This reverts commit 28977f1f.

If select clause is specified and last column has a column alias,
additional column alias causes a statement invalid.

Add test coverage for counting a single column with NULL values.

Fixes #27676, #27682, and #27705.

Follow up to #26266.

The default type of `primary_key` and `references` were changed to
`bigint` since #26266. But legacy migration and sqlite3 adapter should
keep its previous behavior.

`ColumnDumper#migration_keys` was extracted to customize keys for
standardized column arguments widths. But the feature was removed in
df84e986. The internal method is no longer used for that.

Since text default treated as an empty string in non-strict mode has
been removed in #26154, `MySQL::Column` behaves like any other column.
Also, The difference between strict and non-strict mode is covered by
`test_mysql_not_null_defaults_strict` and `test_mysql_not_null_defaults_non_strict`.

```
go get -u github.com/client9/misspell/cmd/misspell
misspell  -w -error -source=text .
```

Some custom primary key tests (added at #17631, #17696, #18220, #18228)
were lost at #26266. Restore the tests to improve test coverage.

The PR #27384 changed migration compatibility behaviour.

```ruby
class CreateMasterData < ActiveRecord::Migration[5.0]
  def change
    create_table :master_data, id: :integer do |t|
      t.string :name
    end
  end
end

```

Previously this migration created non-autoincremental primary key
expected. But after the PR, the primary key changed to autoincremental,
it is unexpected.

This change restores the behaviour of the compatibility layer.

And raise `ArgumentError` when passing string to define callback.

The PR #27384 changed integer-like primary key to be autoincrement
unless an explicit default. This means that integer-like primary key is
restored as autoincrement unless dumping the default nil explicitly.
We should dump integer-like primary key with default nil correctly.

When the association's primary key is manually set as a symbol and called with an array of strings, CollectionAssociation#ids_writer
fails to cast the ids to integers. This is because
AssociationReflection#association_primary_key_type and
ThroughReflection#association_primary_key_type return the incorrect
type, since ModelSchema.type_for_attribute only accepts a string. The
result is an ActiveRecord::RecordNotFound error.

MySQL generated columns: https://dev.mysql.com/doc/refman/5.7/en/create-table-generated-columns.html
MariaDB virtual columns: https://mariadb.com/kb/en/mariadb/virtual-computed-columns/

Declare virtual columns with `t.virtual name, type: …, as: "expression"`.
Pass `stored: true` to persist the generated value (false by default).

Example:

    create_table :generated_columns do |t|
      t.string  :name
      t.virtual :upper_name, type: :string,  as: "UPPER(name)"
      t.virtual :name_length, type: :integer, as: "LENGTH(name)", stored: true
      t.index :name_length  # May be indexed, too!
    end

Closes #22589

Fixes #27666.

Fixes #27826
Refer https://github.com/rails/rails/commit/f7b317175430a2d9300d9c4acfc1f34f4fdb2fbc

Specify complete path to AR::LegacyYamlAdapter

Lambda scopes require a bunch more work.  Ideally the `scope` list would
be a homogeneous collection.  In this case, the reflection knows how to
construct the right reflection for this join, so lets just construct
that relation rather than instance execing to figure it out later.

`PolymorphicReflection` needs to be custom for handling scope lambdas

Each reflection should be responsible for returning the scopes needed to
query against the db.

I can never remember if it returns `self` or `true` (seems to be `true`)

[ci skip]

column_definitions() needs to fetch the collation for every column, if
present. Previously, it did so using a correlated subquery - a subquery
that references results from the outer scope.

This patch updates the query to remove the subquery in favor of a
simpler and more efficient JOIN clause.

Running the two queries through EXPLAIN against Postgres additionally
shows that the original form with a correlated subquery requires a
Nested Loop Left Join, while the new form with a simple JOIN can use a
more efficient Merge Left Join.

This is an alternative of #27714.

If `has_one :through` association has set `nil`, `through_record` is
destroyed but still remain loaded target in `through_proxy` until
`reload` or `reset` explicitly.

If `through_proxy` is not reset (remain destroyed (frozen) target),
setting new record causes `RuntimeError: Can't modify frozen hash`.

To prevent `RuntimeError`, should reload `through_record` that has been
destroyed in `create_through_record`.

this probably fixes #25840

This message could be generated by `assert_predicate`, https://github.com/rails/rails/commit/1853a4f2c8b2bddfbde6aae80abb08310295201c#commitcomment-20546113
but I'd rather handwrite the message string if I could reduce magic from the code by doing so.

`assert_raise` does not check error message. However, in some tests,
it seems like expecting error message checking with `assert_raise`.
Instead of specifying an error message in `assert_raise`, modify to use
another assert to check the error message.

Follow up to 249f71a2

One empty string allocation per column per row.

Since `Migration` module is included in both `MigrationGenerator` and
`ModelGenerator`, no need to define a common method for each class.

These methods are obviously for internal use.

These internal initialize methods are no longer used internally.

This reverts commit 39c77eb1, reversing
changes made to 9f6f51be.