Closes #11026

fixes #10669

While joining_values special treatment is given to string values.
By flattening the array it ensures that string values are detected
as strings and not arrays.

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]]
```

PostgreSQL escapes single quotes by using an additional single quote.
When Rails queries the column information, PostgreSQL returns the
default values with the escaped single quotes.

#extract_value_from_default now converts these to one single quote each.

Fixes #10881.

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

Conflicts:
	activerecord/CHANGELOG.md

Reason: the build broke

If I have a query that produces sql
`WHERE "users"."name" = 'a         b'` then in the log all the
whitespace is being squeezed. So the sql that is printed in the
log is `WHERE "users"."name" = 'a b'`.

This can be confusing. This commit fixes it by ensuring that
whitespace is not squeezed.

fixes #10982

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

As you can also configure your database connection using `ENV["DATABASE_URL"]`,
the fixture setup can't reply on the `.configurations` Hash.

As the fixtures are only loaded when ActiveRecord is actually used
(`rails/test_help.rb`) it should be safe to drop the check for an existing configuration.

recreate the models without mucking with internal caches of the relation
objects, then the test fails.

For example:

class Man < ActiveRecord::Base
  has_many :interests
end

class Interest < ActiveRecord::Base
  belongs_to :man
end

Then do this test:

def test_validate_presence_of_parent_fails_without_inverse_of
  repair_validations(Interest) do
    Interest.validates_presence_of(:man)
    assert_no_difference ['Man.count', 'Interest.count'] do
      man = Man.create(:name => 'John',
                       :interests_attributes => [{:topic=>'Cars'}, {:topic=>'Sports'}])
      assert_not_predicate man.errors[:"interests.man"], :empty?
    end
  end
end

The test will fail.  This is a bad test, so I am removing it.

For example, you need to change this:

  class Author < ActiveRecord::Base
    has_many :posts
    has_many :taggings, :through => :posts
  end

  class Post < ActiveRecord::Base
    has_one :tagging
    has_many :taggings
  end

  class Tagging < ActiveRecord::Base
  end

To this:

  class Author < ActiveRecord::Base
    has_many :posts
    has_many :taggings, :through => :posts, :source => :tagging
  end

  class Post < ActiveRecord::Base
    has_one :tagging
    has_many :taggings
  end

  class Tagging < ActiveRecord::Base
  end