ActiveRecord::Observer was extracted into a separate gem so it
should not be referenced anymore (see https://github.com/rails/rails/commit/ccecab3)

At present, ActiveRecord::Delegation compiles delegation methods on a
global basis. The compiled methods apply to all subsequent Relation
instances. This creates several problems:

1) After Post.all.recent has been called, User.all.respond_to?(:recent)
   will be true, even if User.all.recent will actually raise an error due
   to no User.recent method existing. (See #8080.)

2) Depending on the AR class, the delegation should do different things.
   For example, if a Post.zip method exists, then Post.all.zip should call
   it. But this will then result in User.zip being called by a subsequent
   User.all.zip, even if User.zip does not exist, when in fact
   User.all.zip should call User.all.to_a.zip. (There are various
   variants of this problem.)

We are creating these compiled delegations in order to avoid method
missing and to avoid repeating logic on each invocation.

One way of handling these issues is to add additional checks in various
places to ensure we're doing the "right thing". However, this makes the
compiled methods signficantly slower. In which case, there's almost no
point in avoiding method_missing at all. (See #8127 for a proposed
solution which takes this approach.)

This is an alternative approach which involves creating a subclass of
ActiveRecord::Relation for each AR class represented. So, with this
patch, Post.all.class != User.all.class. This means that the delegations
are compiled for and only apply to a single AR class. A compiled method
for Post.all will not be invoked from User.all.

This solves the above issues without incurring significant performance
penalties. It's designed to be relatively seamless, however the downside
is a bit of complexity and potentially confusion for a user who thinks
that Post.all and User.all should be instances of the same class.

Benchmark
---------

require 'active_record'
require 'benchmark/ips'

class Post < ActiveRecord::Base
  establish_connection adapter: 'sqlite3', database: ':memory:'
  connection.create_table :posts

  def self.omg
    :omg
  end
end

relation = Post.all

Benchmark.ips do |r|
  r.report('delegation')   { relation.omg }
  r.report('constructing') { Post.all }
end

Before
------

Calculating -------------------------------------
          delegation      4392 i/100ms
        constructing      4780 i/100ms
-------------------------------------------------
          delegation   144235.9 (±27.7%) i/s -     663192 in   5.038075s
        constructing   182015.5 (±21.2%) i/s -     850840 in   5.005364s

After
-----

Calculating -------------------------------------
          delegation      6677 i/100ms
        constructing      6260 i/100ms
-------------------------------------------------
          delegation   166828.2 (±34.2%) i/s -     754501 in   5.001430s
        constructing   116575.5 (±18.6%) i/s -     563400 in   5.036690s

Comments
--------

Bear in mind that the standard deviations in the above are huge, so we
can't compare the numbers too directly. However, we can conclude that
Relation construction has become a little slower (as we'd expect), but
not by a huge huge amount, and we can still construct a large number of
Relations quite quickly.

Keying these hashes by klass causes reloadable classes to never get
freed. Thanks to @thedarkone for pointing this out in
the comments on 221571be.

This doesn't seem to make a massive difference to performance.

Benchmark
---------

require 'active_record'
require 'benchmark/ips'

class Post < ActiveRecord::Base
  establish_connection adapter: 'sqlite3', database: ':memory:'
end

GC.disable

Benchmark.ips(20) do |r|
  r.report { Post.connection }
end

Before
------

Calculating -------------------------------------
                          5632 i/100ms
-------------------------------------------------
                       218671.0 (±1.9%) i/s -    4364800 in  19.969401s

After
-----

Calculating -------------------------------------
                          8743 i/100ms
-------------------------------------------------
                       206525.9 (±17.8%) i/s -    4039266 in  19.992590s

These are for internal use only and cannot be relied on as part of the
public API. See discussion on 8c2c6051.

This reverts commit a3024f81.

REASON: Since warning doesn't exist

Move instantiation responsibilities from Inheritance to Persistence. Have Inheritance#discriminate_class_for_record handle STI lookup duties.

Allows you to do BaseClass.new(:type => "SubClass") as well as
parent.children.build(:type => "SubClass") or parent.build_child
to initialize an STI subclass. Ensures that the class name is a
valid class and that it is in the ancestors of the super class
that the association is expecting.

[ci skip]

Rather than just raising a NoMethodError when copying the config, this
commit adds a warning message until either the rails-observers gem is
installed or the relevant config options are removed.

They was extracted from a plugin.

See https://github.com/rails/rails-observers

[Rafael Mendonça França + Steve Klabnik]

Fixes #7418.

This table is being used to verify if the :limit options is being
ignored for text and binary columns

Remove FIXME tag from abstract adapter test.

We are already in the AR namespace, there's no way for it to be
undefined. See the cousin commit 13e72db7

Refactor a bit teardown fixtures to avoid two conditionals.

Setting a nil datetime attribute to a blank string should not cause the
attribute to be dirty.

Fix #8310

Introduced in 53ca22f2.
Thanks @rochefort. [ci skip]

Closes #8265

Since edd94cee, CollectionProxy
delegates all calculation methods - except count - to the scope,
which does basically what this method was doing, but since we're
delegating from the proxy, the association method was never called.

To perform a sum calculation over the array of elements, use to_a.sum(&block).

Please check the discussion in f9cb645d
for more context.

This reverts commit f9cb645d.

Conflicts:
	activerecord/CHANGELOG.md

Revert "Allow blocks for count with ActiveRecord::Relation. Document and test that sum allows blocks"

This reverts commit 9cc2bf69.

Conflicts:
	activerecord/lib/active_record/relation/calculations.rb

This improves memory and performance without having to use symbols which
present DoS problems. Thanks @headius and @tenderlove for the
suggestion.

This was originally committed in
f1765019, and then reverted in
d3494903 due to it causing problems in a
real application. This second attempt should solve that.

Benchmark
---------

require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')

class Post < ActiveRecord::Base
  connection.create_table :posts, force: true do |t|
    t.string :name
  end
end

post = Post.create name: 'omg'

Benchmark.ips do |r|
  r.report('Post.new')          { Post.new name: 'omg' }
  r.report('post.name')         { post.name }
  r.report('post.name=')        { post.name = 'omg' }
  r.report('Post.find(1).name') { Post.find(1).name }
end

Before
------

Calculating -------------------------------------
            Post.new      1419 i/100ms
           post.name      7538 i/100ms
          post.name=      3024 i/100ms
   Post.find(1).name       243 i/100ms
-------------------------------------------------
            Post.new    20637.6 (±12.7%) i/s -     102168 in   5.039578s
           post.name  1167897.7 (±18.2%) i/s -    5186144 in   4.983077s
          post.name=    64305.6 (±9.6%) i/s -     317520 in   4.998720s
   Post.find(1).name     2678.8 (±10.8%) i/s -      13365 in   5.051265s

After
-----

Calculating -------------------------------------
            Post.new      1431 i/100ms
           post.name      7790 i/100ms
          post.name=      3181 i/100ms
   Post.find(1).name       245 i/100ms
-------------------------------------------------
            Post.new    21308.8 (±12.2%) i/s -     105894 in   5.053879s
           post.name  1534103.8 (±2.1%) i/s -    7634200 in   4.979405s
          post.name=    67441.0 (±7.5%) i/s -     337186 in   5.037871s
   Post.find(1).name     2681.9 (±10.6%) i/s -      13475 in   5.084511s

If you accidentally pass a string and a symbol, this breaks. So
we coerce them both to strings.

Fixes #7715