The `add_index` method now supports a `where` option that receives a
string with the partial index criteria.

    add_index(:accounts, :code, :where => "active")

    Generates

    CREATE INDEX index_accounts_on_code ON accounts(code) WHERE active

This is the 'top level' connection, inherited by any models that include
ActiveRecord::Model or inherit from ActiveRecord::Base.

pool.

Only use LOWER for mysql case insensitive uniqueness check when column has a case sensitive collation.

In 1efd8828, ConnectionAdapters was put under eager_autoload. Due to the requires in that file, this caused ConnectionSpecification to be loaded, which references ActiveRecord::Base, which means the database connection is established. We do not want to connect to the database when Active Record is loaded, only when ActiveRecord::Base is first referenced by the user.

Make it the responsibility of the connection to hold onto an ARel visitor for generating SQL. This improves the code architecture generally, and solves some problems with marshalling. Adapter authors please take note: you now need to define an Adapter.visitor_for method, but it degrades gracefully with a deprecation warning for now.

(default is nil meaning unlimited but Oracle imposes a limit of 1000)
Limit is used to make multiple queries when preloading associated has_many or habtm records