Follow up to #23508.

A return value of `schema_type` is used by:

1. primary key type: using as `symbol.inspect`
2. normal column type: using as `symbol.to_s`

It is better to return symbol.

With this addition, you can add a column into the table like:

```
  create_table(:numeric_types) do |t|
    t.numeric :foo, precision: 10, scale: 2, default: 2.0
  end
```

The result of the migration above is same with:

```
  create_table(:numeric_types) do |t|
    t.decimal :foo, precision: 10, scale: 2, default: 2.0
  end
```

Fixes #22980.

It appears first in `lib/active_record.rb`.

This reverts commit 99801c6a.

Ultimately it doesn't matter whether `add_index` or `t.index` are used
in the schema dumper in any meaningful way. There are gems out there
which hook into the old behavior for things like indexing materialized
views. Since the reverted commit doesn't seem to add much benefit,
there's no reason for us to break these gems.

If you set the DATABASE_URL environment variable to `mydatabase` by accident, you end up getting a series of errors that are hard to trace. For example: 

```
warning: already initialized constant ActiveRecord::Base::OrmAdapter
```

Turns out the cascade of errors is due to the error raised by `.tr` being called on `nil`.

This commit makes sure that `scheme` is set before calling `.tr` on it. My previous iteration used `@uri.scheme.try(:tr, '-', '_')` but using the `&&` logical operator is a fair bit faster: http://stackoverflow.com/questions/26655032/try-vs-performance

With this change, the error message becomes much more understandable:

```
FATAL:  database "mydatabase" does not exist (ActiveRecord::NoDatabaseError)
```

https://dev.mysql.com/doc/refman/5.7/en/innodb-fulltext-index.html
https://dev.mysql.com/doc/refman/5.7/en/creating-spatial-indexes.html

Currently some features uses `information_schema` (e.g. foreign key
support). `information_schema` introduced since MySQL 5.0.

This still isn't as separated as I'd like, but it at least moves most of
the burden of alias mapping in one place.

Changed id-writer to save join table records based on association primary key

There was a typo in the variable name leading to infinite recursion

We had previously updated this to attempt to map over whatever was
passed in, so that additional types like range and array could benefit
from this behavior without the time zone converter having to deal with
every known type.

However, the default behavior of a type is to just yield the given value
to `map`, which means that if we don't actually know how to handle a
value, we'll just recurse infinitely. Since both uses of `map` in this
case occur in cases where we know receiving the same object will
recurse, we can just break on reference equality.

Fixes #23241.

`#initialize` of `HasManyReflection`, `HasOneReflection` and
`BelongsToReflection` only pass all arguments to `super` by passed order.
These overriding can be removed.

When a `before_commit` callback raises, the database is rolled back but
AR's record of the current transaction is not, leaving the connection in
a perpetually broken state that affects all future users of the
connection: subsequent requests, jobs, etc. They'll think a transaction
is active when none is, so they won't BEGIN on their own. This manifests
as missing `after_commit` callbacks and broken ROLLBACKs.

This happens because `before_commit` callbacks fire before the current
transaction is popped from the stack, but the exception-handling path
they hit assumes that the current transaction was already popped. So the
database ROLLBACK is issued, but the transaction stack is left intact.

Common cause: deadlocked `#touch`, which is now implemented with
`before_commit` callbacks.

What's next:
* We shouldn't allow active transaction state when checking in or out
  from the connection pool. Verify that conns are clean.
* Closer review of txn manager sad paths. Are we missing other spots
  where we'd end up with incorrect txn state? What's the worst that can
  happen if txn state drifts? How can we guarantee it doesn't and
  contain the fallout if it does?

Thanks for @tomafro for expert diagnosis!

This reverts commit 9f3730a5, reversing
changes made to 2637fb75.

There are additional issues with this commit that need to be addressed
before this change is ready (see #23377). This is a temporary revert in
order for us to have more time to address the issues with that PR,
without blocking the release of beta2.

for those who already migrated to Rails 5.0.0 beta

to support Oracle database which only supports 30 byte identifier length

`initialize_schema_migrations_table` is called in every migrations.

https://github.com/rails/rails/blob/v5.0.0.beta1/activerecord/lib/active_record/migration.rb#L1080
https://github.com/rails/rails/blob/v5.0.0.beta1/activerecord/lib/active_record/schema.rb#L51

This means that extra `show variables` is called regardless of the
existence of `schema_migrations` table.

This change is to avoid extra `show variables` if `schema_migrations`
table exists.

Rails has abstract Reflection classes (`MacroReflection`,
`AssociationReflection` etc.) and concrete Reflection classes
(e.g. `HasManyReflection`, `HasOneReflection` etc.).
In many case `calculate_constructable` returns `true`, so
change `calculate_constructable` to always return `true` and
override this method if necessary.

I've gotten tripped up more than a few times on this, thinking that
using `belongs_to` with `touch: true` would trigger my after_save or
after_update callbacks. The same text is in the documentation for the
touch method itself, but I think its helpful to repeat it again here.
It might save people some time.

- Updated persistence documentation to make it clear that save and save! won't update a record if validation fails.
- Also fixed `update` method's documention to be uniform about this statement.
Fixes #20821

[ci skip]

[Vipul A M & pseidemann ]

    p PostgreSQLAdapter::OID::Money.precision
    # => 19

    p PostgreSQLAdapter::OID::Money.new.precision
    # => nil

This reverts commit ff835f90, reversing
changes made to c4d85dfb.

Reason: This broke the tests. We will add back after investigated.

Follow up to #22896.

`LOST_CONNECTION_ERROR_MESSAGES` was added by f3845822.
But currently unused from anywhere.

`SchemaMigration.drop_table` is only used in tests.
Simply we can use `drop_table if_exists: true`.

`ActiveRecord::ConnectionAdapters::SchemaStatements#columns` is defined
here as an interface method here. So changes to raise `NotImplementedError`
same as `tables`, `views` ...etc.