Currently `object.save` will unfreeze the object, due to
`changes_applied` replaces frozen `@attributes` to new `@attributes`.

Since originally destroyed objects are not allowed to be mutated, `save`
and `save!` should not return success in that case.

Fixes #28563.

If `collation` is given without `charset`, it may generate invalid SQL.
For example `create_database(:matt_aimonetti, collation: "utf8mb4_bin")`:

```
> CREATE DATABASE `matt_aimonetti` DEFAULT CHARACTER SET `utf8` COLLATE `utf8mb4_bin`;
ERROR 1253 (42000): COLLATION 'utf8mb4_bin' is not valid for CHARACTER SET 'utf8'
```

In MySQL, charset is used to find the default collation. If `collation`
is given explicitly, it is not necessary to give extra charset.

Because `Relation` already have Arel `table`.

This is a complete fix to #30995.

Originally alias tracker will only track table aliases on
`Arel::Nodes::Join`, other args are ignored.

Since c5ab6e51, parent aliases hash will be passed then it caused the
regression #30995.

It is enough to pass list of `Arel::Nodes::Join` simply, not need to
pass garbage args which will be ignored.

This option was moved into active_record_deprecated_finders in ac4d101a.

pg-1.0.0 is just released and most Gemfiles don't restrict
it's version. But the version is checked when connecting to
the database, which leads to the following error:

Gem::LoadError: can't activate pg (~> 0.18), already activated pg-1.0.0

See also this pg issue:
https://bitbucket.org/ged/ruby-pg/issues/270/pg-100-x64-mingw32-rails-server-not-start

Preparation for pg-1.0 was done in commit f28a3310,
but the pg version constraint was not yet relaxed.

`find_with_associations` is meaningless name in this point since it just
contain `construct_join_dependency` and `apply_join_dependency`, does
not contain finding anything.
If `apply_join_dependency` returns `relation` and `join_dependency` then
`find_with_associations` is no longer needed.

`relation.exists?` just wants to know if there is a result or not, does
not need the exact records matched. Therefore, an intermediate SELECT
query for eager loading is not necessary.

[ci skip] setting :inverse_of works with :as, :through and :polymorphic options, and is needed for bi-directionality with a scope

[ci skip] Remove :conditions opion from association basics guide

This got replaced by scopes.

Need reloading when through record has replaced.

I removed redundant `Array.wrap(records)` since `Preloader` is nodoc
class and Active Record always pass `records` as an array to
`Preloader`.
But if users relies on that behavior, it is not worth dropping its
behavior.

Fixes #31661.

Currently deleting through records doesn't respect `source_type`. It
should not be ignored in that case.

Related #23209.
Fixes #24116.

We can use `relation.last(index)[-index]` instead of loading all records
when using reversible order because `last` will call `reverse_order`.

Currently `last` with `offset` behaves incorrectly because `offset` can
not be reversed like `limit`. Therefore, `offset` should also be handled
like `limit`.

Commit 4ec5b0d6 was for fixing the regression #18787, but #28379 fixes
#18787 as well. So 4ec5b0d6 is no longer necessary.

Follow up of 09cac8c6.

This is a partial revert of #26182. There is no reason to change the
default value.

Currently reflections keeps the order when first added even if when
redefining association. As a result of the order, redefining through
association which use newly added association will raise
`HasManyThroughOrderError`. We need to redefine reflection order as well
when redefining association.

Fixes #31068.

This method which is used only in the internal was introduced in
ac384820 and was renamed in #17579. It does not need to be exposed.

An empty string is an invalid value in Ruby's range class.
So need to handle `Float::INFINITY` as it is and cast it in
`encode_range`.

Fixes #31612

Follow up of #31606.

This method was introduced in #26718, which is internally used only in
`CollectionAssociation`. There is no need to be in the reflection
classes.

Refactor delegating `join_primary_key` instead of `join_keys` and `association_primary_key` in `TableMetadata`

Because `join_primary_key` is called by `join_keys` and it is to
abstract calling `association_primary_key`.

We can't replace existing SELECT list as long as having DISTINCT, it
will cause incorrect result.

And also, PostgreSQL has a limitation that ORDER BY expressions must
appear in select list for SELECT DISTINCT.

Therefore, we should not replace existing SELECT list when using
DISTINCT.

Fixes #29779.

We can't replace existing select list as long as referenced by ORDER BY.

when model is added to collection association

Originally I tried to add `assert_equal pk.sql_type, ref.sql_type`. But
the assert failed even though the same type due to `sql_type` of primary
key in SQLite is upper case. Prefer lower case like other types.

update_attribute previously stopped execution, before saving and before
running callbacks, if the record's attributes hadn't changed. [The
documentation](http://api.rubyonrails.org/classes/ActiveRecord/Persistence.html#method-i-update_attribute)
says that "Callbacks are invoked", which was not happening if the
persisted attributes hadn't changed.

Follow up of #31177.

Like other query bang methods.

The `app:update` rake task will regenerate `development.rb` so that it
contains this option; that means we're currently adding it to existing
apps in two places, which is unnecessary and confusing.

Also:
 - Remove inaccurate comment about which stack frames are ignored
 - Clarify that the feature uses `caller_locations`, not `caller`
 - Remove unused return value in `extract_callstack`

Currently `count(:all)` with `distinct` doesn't work correctly because
SELECT list is always replaced to `*` or primary key in that case even
if having custom SELECT list.

And also, PostgreSQL has a limitation that ORDER BY expressions must
appear in select list for SELECT DISTINCT.

Therefore, we should not replace custom SELECT list when using
`count(:all)` with `distinct`.

Closes #31277.

Otherwise it will occur syntax error if primary key is a reserved word.