Our general contract in Active Record is that strings are assumed to be
SQL literals, and symbols are assumed to reference a column. If a from
clause is given, we shouldn't include the table name, but we should
still quote the value as if it were a column.

Upon fixing this, the tests were still failing on SQLite. This was
because the column name being returned by the query was `"\"join\""`
instead of `"join"`. This is actually a bug in SQLite that was fixed a
long time ago, but I was using the version of SQLite included by OS X
which has this bug. Since I'm guessing this will be a common case for
contributors, I also added an explicit check with a more helpful error
message.

Fixes #20360

This allows easier integration with ActiveRecord, such that
AR#pluck will now use Enumerable#pluck if the relation is loaded,
without needing to hit the database.

For certain column types, using `serialize` is unnecessary, or the user
may get unexpected contents back from the DB adapter (which is handling
some basic deserialization for them). Call this out in the
documentation.

For background, see:

    https://gist.github.com/ernie/33f75f2294885b9806f9
    https://twitter.com/erniemiller/status/604262907442905090

This is a usability change to fix a quirk from our definition of partial
writes. By default, we only persist changed attributes. When creating a
new record, this is assumed that the default values came from the
database. However, if the user provided a default, it will not be
persisted, since we didn't see it as "changed". Since this is a very
specific case, I wanted to isolate it with the other quirks that come
from user provided default values. The number of edge cases which are
presenting themselves are starting to make me wonder if we should just
remove the ability to assign a default, in favor of overriding
`initialize`. For the time being, this is required for the attributes
API to not have confusing behavior.

We had to delete one test, since this actually changes the meaning of
`.changed?` on Active Record models. It now specifically means
`changed_from_database?`. While I think this will make the attributes
API more ergonomic to use, it is a subtle change in definition (though
not a backwards incompatible one). We should probably figure out the
right place to document this. (Feel free to open a PR doing that if
you're reading this).

/cc @rafaelfranca @kirs @senny

This is an alternate implementation of #19921.

Close #19921.

[Sean Griffin & Kir Shatrov]

This is a variant implementation of the changes proposed in #19914.
Unlike that PR, the change in behavior is isolated in its own class.
This is to prevent wonky behavior if a Proc is assigned outside of the
default, and it is a natural place to place the behavior required by #19921
as well.

Close #19914.

[Sean Griffin & Kir Shatrov]

Couldn't find other way to get the association name from a given class
other than looping through `reflect_on_all_associations` reflections ..

Noticed this one while looking at this example:

```ruby
class Product < ActiveRecord::Base
  has_many :variants
  has_many :translations
end

class Translation < ActiveRecord::Base
  belongs_to :product
end

class Variant < ActiveRecord::Base
  belongs_to :product
end

class BugTest < Minitest::Test
  def test_merge_stuff
    product = Product.create! name: 'huhu'
    variant = Variant.create! product_id: product.id
    Translation.create! locale: 'en', product_id: product.id

    product_relation = Product.all
                              .preload(:translations)
                              .joins(:translations)
                              .merge(Translation.where(locale: 'en'))
                              .where(name: 'huhu')

    assert_equal variant, Variant.joins(:product).merge(product_relation).first
  end
end
```

Currently, values for columns backing Active Record enums must be
specified as integers in test fixtures:

    awdr:
      title: "Agile Web Development with Rails"
      status: 2

    rfr:
      title: "Ruby for Rails"
      status: <%= Book.statuses[:proposed] %>

This is potentially confusing, since enum values are typically
specified as symbols or strings in application code. To resolve the
confusion, this change permits the use of symbols or strings to specify
enum values:

    awdr:
      status: :published

It is compatible with fixtures that specify enum values as integers.

Related with #17370.

See #9683 for the reasons we switched to `distinct`.

Here is the discussion that triggered the actual deprecation #20198.

`uniq`, `uniq!` and `uniq_value` are still around.
They will be removed in the next minor release after Rails 5.

we should ask the reflection for its name rather than storing the
reflection name in two places (an array and a reflection object)

This line introduced by the commit fd398475 for using
`Arel::Visitors::BindVisitor`. Currently it is not used.

Related with #20028.

skip]

This patch
  - reduces the duplication among the `reference`-family methods.
  - better explains all the optians available for `add_reference`.
  - redirects to user from `references` to `add_reference`.

Originated by #20184.

Active Record detects when the process has forked and automatically
creates a new connection pool to avoid sharing file descriptors.

If the existing connection pool had a schema cache associated with it,
the new pool should copy it to avoid unnecessarily querying the database
for its schema.

The code to detect that the process has forked is in ConnectionHandler,
but the existing test for it was in the ConnectionManagement test file.
I moved it to the right place while I was writing the new test for this
change.

Renamed `@reserved_connections` -> `@thread_cached_conns`. New name
clearly conveys the purpose of the cache, which is to speed-up
`#connection` method.

The new `@thread_cached_conns` now also uses `Thread` objects as keys
(instead of previously `Thread.current.object_id`).

Since there is no longer any synchronization around
`@thread_cached_conns`, `disconnect!` and `clear_reloadable_connections!`
methods now pre-emptively obtain ownership (via `checkout`) of all
existing connections, before modifying internal data structures.

A private method `release` has been renamed `thread_conn_uncache` to
clear-up its purpose.

Fixed some brittle `thread.status == "sleep"` tests (threads can go
into sleep even without locks).

Move post checkout connection verification out of mutex.synchronize.

This can resolve confusing situation when a top level constant exists
but a namespaced version is identified.

Related to #19531.

Add full set of MySQL CLI options to support SSL authentication when using db:structure dump and load