Record saved in save_has_one_association already make call to association.loaded! via record's before_save callback of save_belongs_to_association,
 but this will reload object if accessed in record's validation.

`:authors` has a foreign key to `:author_addresses`.
If only `:authors` fixture loaded into the database which supports
foreign key and checks the existing data when enabling foreien keys
like Oracle, it raises the following error

`ORA-02298: cannot validate (ARUNIT.FK_RAILS_94423A17A3) - parent keys not found`

It is because there is no parent data exists in `author_addresses` table.

Here are how other database with foreign key support works:

- MySQL does not check the existing data when enabling foreign key by `foreign_key_checks=1`
https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_foreign_key_checks
> Setting foreign_key_checks to 1 does not trigger a scan of the existing table data. Therefore, rows added to the table while foreign_key_checks=0 will not be verified for consistency.

- PostgreSQL database itself has a feature to check existing data when
enabling foreign key and discussed at #27636, which is reverted.

ActiveRecord associations automatically guess the inverse associations.
But this feature does not work correctly on assoctions for STI.
For example, before this commit

```
class Post < ActiveRecord::Base
  belongs_to :author
end

class SpecialPost < Post; end

class Author < ActiveRecord::Base
  has_many :posts
  has_many :special_posts
end
```

`author.posts.first.author` works correctly, but
`author.special_posts.first.author` does not work correctly.

It is covered by following assertion.

This reverts commit 3420a145, reversing
changes made to afb66a5a.

This makes automatic inverse detection possible for polymorphic
:has_one & :has_many possible.

This resolves a number of issues, eg. `touch: true` on polymorphic relationships (#16446) and automatically setting inverse associations on newly built objects (#15028, #21843).

These are followups for 307065f9,
but TBH I'm personally not very much confortable with this style.
Maybe we could override assert_equal in our test_helper not to warn?

If a parent association was accessed in an `after_find` or
`after_initialize` callback, it would always end up loading the
association, and then immediately overwriting the association we just
loaded. If this occurred in a way that the parent's `current_scope` was
set to eager load the child, this would result in an infinite loop and
eventually overflow the stack.

For records that are created with `.new`, we have a mechanism to
perform an action before the callbacks are run. I've introduced the same
code path for records created with `instantiate`, and updated all code
which sets inverse instances on newly loaded associations to use this
block instead.

Fixes #26320.

Style/SpaceBeforeBlockBraces
Style/SpaceInsideBlockBraces
Style/SpaceInsideHashLiteralBraces

Fix all violations in the repository.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

`inverse_of` on through associations was accidently removed/caused to
stop working in commit f8d2899d which was part of a refactoring on
`ThroughReflection`.

To fix we moved `inverse_of` and `check_validity_of_inverse!` to the
`AbstractReflection` so it's available to the `ThroughReflection`
without having to dup any methods. We then need to delegate `inverse_name`
method in `ThroughReflection`. `inverse_name` can't be moved to
`AbstractReflection` without moving methods that set the instance
variable `@automatic_inverse_of`.

This adds a test that ensures that `inverse_of` on a `ThroughReflection`
returns the correct class name, and the correct record for the inverse
relationship.

Fixes #21692

use name specified by 'as' for automatic inverse association to avoid reflecting on wrong association

`InverseHasManyTests#test_parent_instance_should_be_shared_within_create_block_of_new_child`
was mistakenly the same as
`InverseHasManyTests#test_parent_instance_should_be_shared_within_build_block_of_new_child`.

check at association reader that record is inverted and should not be reloaded because of stale was changed at target record

Currently, ActiveRecord models with multiple words cannot have their
inverse associations detected automatically.

currently `post.comments.find(Comment.first.id)` would load all
comments for the given post to set the inverse association.

This has a huge performance penalty. Because if post has 100k
records and all these 100k records would be loaded in memory
even though the comment id was supplied.

Fix is to use in-memory records only if loaded? is true. Otherwise
load the records using full sql.

Fixes #10509

PR https://github.com/rails/rails/pull/10566 had to be reverted
because after applying the fix test
"test_raise_record_not_found_error_when_invalid_ids_are_passed"
started failing.

In this test invalid_id is being assigned a really large number
which was causing following failure when PR #10566 was applied.

```
RangeError: bignum too big to convert into `long long'
SELECT  `interests`.* FROM `interests`
WHERE `interests`.`man_id` = ? AND `interests`.`id` = ?
LIMIT 1  [["man_id", 970345987], ["id", 2394823094892348920348523452345]]
```

This test is not failing in master because when test code
`man.interests.find(invalid_id)` is executed then interests
are fully loaded in memory and no database query is executed.

After PR #10566 was merged then test code
`man.interests.find(invalid_id)` started executing sql query
and hence the error.

In case someone is wondering why the second part of query is not
failing, then that's because the actual query does not require
any variable substituation where the number is large. In that
case the sql generate is following.

```
SELECT `interests`.* FROM `interests`
WHERE `interests`.`man_id` = ? AND `interests`.`id`
IN (8432342, 2390102913, 2453245234523452)  [["man_id", 970345987]]
```

This reverts commit 2b817a5e, reversing
changes made to 353a398b.

Conflicts:
	activerecord/CHANGELOG.md

Reason: the build broke

currently `post.comments.find(Comment.first.id)` would load all
comments for the given post to set the inverse association.

This has a huge performance penalty. Because if post has 100k
records and all these 100k records would be loaded in memory
even though the comment id was supplied.

Fix is to use in-memory records only if loaded? is true. Otherwise
load the records using full sql.

Fixes #10509

the results. Added tests to check to make sure that inverse associations are
automatically found when has_many, has_one, or belongs_to associations
are defined.

so that it is consistent with the error thrown for +find+ without an
inverse_of association.

inverse_of option. I've also refactored the code for raising a
RecordNotFound exception when searching for records with ids.

This fixes inconsistency when building children of association
which has inverse_of set properly.

When creating new association object with a block:

    parent.association.build do |child|
      child.parent.equal?(parent) # false
    end

So the block the `child.parent` did not point to the same object.
But when the object is created it points to same instance:

    child = parent.association.build
    child.parent.equal?(parent) # true

if the association already holds that record in memory before checking
the database for the specified ids.

When calling first(n) or last(n) in a collection, Active Record was
improperly trying to set the inverse of instance in case that option
existed. This change was introduced by
fdf4eae5.

In such cases we don't need to do that "manually", since the way
collection will be loaded will already handle that, so we just skip
setting the inverse association when any argument is given to
first(n)/last(n).

The test included ensures that these scenarios will have the inverse of
instance set properly.

Fixes #8087, Closes #8094.

Squashed cherry-pick from d37d40b2 and c368b660.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/associations/collection_association.rb

It doesn't serve much purpose now that ActiveRecord::Base.all returns a
Relation.

The code is moved to active_record_deprecated_finders.