Fixes #19568

- ‘dection’ -> ‘detection’

- ‘exceeds threshold’ -> ‘exceeds the threshold’

- Other minor improvements.

When set to an integer, a warning will be logged whenever a result set
larger than the specified size is returned by a query. Fixes #16463

The warning is outputed a module which is prepended in an initializer,
so there will be no performance impact if
`config.active_record.warn_on_records_fetched_greater_than` is not set.

This reverts commit 796cab45.

Fixes db:structure:dump when using schema_search_path and PostgreSQL
extensions.

Closes #17157.

Instead use .scope_attributes? consistently in ActiveRecord to check whether
there are attributes currently associated with the scope.

Move the implementation of .scope_attributes? and .scope_attributes to
ActiveRecord::Scoping because they don't particularly have to do specifically
with Named scopes and their only dependency, in the case of
.scope_attributes?, and only caller, in the case of .scope_attributes is
contained in Scoping.

I should have done this in the first place. We are now serializing an
explicit version so we can make more careful changes in the future. This
will load Active Record objects which were serialized in Rails 4.1.

There will be bugs, as YAML serialization was at least partially broken
back then. There will also be edge cases that we might not be able to
handle, especially if the type of a column has changed.

In addition, we're passing this as `from_database`, since that is
required for serialized columns at minimum. All other types were
serializing the cast value. At a glance, there should be no types for
which this is a problem.

Finally, dirty checking information will be lost on records serialized
in 4.1, so no columns will be marked as changed.

This reverts commit 393e65b4 and
ec51c3fe

We don't want the records to hold hard references to transactions
because they point at records that have callbacks.

this way we don't have to mutate a state object, we can just change the
state of the txn

As far as I can tell nobody is setting this variable.

`@reflects_state[depth+1]` will always be nil because we haven't made a
method call that would make it true yet.

[fixes #18903]

Deprecate `required` option in favor of `optional` for belongs_to.

They are implementation details

Deprecated finders are not supported anymore

Fix Issue #15549, unbounded memory growth when saving records that have any after_create callbacks (or any associations, which makes after_create callbacks for you)

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.

Remaining are `limit`, `precision`, `scale`, and `type` (the symbol
version). These will remain on the column, since they mirror the options
to the `column` method in the schema definition DSL

Related to #10690.

`initialize_internals_callback` will attempt to assign attributes from
the current scope, which will fail if something defined the method
and calls super (meaning it won't hit `method_missing`).

Fixes #18339

This API will require much less consuming code to change to accomodate
the removal of automatic type casting from Arel. As long as the
predicates are constructed using the `arel_table` off of an AR subclass,
there will be no changes that need to happen.

This class cares far too much about the internals of other parts of
Active Record. This is an attempt to break out a meaningful object which
represents the needs of the predicate builder. I'm not fully satisfied
with the name, but the general concept is an object which represents a
table, the associations to/from that table, and the types associated
with it. Many of these exist at the `ActiveRecord::Base` class level,
not as properties of the table itself, hence the need for another
object. Currently it provides these by holding a reference to the class,
but that will likely change in the future. This allows the predicate
builder to remain wholy concerned with building predicates.

/cc @mrgilman

Construction of relations can be a hotspot, we don't want to create one
of these in the constructor. This also allows us to do more expensive
things in the predicate builder's constructor, since it's created once
per AR::Base subclass

If there is a method defined such as `find_and_do_stuff(id)`, which then
gets called on an association, we will perform statement caching and the
parent ID will not change on subsequent calls.

Fixes #18117

This `# :nodoc:` had the effect of hiding every method that follows.
This meant that the API page for `ActiveRecord::Core` only contained
`configurations` and none of the following methods.

Furthermore this `# :nodoc:` had no effect on `maintain_test_schema`.
Those `mattr_accessor` inside the `included` block are not picked up
by rdoc.

/cc @zzak

We never actually make use of it on the table, since we're constructing
the select manager manually. It looks like if we ever actually were
grabbing it from the table, we're grossly misusing it since it's meant
to vary by AR class.

Its existence on `Arel::Table` appears to be purely for convenience
methods that are never used outside of tests. However, in production
code it just complicates construction of the tables on the rails side,
and the plan is to remove it from `Arel::Table` entirely. I'm not
convinced it needs to live on `SelectManager`, etc either.

We don't know which attributes will or won't be used, and we don't want
to create massive bottlenecks at instantiation. Rather than doing *any*
iteration over types and values, we can lazily instantiate the object.

The lazy attribute hash should not fully implement hash, or subclass
hash at any point in the future. It is not meant to be a replacement,
but instead implement its own interface which happens to overlap.

Sufficiently large integers cause `find` and `find_by` to raise
`StatementInvalid` instead of `RecordNotFound` or just returning `nil`.
Given that we can't cast to `nil` for `Integer` like we would with junk
data for other types, we raise a `RangeError` instead, and rescue in
places where it would be highly unexpected to get an exception from
casting.

Fixes #17380

It is internal use only. This is to avoid conflicting with users' method
names. Fixes #17458

The current style for warning messages without newlines uses
concatenation of string literals with manual trailing spaces
where needed.

Heredocs have better readability, and with `squish` we can still
produce a single line.

This is a similar use case to the one that motivated defining
`strip_heredoc`, heredocs are super clean.

This will avoid naming clash with user defined methods

Isolate access to @associations_cache and @aggregations cache to the Associations and Aggregations modules, respectively.

This includes replacing the `association_cache` accessor with a more
limited `association_cached?` accessor and making `clear_association_cache`
and `clear_aggregation_cache` private.

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

Simplify `find_by_statement_cache` interaction down to a class-level ivar with a single accessor `cached_find_by_statement`.