The **(11)** does not affect the storage size of the data type, which for an
INT will always be 4 bytes. It affects the **display width**.

http://www.tocker.ca/2015/07/02/proposal-to-deprecate-mysql-integer-display-width-and-zerofill.html

Some databases like MySQL allow defining collation charset for specific
columns.

Example:

    create_table :foos do |t|
      t.string :string_utf8_bin, charset: 'utf8', collation: 'utf8_bin'
      t.text   :text_ascii,      charset: 'ascii'
    end

The type from the column is never used, except when being passed to the
attributes API. While leaving the type on the column wasn't necessarily
a bad thing, I worry that it's existence there implies that it is
something which should be used.

During the design and implementation process of the attributes API,
there have been plenty of cases where getting the "right" type object
was hard, but I had easy access to the column objects. For any
contributor who isn't intimately familiar with the intents behind the
type casting system, grabbing the type from the column might easily seem
like the "correct" thing to do.

As such, the goal of this change is to express that the column is not
something that should be used for type casting. The only places that are
"valid" (at the time of this commit) uses of acquiring a type object
from the column are fixtures (as the YAML file is going to mirror the
database more closely than the AR object), and looking up the type
during schema detection to pass to the attributes API

Many of the failing tests were removed, as they've been made obsolete
over the last year. All of the PG column tests were testing nothing
beyond polymorphism. The Mysql2 tests were duplicating the mysql tests,
since they now share a column class.

The implementation is a little hairy, and slightly verbose, but it felt
preferable to going back to 20 constructor options for the columns. If
you are git blaming to figure out wtf I was thinking with them, and have
a better idea, go for it. Just don't use a type object for this.

Now that type casting has nothing to do with columns, our only usage of
this method is gone.

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

Before this change we had a small set of "truthy", and all others
are "falsy".

Now, we have a small set of "falsy" values and all others are
"truthy" matching Ruby's semantics.

This reverts commit ae96f229.

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

Slightly refactoring `PostgreSQLColumn`. `array` should be readonly.
`default_function` should be initialized by `super`. `sql_type` has been
removed `[]`. Since we already choose to remove it we should not change.

I think we should deprecate this behavior and just error if you tell us
to do a case insensitive comparison for types which are not case
sensitive. Partially reverts 35592307

Fixes #18195

The query cache uses bind values as hash keys. The current
implementation relies on reference equality for hash equality. This is
brittle, and can easily break in the future.

We shouldn't rely on reference equality of these objects in tests

The method has been removed in 09206716
(PR #16074), but the delegation was apparently missed, and one instance
of the method was added back with the addition of OID::Xml in
336be2bd (PR #16072), so we can safely
rm both.

This is public API, and `simple_form` depends on the `nil` return value.
We need to go through a deprecation cycle to return a null object. If
people want hash access, they can access the hash.

If we want to have type decorators mess with the attribute, but not the
column, we need to stop type casting on the column. Where possible, we
changed the tests to test the value of `column_defaults`, which is
public API. `Column#default` is not.

We are no longer including column objects in YAML serialization, thanks
to https://github.com/rails/rails/pull/15621

On MySQL and PostgreSQL, the adapter does not type cast virtual columns
for us.

In some cases there is a difference between the two, we should always
be doing one or the other. For convenience, `type_cast` is still a
private method on type, so new types that do not need different behavior
don't need to implement two methods, but it has been moved to private so
it cannot be used accidentally.

- The following is now true for all types, all the time
  - `model.attribute_before_type_cast == given_value`
  - `model.attribute == model.save_and_reload.attribute`
  - `model.attribute == model.dup.attribute`
  - `model.attribute == YAML.load(YAML.dump(model)).attribute`
- Removes the remaining types implementing `type_cast_for_write`
- Simplifies the implementation of time zone aware attributes
- Brings tz aware attributes closer to being implemented as an attribute
  decorator
- Adds additional point of control for custom types

We need to decorate the types lazily. This is extracted to a separate
API, as there are other refactorings that will be able to make use of
it, and to allow unit testing the finer points more granularly.

Follow up to #15438 and #15502.

/cc @sgrif

The contract of `_field_changed?` assumes that the old value is always
type cast. That is not the case for the value in `Column#default` as
things are today. It appears there are other public methods that
assume that `Column#default` is type cast, as well. The reason for this
change originally was because the value gets put into `@raw_attributes`
in initialize. This reverts to the old behavior on `Column`, and updates
`initialize` to make sure that the values are in the right format.

The types know more about what is going on than the dirty module. Let's
ask them!

Nearly completely implemented in terms of custom properties.
`_before_type_cast` now stores the raw serialized string consistently,
which removes the need to keep track of "state". The following is now
consistently true:

- `model.serialized == model.reload.serialized`
- A model can be dumped and loaded infinitely without changing
- A model can be saved and reloaded infinitely without changing

This removes the case statement in `SchemaDumper` and gives every `Type`
the possibility to control the SchemaDumper default value output.

/cc @sgrif

Many of the methods defined in `AttributeMethods::Serialization` can be
refactored onto this type as well, but this is a reasonable small step.
Removes the `Type` class, and the need for `decorate_columns` to handle
serialized types.

It appears this property was added, but never actually used. It would be
broken if it were, as it only type casts one way.

Adds the ability to save custom types, which type cast to non-primitive
ruby objects.

It appears to have been used at some point in the past, but is no longer
used in any meaningful way. Whether a column is considered primary is
a property of the model, not the schema/column. This also removes the
need for yet another layer of caching of the model's schema, and we can
leave that to the schema cache.

Columns and injected types no longer have any conditionals based on the
format of SQL type strings! Hooray!

Ideally types will be usable without having to specify a sql type
string, so we should keep the information related to parsing them on the
adapter or another object.

  - `extract_precision`, `extract_limit`, and `extract_default` probably need to follow.
  - would be good to remove the delegation `Column#extract_scale`.

/cc @sgrif