Hash#keys.each allocates an array of keys; Hash#each_key iterates through the
keys without allocating a new array. This is the reason why Hash#each_key
exists.

@column_names_with_alias, @dynamic_methods_hash, @time_zone_column_names, and @cached_time_zone

Inspired by @tenderlove's work in
c363fff2, this reduces the number of
strings allocated when running callbacks for ActiveRecord instances. I
measured that using this script:

```
require 'objspace'
require 'active_record'
require 'allocation_tracer'

ActiveRecord::Base.establish_connection adapter: "sqlite3",
                                        database: ":memory:"

ActiveRecord::Base.connection.instance_eval do
  create_table(:articles) { |t| t.string :name }
end

class Article < ActiveRecord::Base; end
a = Article.create name: "foo"
a = Article.find a.id

N = 10
result = ObjectSpace::AllocationTracer.trace do
  N.times { Article.find a.id }
end

result.sort.each do |k,v|
  p k => v
end
puts "total: #{result.values.map(&:first).inject(:+)}"
```

When I run this against master and this branch I get this output:

```
pete@balloon:~/projects/rails/activerecord$ git checkout master
M Gemfile
Switched to branch 'master'
pete@balloon:~/projects/rails/activerecord$ bundle exec ruby benchmark_allocation_with_callback_send.rb > allocations_before
pete@balloon:~/projects/rails/activerecord$ git checkout remove-dynamic-send-on-built-in-callbacks
M Gemfile
Switched to branch 'remove-dynamic-send-on-built-in-callbacks'
pete@balloon:~/projects/rails/activerecord$ bundle exec ruby benchmark_allocation_with_callback_send.rb > allocations_after
pete@balloon:~/projects/rails/activerecord$ diff allocations_before allocations_after
39d38
<
{["/home/pete/projects/rails/activesupport/lib/active_support/callbacks.rb",
81]=>[40, 0, 0, 0, 0, 0]}
42c41
< total: 630
---
> total: 590

```

In addition to this, there are two micro-optimizations present:

* Using `block.call if block` vs `yield if block_given?` when the block was being captured already.

```
pete@balloon:~/projects$ cat benchmark_block_call_vs_yield.rb
require 'benchmark/ips'

def block_capture_with_yield &block
  yield if block_given?
end

def block_capture_with_call &block
  block.call if block
end

def no_block_capture
  yield if block_given?
end

Benchmark.ips do |b|
  b.report("block_capture_with_yield") { block_capture_with_yield }
  b.report("block_capture_with_call") { block_capture_with_call }
  b.report("no_block_capture") { no_block_capture }
end
pete@balloon:~/projects$ ruby benchmark_block_call_vs_yield.rb
Calculating -------------------------------------
block_capture_with_yield
                        124979 i/100ms
block_capture_with_call
                        138340 i/100ms
    no_block_capture    136827 i/100ms
-------------------------------------------------
block_capture_with_yield
                      5703108.9 (±2.4%) i/s -   28495212 in   4.999368s
block_capture_with_call
                      6840730.5 (±3.6%) i/s -   34169980 in   5.002649s
    no_block_capture  5821141.4 (±2.8%) i/s -   29144151 in   5.010580s
```

* Defining and calling methods instead of using send.

```
pete@balloon:~/projects$ cat benchmark_method_call_vs_send.rb
require 'benchmark/ips'

class Foo
  def tacos
    nil
  end
end

my_foo = Foo.new

Benchmark.ips do |b|
  b.report('send') { my_foo.send('tacos') }
  b.report('call') { my_foo.tacos }
end
pete@balloon:~/projects$ ruby benchmark_method_call_vs_send.rb
Calculating -------------------------------------
                send     97736 i/100ms
                call    151142 i/100ms
-------------------------------------------------
                send  2683730.3 (±2.8%) i/s -   13487568 in   5.029763s
                call  8005963.9 (±2.7%) i/s -   40052630 in   5.006604s
```

The result of this is making typical ActiveRecord operations slightly faster:

https://gist.github.com/phiggins/e46e51dcc7edb45b5f98

Since we want this flag to be enabled anytime we are running the tests
under JRuby, let's enable this at the Rakefile level so people get the
performance boost on their local checkout.

Moreover, we avoid having to update this particular line anytime the
option changes on the JRuby side.

The only drawback is that we have to define it in every Rakefile but
there's no big deal, this is already the case for other options.

- middlware -> middleware

Benchmark:

```ruby
require 'objspace'
require 'active_record'

ActiveRecord::Base.establish_connection adapter: "sqlite3",
                                        database: ":memory:"

ActiveRecord::Base.connection.instance_eval do
  create_table(:articles) { |t| t.string :name }
end

class Article < ActiveRecord::Base; end
a = Article.create name: "foo"
a = Article.find a.id

N = 10
ObjectSpace::AllocationTracer.trace do
  N.times { Article.find a.id }
end
ObjectSpace::AllocationTracer.allocated_count_table
table.sort_by { |_,x| x }.each do |k,v|
  p k => (v / N)
end
```

Honoring an overidden `rack.test` allows testing closed connection between
multiple requests. This is useful if you're working on database resiliency, to
ensure the connection is in the expected state from one request to another on
the same worker.

[ci skip]

it doesn't work on SQLite3 since it doesn't support truncate, but that's
OK.  If you call truncate on the connection, you're now bound to that
database (same as if you use hstore or any other db specific feature).

Previously the documentation stated that `primary_key` should be the name of
a *method* that returns the primary key used for the association. This is
incorrect. This changes the documentation to say that the value must be the
name of a column.

fix fixtures not loading if a belongs_to association is defined with a :foreign_key option that's a symbol

This test has always been green because it uses "assert" and the first
argument is an truthy class/object.

The hash is now string-keyed, and [_]reflect_on_association calls `to_s` on the
argument anyway.

For now, we will just skip the cache when a non-column key is used in the hash.
If the future, we can probably move some of the logic in PredicateBuilder.expand
up the chain to make caching possible for association queries.

Closes #16903
Fixes #16884

This is pretty lame though, so feel free to send a PR with a better fix! ;)

cc @zev @tenderlove

 - Added a note about some features which are not dependent on
   auto-generated ID's.
 - [ci skip]

While investigating #16951 I found that another library's monkey-patching of
`Enumerable` was causing the test migrations helper to break when trying to
build the `CREATE DATABASE` statement. The prior approach used `#sum` to build
the string from the options hash.

As the code that combines the options to build the database statement is not
user-facing, using `#inject` here instead will remove the only place where the
database creation/migration code is dependent on ActiveSupport's monkey-patching
of `Enumerable`.

Closes #16907.

[Matthew Draper & Yves Senn]

Maintain compatibility for:
* ActiveRecord::Base#find(:all)
* ActiveRecord::Base#find(:first)
* ActiveRecord::Base#find(:last)

Working with two different machines is hard 😓

[Matthew Draper & Yves Senn]

Closes #16860. (pull request to discuss the implementation)

Changed the DangerousAttributeError exception message to include messaging that the conflict could be because of a conflicting attribute.

Dump the standard schema once instead of redoing it per test

Allows :limit defaults to be changed without pulling the rug out from
under old migrations that omitted :limit because it matched the default
at the time.

to drop sequences at the same time each tables dropped for Oracle