ConnectionHandler will not have any knowlodge of AR models now, it will
only know about the specs.
Like that we can decouple the two, and allow the same model to use more
than one connection.

Historically, folks used to create abstract AR classes on the fly in
order to have multiple connections for the same model, and override the
connection methods.

With this, now we can override the `specificiation_id` method in the
model, to return a key, that will be used to find the connection_pool
from the handler.

[ci skip]

Unlike `one?` and `none?`, `empty?` has interactions with methods
outside of enumerable. It also doesn't fit in the same vein.
`Topic.any?` makes sense. `Topic.empty?` does not, as `Topic` is not a
container.

Fixes #24808
Close #24812

`schema_creation` is not public API.

https://github.com/rails/rails/blob/v5.0.0.beta4/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb#L121
https://github.com/rails/rails/blob/v5.0.0.beta4/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb#L78

Rails should not be explicity mentioned within Active Record, since
railties and the Rails ecosystem is not required for use.

Refactor of #22911.
Signed-off-by: NJeremy Daer <jeremydaer@gmail.com>

Follow up of 1683410c.
Signed-off-by: NJeremy Daer <jeremydaer@gmail.com>

`ActiveRecord::Migration` needn't know about migration version
compatibility lookup. Delegate it to the Compatibility module.
Signed-off-by: NJeremy Daer <jeremydaer@gmail.com>

Currently, loading the schema (schema_load)
accesses the locking column (locking_column)
which defaults the value (reset_locking_column)
which invalidates the schema (reload_schema_from_cache)
which forces another schema load.

Good news:
The second schema_load does accesses locking_column,
but locking_column is set, so it does not reset_locking_column
and it does not trigger an infinite loop.

The solution is not invalidate the cache while default locking_column

Others adapters need to perform a check for validity.
Add coverage for mysql2 db type validation

Example:

    create_table :users do |t|
      t.string :name
      t.index 'lower(name) varchar_pattern_ops'
    end

Fixes #19090.
Fixes #21765.
Fixes #21819.
Fixes #24359.
Signed-off-by: NJeremy Daer <jeremydaer@gmail.com>

There is no need to fetch all table indexes in remove_index if name is specified. If name is wrong, then StatementInvalid will be raised.
Signed-off-by: NJeremy Daer <jeremydaer@gmail.com>

- Rename max to statement_limit
- Remove magic number 1000 from everywhere
- Defined StatementPool::DEFAULT_STATEMENT_LIMIT and started using it everywhere
Signed-off-by: NJeremy Daer <jeremydaer@gmail.com>

Adapters override `#supports_savepoints?` to return `true` if they
support transaction savepoints. Defaults to `false`.

 [ci skip]

`IPAddr` is used in `OID::Cidr`.

Follow up to #23458.
Active Record supports MySQL >= 5.0 now.

Follow up to #24685. `insert_versions_sql` is not public API.

Make sure we handle explicitly passed nil's to lock_version as well.
An explicitly passed nil value is now converted to 0 on LockingType,
so that we don't end up with ActiveRecord::StaleObjectError in update record
optimistic locking

Fixes #24695

Re. 6e098284

https://github.com/rails/rails/commit/42dd2336b31a8d98776d039a2b9fd7f834156a78 changed INSERT INTO versions to run in 1 single query.
This breaks for sqlite versions < 3.7.11, which is especially the case on Ubuntu 12.04 LTS, that has SQLite version 3.7.9 as default.

So we check for support for multi insert, before performing single query inserts, else fallback to older version of running multiple queries.
[Vipul A M & Yasuo Honda]

When the query cache completes, if Active Record is still inside of a
transaction, it is because the transaction is meant to be left open
above this unit of work (such as transactional fixtures in tests). There
were several tests around the behavior of "tests" that were invalid, as
tests are not run through the executor. They have been changed to
reflect the new behavior, which is closer to what actually occurs in
Rails tests.

Fixes #23989
Fixes #24491
Close #24500

Follow up to #24221.