`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

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.

Arel doesn't support subselect generation for DELETE unlike UPDATE yet,
but we already have that generation in connection adapters. We can
simply use the subselect generated by that one.

Most RDBMS (except SQLite) requires subselect for UPDATE with OFFSET,
but Arel doesn't support executable subselect generation for MySQL's
UPDATE yet. We need to use the subselect generated by the connection
adapter for now, it works well.

Fixes #30148.

Previously table name qualified `*` is used in that case. If it is not
qualified with a table name, an ambiguous column name error will occur
when using JOINs.

If we don't do this, then we end up with an inconsistent situation where
a parent class may e.g. reset column information, but child classes will
contine to see attribute methods as already generated, and thus not pick
up this new column (falling through to method_missing).

This new ActiveRecord configuration option allows you to easily
pinpoint what line of application code is triggering SQL queries in the
development log by appending below each SQL statement log the line of
Ruby code that triggered it.

It’s useful with N+1 issues, and to locate stray queries.

By default this new option ignores Rails and Ruby code in order to
surface only callers from your application Ruby code or your gems.

It is enabled on newly generated Rails 5.2 applications and can be
enabled on existing Rails applications:

```ruby
Rails.application.configure do
  # ...
  config.active_record.verbose_query_logs = true
end
```

The `rails app:upgrade` task will also add it to
`config/development.rb`.

This feature purposely avoids coupling with
ActiveSupport::BacktraceCleaner since ActiveRecord can be used without
ActiveRecord. This decision can be reverted in the future to allow more
configurable backtraces (the exclusion of gem callers for example).

`BigDecimal.new` has been deprecated in BigDecimal 1.3.3
 which will be a default for Ruby 2.5.

Refer https://github.com/ruby/bigdecimal/commit/533737338db915b00dc7168c3602e4b462b23503

```
$ cd rails/activerecord/
$ git grep -l BigDecimal.new | grep \.rb | xargs sed -i -e "s/BigDecimal.new/BigDecimal/g"
```

- Changes made only to Active Record. Will apply the same change to
other module once this commit is merged.

- The following deprecation has not been addressed because it has been
reported at `ActiveRecord::Result.new`. `ActiveRecord::Result.ancestors`
did not show `BigDecimal`.

* Not addressed

```ruby
/path/to/rails/activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb:34:
warning: BigDecimal.new is deprecated
```

* database_statements.rb:34

```ruby
ActiveRecord::Result.new(result.fields, result.to_a) if result
```

* ActiveRecord::Result.ancestors

```ruby
[ActiveRecord::Result,
 Enumerable,
 ActiveSupport::ToJsonWithActiveSupportEncoder,
 Object,
 Metaclass::ObjectMethods,
 Mocha::ObjectMethods,
 PP::ObjectMixin,
 ActiveSupport::Dependencies::Loadable,
 ActiveSupport::Tryable,
 JSON::Ext::Generator::GeneratorMethods::Object,
 Kernel,
 BasicObject]
```

This commit has been tested with these Ruby and BigDecimal versions

- ruby 2.5 and bigdecimal 1.3.3

```
$ ruby -v
ruby 2.5.0dev (2017-12-14 trunk 61217) [x86_64-linux]
$ gem list |grep bigdecimal
bigdecimal (default: 1.3.3, default: 1.3.2)
```

- ruby 2.4 and bigdecimal 1.3.0

```
$ ruby -v
ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-linux-gnu]
$ gem list |grep bigdecimal
bigdecimal (default: 1.3.0)
```

- ruby 2.3 and bigdecimal 1.2.8

```
$ ruby -v
ruby 2.3.5p376 (2017-09-14 revision 59905) [x86_64-linux]
$ gem list |grep -i bigdecimal
bigdecimal (1.2.8)
```

- ruby 2.2 and bigdecimal 1.2.6
```
$ ruby -v
ruby 2.2.8p477 (2017-09-14 revision 59906) [x86_64-linux]
$ gem list |grep bigdecimal
bigdecimal (1.2.6)
```

We need to pass scope attributes to `klass.new` to detect subclass.
Otherwise `subclass_from_attributes` can't detect subclass which is had
in scope attributes.

Fixes #18062.
Closes #18227.
Closes #30720.

Use CONSTRAINT_SCHEMA key for information_schema.referential_constraints.
See https://dev.mysql.com/doc/refman/5.7/en/information-schema-optimization.html.

```
> EXPLAIN SELECT fk.referenced_table_name AS 'to_table', fk.referenced_column_name AS 'primary_key', fk.column_name AS 'column', fk.constraint_name AS 'name', rc.update_rule AS 'on_update', rc.delete_rule AS 'on_delete' FROM information_schema.referential_constraints rc JOIN information_schema.key_column_usage fk USING (constraint_schema, constraint_name) WHERE fk.referenced_column_name IS NOT NULL AND fk.table_schema = 'activerecord_unittest' AND fk.table_name = 'fk_test_has_pk' AND rc.constraint_schema = 'activerecord_unittest' AND rc.table_name = 'fk_test_has_pk'\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: rc
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: CONSTRAINT_SCHEMA,TABLE_NAME
      key_len: NULL
          ref: NULL
         rows: NULL
     filtered: NULL
        Extra: Using where; Open_full_table; Scanned 0 databases
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: fk
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: TABLE_SCHEMA,TABLE_NAME
      key_len: NULL
          ref: NULL
         rows: NULL
     filtered: NULL
        Extra: Using where; Open_full_table; Scanned 0 databases; Using join buffer (Block Nested Loop)
2 rows in set, 1 warning (0.00 sec)
```