1#commitcomment-9145960

addresses https://github.com/rails/rails/commit/91949e48cf41af9f3e4ffba3e5eecf9b0a08bfc3#commitcomment-9144563

This adresses https://github.com/rails/rails/commit/1b7aa62b184c4410c99208f71b59bbac5c5f03be#commitcomment-9147803

When associations checked for autosave have a cycle, and
none of them is dirty, then changed_for_autosave? will be an
infinite loop. We now remember if we're in the check and
will short circuit the recursion.

The hack so skip a Thread test for Ruby 1.9 can be removed now that
Rails requires Ruby >= 2.0.

Conflicts:
	activerecord/test/cases/transactions_test.rb

Update SecureToken Docs

Add Changelog entry for has_secure_token [ci skip]

Before this change we had a small set of "truthy", and all others
are "falsy".

Now, we have a small set of "falsy" values and all others are
"truthy" matching Ruby's semantics.

Before this change any error raised inside a transaction callback
are rescued and printed in the logs.

Now these errors are not rescue anymore and just bubble up,
as the other callbacks.