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

`display_deprecation_warning_for_false_terminator` was removed since
3a25cdca.

All indentation was normalized by rubocop auto-correct at 80e66cc4.
But comments was still kept absolute position. This commit aligns
comments with method definitions for consistency.

Some case expressions remain, need to think about those ones.

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

Do not specal case inspecting associated arrays of over 10 elements, preventing infinite looping in some cases.

The `#[]` method *used to be* an alias of `#read_attribute`, but since Rails 4
(10f6f90d), it will raise an exception for
missing attributes. Saying that it is an alias is confusing.

Ruby 2.4 unifies Fixnum and Bignum into Integer: https://bugs.ruby-lang.org/issues/12005

* Forward compat with new unified Integer class in Ruby 2.4+.
* Backward compat with separate Fixnum/Bignum in Ruby 2.2 & 2.3.
* Drops needless Fixnum distinction in docs, preferring Integer.

`has_attribute?` method to check wether a given attribute has been defined.

The focus of this change is to make the API more accessible.
References to method and classes should be linked to make it easy to
navigate around.

This patch makes exzessiv use of `rdoc-ref:` to provide more readable
docs. This makes it possible to document `ActiveRecord::Base#save` even
though the method is within a separate module
`ActiveRecord::Persistence`. The goal here is to bring the API closer to
the actual code that you would write.

This commit only deals with Active Record. The other gems will be
updated accordingly but in different commits. The pass through Active
Record is not completely finished yet. A follow up commit will change
the spots I haven't yet had the time to update.

/cc @fxn

The thread_safe gem is being deprecated and all its code has been merged
into the concurrent-ruby gem. The new class, Concurrent::Map, is exactly
the same as its predecessor except for fixes to two bugs discovered
during the merge.

When a symbol is passed in, we call `to_s` on it which allocates a string. The two hardcoded symbols that are used internally are `:to_partial_path` and `:to_model`.

This change buys us 71,136 bytes of memory and 1,777 fewer objects per request.

I wrote a utility that helps find areas where you could optimize your program using a frozen string instead of a string literal, it's called [let_it_go](https://github.com/schneems/let_it_go). After going through the output and adding `.freeze` I was able to eliminate the creation of 1,114 string objects on EVERY request to [codetriage](codetriage.com). How does this impact execution?

To look at memory:

```ruby
require 'get_process_mem'

mem = GetProcessMem.new
GC.start
GC.disable
1_114.times { " " }
before = mem.mb

after = mem.mb
GC.enable
puts "Diff: #{after - before} mb"

```

Creating 1,114 string objects results in `Diff: 0.03125 mb` of RAM allocated on every request. Or 1mb every 32 requests.

To look at raw speed:

```ruby
require 'benchmark/ips'

number_of_objects_reduced = 1_114

Benchmark.ips do |x|
  x.report("freeze")    { number_of_objects_reduced.times { " ".freeze } }
  x.report("no-freeze") { number_of_objects_reduced.times { " " } }
end
```

We get the results

```
Calculating -------------------------------------
              freeze     1.428k i/100ms
           no-freeze   609.000  i/100ms
-------------------------------------------------
              freeze     14.363k (± 8.5%) i/s -     71.400k
           no-freeze      6.084k (± 8.1%) i/s -     30.450k
```

Now we can do some maths:

```ruby
ips = 6_226k # iterations / 1 second
call_time_before = 1.0 / ips # seconds per iteration 

ips = 15_254 # iterations / 1 second
call_time_after = 1.0 / ips # seconds per iteration 

diff = call_time_before - call_time_after

number_of_objects_reduced * diff * 100

# => 0.4530373333993266 miliseconds saved per request
```

So we're shaving off 1 second of execution time for every 220 requests. 

Is this going to be an insane speed boost to any Rails app: nope. Should we merge it: yep. 

p.s. If you know of a method call that doesn't modify a string input such as [String#gsub](https://github.com/schneems/let_it_go/blob/b0e2da69f0cca87ab581022baa43291cdf48638c/lib/let_it_go/core_ext/string.rb#L37) please [give me a pull request to the appropriate file](https://github.com/schneems/let_it_go/blob/b0e2da69f0cca87ab581022baa43291cdf48638c/lib/let_it_go/core_ext/string.rb#L37), or open an issue in LetItGo so we can track and freeze more strings. 

Keep those strings Frozen

![](https://www.dropbox.com/s/z4dj9fdsv213r4v/let-it-go.gif?dl=1)

It's finally finished!!!!!!! The reason the Attributes API was kept
private in 4.2 was due to some publicly visible implementation details.
It was previously implemented by overloading `columns` and
`columns_hash`, to make them return column objects which were modified
with the attribute information.

This meant that those methods LIED! We didn't change the database
schema. We changed the attribute information on the class. That is
wrong! It should be the other way around, where schema loading just
calls the attributes API for you. And now it does!

Yes, this means that there is nothing that happens in automatic schema
loading that you couldn't manually do yourself. (There's still some
funky cases where we hit the connection adapter that I need to handle,
before we can turn off automatic schema detection entirely.)

There were a few weird test failures caused by this that had to be
fixed. The main source came from the fact that the attribute methods are
now defined in terms of `attribute_names`, which has a clause like
`return [] unless table_exists?`. I don't *think* this is an issue,
since the only place this caused failures were in a fake adapter which
didn't override `table_exists?`.

Additionally, there were a few cases where tests were failing because a
migration was run, but the model was not reloaded. I'm not sure why
these started failing from this change, I might need to clear an
additional cache in `reload_schema_from_cache`. Again, since this is not
normal usage, and it's expected that `reset_column_information` will be
called after the table is modified, I don't think it's a problem.

Still, test failures that were unrelated to the change are worrying, and
I need to dig into them further.

Finally, I spent a lot of time debugging issues with the mutex used in
`define_attribute_methods`. I think we can just remove that method
entirely, and define the attribute methods *manually* in the call to
`define_attribute`, which would simplify the code *tremendously*.

Ok. now to make this damn thing public, and work on moving it up to
Active Model.

[ci skip]

This method can be used to see all of the fields on a model which have
been read. This can be useful during development mode to quickly find
out which fields need to be selected. For performance critical pages, if
you are not using all of the fields of a database, an easy performance
win is only selecting the fields which you need. By calling this method
at the end of a controller action, it's easy to determine which fields
need to be selected.

While writing this, I also noticed a place for an easy performance win
internally which I had been wanting to introduce. You cannot mutate a
field which you have not read. Therefore, we can skip the calculation of
in place changes if we have never read from the field. This can
significantly speed up methods like `#changed?` if any of the fields
have an expensive mutable type (like `serialize`)

```
Calculating -------------------------------------
 #changed? with serialized column (before)
                       391.000  i/100ms
 #changed? with serialized column (after)
                         1.514k i/100ms
-------------------------------------------------
 #changed? with serialized column (before)
                          4.243k (± 3.7%) i/s -     21.505k
 #changed? with serialized column (after)
                         16.789k (± 3.2%) i/s -     84.784k
```

This reverts commit ae96f229.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/attribute_methods.rb

[ci skip]

[ci skip]

We added a comparison to "id", and call to `self.class.primary_key` a
*lot*. We also have performance hits from `&block` all over the place.
We skip the check in a new method, in order to avoid breaking the
behavior of `read_attribute`

This patch uniformizes warning messages. I used the most common style
already present in the code base:

* Capitalize the first word.

* End the message with a full stop.

* "Rails 5" instead of "Rails 5.0".

* Backticks for method names and inline code.

Also, converted a few long strings into the new heredoc convention.