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 probably fixes #25840

Follow up to 249f71a2

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

These internal initialize methods are no longer used internally.

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

* Don't force PKs on tables that have explicitly opted out
* All integer-like PKs are autoincrement unless they have an explicit
  default

Remove `initialize_schema_migrations_table` and `initialize_internal_metadata_table` internal public methods

These internal methods accidentally appeared in the doc, and so almost
useless. It is enough to create these internal tables directly, and
indeed do so in several places.

https://github.com/rails/rails/blob/v5.0.1/activerecord/lib/active_record/schema.rb#L55
https://github.com/rails/rails/blob/v5.0.1/activerecord/lib/active_record/railties/databases.rake#L6
https://github.com/rails/rails/blob/v5.0.1/activerecord/lib/active_record/tasks/database_tasks.rb#L230

https://www.sqlite.org/foreignkeys.html

Raise `ActiveRecord::InvalidForeignKey` when a record cannot be inserted
or updated because it references a non-existent record for SQLite3
adapter.

`timestamp_attributes_for_updates_in_model` returns an empty
array when a model has no `updated_at` or `updated_on`.

So my previously thought uncommon case is a lot more likely
now.

This reverts commit a0a1ede8.

Closes #27683.

Seeing a code sample that leads to what we're guarding against:

```ruby
Topic.update_counters(1, replies_count: 1, touch: [])
```

It doesn't look like a case people would ever intentionally end
up with. Thus we're better off sparing the conditional.

Note: it could happen if a method returns an empty array
that's then passed to `update_counters` and its touchy friends.
But `[].presence` can fix that once people see their query blow
up.

[ Eugene Kenny & Kasper Timm Hansen ]

`type_condition` should be overwritten by `create_with_value`. So `type`
in `create_with_value` should be a string because `where_values_hash`
keys are converted to string.

Fixes #27600.

If a process is forked more than once, the pool was grabbing the oldest
spec, not the most recent spec. This wasn't noticed before because most
folks are lilely forking the process only once.

If you're forking the process multiple times however the wrong spec name
will be returned and an incorrect connection will be used for the
process.

This fixes the issue by reversing the list of spec names so we can grab
the most recent spec rather than the oldest spec.

Supports the `touch` option from update_counters.
The default behavior is not to update timestamp columns.

because Struct.new returns a Class, we just can give it a name and use it directly without inheriting from it

(I personally prefer writing one string in one line no matter how long it is, though)

Passing a class to `has_and_belongs_to_many` show deprecation message
three times. It is enough only once.

The idea of `class_name` as an option of reflection is that passing a
string would allow us to lazy autoload the class.

Using `belongs_to :client, class_name: Customer` is eagerloading models more than necessary
and creating possible circular dependencies.

UUID primary key with no default value feature (#10404, #18206) was lost
in legacy migration caused by #25395 got merged. Restore the feature
again in legacy migration.

Avoids a NoMethodError when table_name is a symbol instead of a string.

In Rails 4.2 calling `ActiveRecord::Migrator.migrate` would return an array of results. Without realizing that this return type was expected I accidentally introduced a change in https://github.com/rails/rails/commit/4d60e93174a3d6d90b1a06fc7515cb5cd749a6f3

This PR preserves the previous behavior and adds a test on the return type. This will need a backport to 5.0 branch.

Actually, private methods cannot be called with `self.`, so it's not just redundant, it's a bad habit in Ruby

Need `table_name` to avoid all databases scan.
See https://dev.mysql.com/doc/refman/5.7/en/information-schema-optimization.html.

```
> EXPLAIN SELECT fk.referenced_table_name AS 'to_table', fk.referenced_column_name AS 'primary_key', fk.column_name AS 'column', fk.constraint_name AS 'name', rc.update_rule AS 'on_update', rc.delete_rule AS 'on_delete' FROM information_schema.key_column_usage fk JOIN information_schema.referential_constraints rc USING (constraint_schema, constraint_name) WHERE fk.referenced_column_name IS NOT NULL AND fk.table_schema = 'activerecord_unittest' AND fk.table_name = 'fk_test_has_pk' AND rc.table_name = 'fk_test_has_pk'\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: fk
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: TABLE_SCHEMA,TABLE_NAME
      key_len: NULL
          ref: NULL
         rows: NULL
     filtered: NULL
        Extra: Using where; Open_full_table; Scanned 0 databases
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: rc
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: TABLE_NAME
      key_len: NULL
          ref: NULL
         rows: NULL
     filtered: NULL
        Extra: Using where; Open_full_table; Scanned 1 database; Using join buffer (Block Nested Loop)
2 rows in set, 1 warning (0.00 sec)
```

Fixes #27579.