Calling default_scope without a proc will now raise `ArgumentError`.

Deprecated options `delete_sql`, `insert_sql`, `finder_sql` and `counter_sql`
have been deleted.

Removed deprecated methods `partial_updates`, `partial_updates?` and
`partial_updates=`

For example:

    class Post < ActiveRecord::Base
      default_scope -> { where published: true }
    end

    class Comment
      belongs_to :post
    end

When calling `Comment.join(:post)`, we expect to receive only
comments on published posts, since that is the default scope for
posts.

Before this change, the default scope from `Post` was not applied,
so we'd get comments on unpublished posts.

They were deprecated in 4.0, planned to remove in 4.1

[ci skip]

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

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

This lets us do things like call: .find_each.with_index

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 with #10992, this lets us call #with_index, etc on the results.

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.

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

of using +inverse_of: false+ option. Changing the documentation and
adding a CHANGELOG entry for the automatic inverse detection feature.

When then PostgreSQL visitor was [added](https://github.com/rails/rails/commit/6b7fdf3bf3675a14eae74acc5241089308153a34)
`add_column` was no longer receiving the column options directly. This
caused the options to be lost along the way.

When removing records from a `has_many` association it used
the `primary_key` defined on the association.

Our test suite didn't fail because on all occurences of `:primary_key`,
the specified column was available in both tables. This prevented the
code from raising an exception but it still behaved badly.

I added a test-case to prevent regressions that failed with:

```
  1) Error:
HasManyAssociationsTest#test_has_many_assignment_with_custom_primary_key:
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: essays.first_name: UPDATE "essays" SET "writer_id" = NULL WHERE "essays"."writer_id" = ? AND "essays"."first_name" IS NULL
```