We sometimes say "✂️ newline after `private`" in a code review (e.g.
https://github.com/rails/rails/pull/18546#discussion_r23188776,
https://github.com/rails/rails/pull/34832#discussion_r244847195).

Now `Layout/EmptyLinesAroundAccessModifier` cop have new enforced style
`EnforcedStyle: only_before` (https://github.com/rubocop-hq/rubocop/pull/7059).

That cop and enforced style will reduce the our code review cost.

This fixes a regression for #35864.

Usually, stashed joins (mainly eager loading) are performed as LEFT
JOINs.
But the case of merging joins/left_joins of different class, that
(stashed) joins are performed as the same `join_type` as the parent
context for now.
Since #35864, both (joins/left_joins) stashed joins might be contained
in `joins_values`, so each stashed joins should maintain its own
`join_type` context.

Fixes #36103.

The default log messages for Model.exists?, when called from .save
on an object which uses scoped uniqueness validation like:

    class Example < ApplicationRecord
      validates :field, uniqueness: {scope: parent_id}
    end

can result in slightly misleading logs.

An example case:

    ↳ app/controllers/example_controller.rb:23
    (0.2ms)  begin transaction
    ↳ app/controllers/example_controller.rb:39
    Example Exists (0.2ms)  SELECT  1 AS one FROM "examples" WHERE "examples"."field" IS NULL AND "examples"."parent_id" = ? LIMIT ?  [["parent_id", 123], ["LIMIT", 1]]
    ↳ app/controllers/example_controller.rb:39
    (0.1ms)  rollback transaction

To me, a Rails newbie, this parsed as the following:

- started the transaction to create a thing
- found that your object exists already!
- so we rolled back the transaction

(even though the actual cause of the transaction is something that happens
after the Exists check.)

All this does is add a question mark to the message, to make it clear in the
log that this is a check, not a confirmation.

This may be kind of silly, but it may save some future goofs by newbs like me.

Originally the `JoinDependency` has the deduplication for eager loading
(LEFT JOIN). This re-uses that deduplication for `left_joins`.

And also, This makes left join order into part of joins, i.e.:

Before:

```
association joins -> stash joins (eager loading, etc) -> string joins -> left joins
```

After:

```
association joins -> stash joins (eager loading, left joins, etc) -> string joins
```

Now string joins are able to refer left joins.

Fixes #34325.
Fixes #34332.
Fixes #34536.

It's mentioned everywhere as `ActiveRecord::RecordNotFound` so to be
coherent with the rest of the documentation I've applied it here.
Also doc was saying if the parameter is integer it coerces it which is
other way around.

The `distinct` affects (reduces) rows of the result, so it is important
part when both `distinct` and `offset` are given.

Replacing SELECT clause to `1 AS one` and removing `distinct` and
`order` is just optimization for the `exists?`, we should not apply the
optimization for that case.

Fixes #35191.

Currently several queries cannot return correct result due to incorrect
`RangeError` handling.

First example:

```ruby
assert_equal true, Topic.where(id: [1, 9223372036854775808]).exists?
assert_equal true, Topic.where.not(id: 9223372036854775808).exists?
```

The first example is obviously to be true, but currently it returns
false.

Second example:

```ruby
assert_equal topics(:first), Topic.where(id: 1..9223372036854775808).find(1)
```

The second example also should return the object, but currently it
raises `RecordNotFound`.

It can be seen from the examples, the queries including large number
assuming empty result is not always correct.

Therefore, This change handles `RangeError` to generate executable SQL
instead of raising `RangeError` to users to always return correct
result. By this change, it is no longer raised `RangeError` to users.

Allow `ActionController::Params` as argument of
`ActiveRecord::Base#exists?`

When calling ordered finder methods such as +first+ or +last+ without an
explicit order clause, ActiveRecord sorts records by primary key. This
can result in unpredictable and surprising behaviour when the primary
key is not an auto-incrementing integer, for example when it's a UUID.
This change makes it possible to override the column used for implicit
ordering such that +first+ and +last+ will return more predictable
results. For Example:

  class Project < ActiveRecord::Base
    self.implicit_order_column = "created_at"
  end

At https://github.com/rails/rails/commit/fc0e3354af7e7878bdd905a95ce4c1491113af9a,

```rb
relation = relation.where(conditions)
```

was rewritten to:

```rb
relation.where!(condition)
```

This change accidentally changed the result of `Topic.exists?({})` from true to false.

To fix this regression, first I moved the blank check logic (`opts.blank?`) from `#where` to `#where!`,
because I thought `#where!` should be identical to `#where`, except that instead of returning a new relation,
it adds the condition to the existing relation.

But on second thought after some discussion on https://github.com/rails/rails/pull/34329,
I started to think that just fixing `#construct_relation_for_exists` is more preferable
than changing `#where` and `#where!`.

In Ruby 2.3 or later, `String#+@` is available and `+@` is faster than `dup`.

```ruby
# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "benchmark-ips"
end

Benchmark.ips do |x|
  x.report('+@') { +"" }
  x.report('dup') { "".dup }
  x.compare!
end
```

```
$ ruby -v benchmark.rb
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
Warming up --------------------------------------
                  +@   282.289k i/100ms
                 dup   187.638k i/100ms
Calculating -------------------------------------
                  +@      6.775M (± 3.6%) i/s -     33.875M in   5.006253s
                 dup      3.320M (± 2.2%) i/s -     16.700M in   5.032125s

Comparison:
                  +@:  6775299.3 i/s
                 dup:  3320400.7 i/s - 2.04x  slower

```

When you pass an empty array to find we know we shoudl return an empty
array but it is surprising that we are returning the original empty
array instead of a new one.

This reverts commit d162188d, reversing
changes made to 35767828.

Reason: #24131 conflicts the #5153's default order contract, it means
that existing apps would be broken by that change.

We don't want to break existing apps without a deprecation cycle.

Currently, column aliases which is used for eager loading are calculated
before constructing all table aliases in FROM clause.

`JoinDependency#join_constraints` constructs table aliases for `joins`
first, and then always re-constructs table aliases for eager loading.

If both `joins` and eager loading are given a same table association,
the re-construction would cause the discrepancy between column aliases
and table aliases.

To avoid the discrepancy, the column aliases should be calculated after
all table aliases are constructed.

Fixes #30603.

If `eager_loading` is true, `apply_join_dependency` force applies
LIMIT/OFFSET before JOINs by `limited_ids_for` to keep parent records
count. But for aggregation queries, LIMIT/OFFSET should be applied after
aggregations the same as SQL semantics.

And also, we could not replace SELECT list by `limited_ids_for` when a
query has a GROUP BY clause. It had never been worked since it will
causes generating invalid SQL for MySQL, PostgreSQL, and probably most
backends.

```
% ARCONN=postgresql be ruby -w -Itest test/cases/calculations_test.rb -n test_group_by_with_limit
Using postgresql
Run options: -n test_group_by_with_limit --seed 20925

# Running:

E

Error:
CalculationsTest#test_group_by_with_limit:
ActiveRecord::StatementInvalid: PG::GroupingError: ERROR:  column "posts.id" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: SELECT  DISTINCT "posts"."id", "posts"."type" AS alias_0 FRO...                         ^
: SELECT  DISTINCT "posts"."id", "posts"."type" AS alias_0 FROM "posts" LEFT OUTER JOIN "comments" ON "comments"."post_id" = "posts"."id" GROUP BY "posts"."type" ORDER BY "posts"."type" ASC LIMIT $1
```

Fixes #8103.
Closes #27249.

This reverts ignoring polymorphic error introduced at 02da8aea.

What the ignoring want to solve was caused by force eager loading
regardless of whether it is necessary, but it has been fixed by #29043.

The ignoring is now only causing a mismatch of `exists?` behavior with
`to_a`, `count`, etc. It should behave consistently.

This is a complete fix to #30995.

Originally alias tracker will only track table aliases on
`Arel::Nodes::Join`, other args are ignored.

Since c5ab6e51, parent aliases hash will be passed then it caused the
regression #30995.

It is enough to pass list of `Arel::Nodes::Join` simply, not need to
pass garbage args which will be ignored.

`find_with_associations` is meaningless name in this point since it just
contain `construct_join_dependency` and `apply_join_dependency`, does
not contain finding anything.
If `apply_join_dependency` returns `relation` and `join_dependency` then
`find_with_associations` is no longer needed.

`relation.exists?` just wants to know if there is a result or not, does
not need the exact records matched. Therefore, an intermediate SELECT
query for eager loading is not necessary.

We can use `relation.last(index)[-index]` instead of loading all records
when using reversible order because `last` will call `reverse_order`.

Currently `last` with `offset` behaves incorrectly because `offset` can
not be reversed like `limit`. Therefore, `offset` should also be handled
like `limit`.

ref https://github.com/rails/rails/pull/22653

Make it clear that `exists?` can be chained onto a relation

Fixes #30315.

I don't think this is a good abstraction because the internal method is
used only if the relation need to be applied join dependency.

`relation.exists?` should reference correct aliases while joining tables
of has_many through associations.

This is preparation to respect parent relation's alias tracking for
fixing #30681.

Because `quoted_table_name` doesn't respect table alias. We should use
`arel_attribute` for that, so I added `column_name_from_arel_node` to
generate column name from an arel node.

This regression was caused by caa178c1. The block for
`set_inverse_instance` should also be passed to join dependency.

Fixes #30402.

This builds on top of 15e2da65.
now it also returns exact Ids which were not found which will be debugging simple.

A common source of bugs and code bloat within Active Record has been the
need for us to maintain the list of bind values separately from the AST
they're associated with. This makes any sort of AST manipulation
incredibly difficult, as any time we want to potentially insert or
remove an AST node, we need to traverse the entire tree to find where
the associated bind parameters are.

With this change, the bind parameters now live on the AST directly.
Active Record does not need to know or care about them until the final
AST traversal for SQL construction. Rather than returning just the SQL,
the Arel collector will now return both the SQL and the bind parameters.
At this point the connection adapter will have all the values that it
had before.

A bit of this code is janky and something I'd like to refactor later. In
particular, I don't like how we're handling associations in the
predicate builder, the special casing of `StatementCache::Substitute` in
`QueryAttribute`, or generally how we're handling bind value replacement
in the statement cache when prepared statements are disabled.

This also mostly reverts #26378, as it moved all the code into a
location that I wanted to delete.

/cc @metaskills @yahonda, this change will affect the adapters

Fixes #29766.
Fixes #29804.
Fixes #26541.
Close #28539.
Close #24769.
Close #26468.
Close #26202.

There are probably other issues/PRs that can be closed because of this
commit, but that's all I could find on the first few pages.