* add newline for display the fenced code block
* add "#" in the comments section

This reverts commit 52439460.

This fixes an issue with the build where tests would fail on mysql and
postgresql due to different ordering.

This reverts commit 6dc6a0b1, reversing
changes made to ec94f00b.

This pull request broke the build.

We should call `scope.order!` and set `scope.reordering_value` to `true` if :reordering values are specified

Fixes #21886

Closes #21922

Let `Book(id, author_id)`, `Photo(id, book_id, author_id)` and `Author(id)`

Running `Book.group(:author_id).joins(:photos).count` will produce:

* Rails 4.2 - conflicts `author_id` in both projection and group by:
```sql
SELECT COUNT(*) AS count_all, author_id AS author_id
  FROM "books" INNER JOIN "photos" ON "photos"."book_id" = "books"."id"
 GROUP BY author_id
```

* Master (9d02a25d) - conflicts `author_id` only in projection:
```sql
SELECT COUNT(*) AS count_all, author_id AS author_id
  FROM "books" INNER JOIN "photos" ON "photos"."book_id" = "books"."id"
 GROUP BY "books"."author_id"
```

* With this fix:
```sql
SELECT COUNT(*) AS count_all, "books"."author_id" AS books_author_id
  FROM "books" INNER JOIN "photos" ON "photos"."book_id" = "books"."id"
 GROUP BY "books"."author_id"
```

Column names inserted via `group` have to be qualified with table name.

Prior to this commit, Rails makes no differentiation between whether a
query uses bind parameters, and whether or not we cache that query as a
prepared statement. This leads to the cache populating extremely fast in
some cases, with the statements never being reused.

In particular, the two problematic cases are `where(foo: [1, 2, 3])` and
`where("foo = ?", 1)`. In both cases we'll end up quoting the values
rather than using a bind param, causing a cache entry for every value
ever used in that query.

It was noted that we can probably eventually change `where("foo = ?",
1)` to use a bind param, which would resolve that case. Additionally, on
PG we can change our generated query to be `WHERE foo = ANY($1)`, and
pass an array for the bind param. I hope to accomplish both in the
future.

For SQLite and MySQL, we still end up preparing the statements anyway,
we just don't cache it. The statement will be cleaned up after it is
executed. On postgres, we skip the prepare step entirely, as an API is
provided to execute with bind params without preparing the statement.

I'm not 100% happy on the way this ended up being structured. I was
hoping to use a decorator on the visitor, rather than mixing a module
into the object, but the way Arel has it's visitor pattern set up makes
it very difficult to extend without inheritance. I'd like to remove the
duplication from the various places that are extending it, but that'll
require a larger restructuring of that initialization logic. I'm going
to take another look at the structure of it soon.

This changes the signature of one of the adapter's internals, and will
require downstream changes from third party adapters. I'm not too
worried about this, as worst case they can simply add the parameter and
always ignore it, and just keep their previous behavior.

Fixes #21992.

[#20473]

This type adds an escape hatch to apps for which string duping causes
unacceptable memory growth. The reason we are duping them is in order to
detect mutation, which was a feature added to 4.2 in #15674. The string
type was modified to support this behavior in #15788.

Memory growth is really only a concern for string types, as it's the
only mutable type where the act of coersion does not create a new object
regardless (as we're usually returning an object of a different class).

I do feel strongly that if we are going to support detecting mutation,
we should do it universally for any type which is mutable. While it is
less common and ideomatic to mutate strings than arrays or hashes, there
shouldn't be rules or gotchas to understanding our behavior.

However, I also appreciate that for apps which are using a lot of string
columns, this would increase the number of allocations by a large
factor. To ensure that we keep our contract, if you'd like to opt out of
mutation detection on strings, you'll also be option out of mutation of
those strings.

I'm not completely married to the thought that strings coming out of
this actually need to be frozen -- and I think the name is correct
either way, as the purpose of this is to provide a string type which
does not detect mutation.

In the new implementation, I'm only overriding `cast_value`. I did not
port over the duping in `serialize`. I cannot think of a reason we'd
need to dup the string there, and the tests pass without it.
Unfortunately that line was introduced at a time where I was not nearly
as good about writing my commit messages, so I have no context as to
why I added it. Thanks past Sean. You are a jerk.

When passing an instance of `ActiveRecord::Base` to `#update`, it would
internally call `#find`, resulting in a misleading deprecation warning.

This change gives this deprecated use of `#update` its own, meaningful
warning.

Closes #21563.

The `name` argument of `add_references` was both used to generate the
column name `<name>_id` and as the target table for the foreign key
`name.pluralize`.

It's primary purpose is to define the column name. In cases where the
`to_table` of the foreign key is different than the column name we
should be able to specify it individually.

With the previous implementation, the block passed to
define_singleton_method, which will live forever as the method body,
captures the parameters (args and block) in its enclosure.

For the current_scope registry, that can include an AR::Relation.

`pk_and_sequence_for` is implemented for PG and MySQL adapters (not
implemented for Sqlite3 adapter). But MySQL adapters are not using
`pk_and_sequence_for` already.

In this commit, find_by doesn't cache arguments
so that find_by with association subquery works correctly.

Fixes #20817

Move from `AS::Callbacks::CallbackChain.halt_and_display_warning_on_return_false`
to `AS::Callbacks.halt_and_display_warning_on_return_false` base on
[this
discussion](https://github.com/rails/rails/pull/21218#discussion_r39354580)

Fix the documentation broken by 0a120a81

The implementation of `attribute_method?` on Active Record requires
establishing a database connection and querying the schema. As a general
rule, we don't want to require database connections for any class macro,
as the class should be able to be loaded without a database (e.g. for
things like compiling assets).

Instead of eagerly defining these methods, we do it lazily the first
time they are accessed via `method_missing`. This should not cause any
performance hits, as it will only hit `method_missing` once for the
entire class.

Timestamp column can have less precision than ruby timestamp
In result in how big a fraction of a second can be stored in the
database.

  m = Model.create!
  m.created_at.usec == m.reload.created_at.usec
    # => false
    # due to different seconds precision in Time.now and database column

If the precision is low enough, (mysql default is 0, so it is always low
enough by default) the value changes when model is reloaded from the
database. This patch fixes that issue ensuring that any timestamp
assigned as an attribute is converted to column precision under the
attribute.

These new methods are used from the Active Record model layer to
determine which relations are viable to back a model. These new methods
allow us to change `conn.tables` in the future to only return tables and
no views. Same for `conn.table_exists?`.

The goal is to provide the following introspection methods on the
connection:

* `tables`
* `table_exists?`
* `views`
* `view_exists?`
* `data_sources` (views + tables)
* `data_source_exists?` (views + tables)

Fixes #21488

[Sean Griffin & johanlunds]

Example:

    create_table :barcodes, primary_key: ["region", "code"] do |t|
      t.string :region
      t.integer :code
    end

In the case of using `unsigned` as the type:

    create_table :foos do |t|
      t.unsigned_integer :unsigned_integer
      t.unsigned_bigint  :unsigned_bigint
      t.unsigned_float   :unsigned_float
      t.unsigned_decimal :unsigned_decimal, precision: 10, scale: 2
    end

Example:

    create_table :foos do |t|
      t.integer :unsigned_integer, unsigned: true
      t.bigint  :unsigned_bigint,  unsigned: true
      t.float   :unsigned_float,   unsigned: true
      t.decimal :unsigned_decimal, unsigned: true, precision: 10, scale: 2
    end

Currently, `set_fixture_class` is only available using the
`TestFixtures` concern and it is ignored for `rake db:fixtures:load`.
Using the correct model class, it is possible for the fixture load
to also load the associations from the YAML files (e.g., `:belongs_to`
and `:has_many`).

Generic cast-to-text was only added in 8.3.