i.e. Oracle database does not support these isolation levels.
`:read_uncommitted` `:repeatable_read`
This commit also works with other databases which do not support
these isolation levels.
If your database supports setting the isolation level for a transaction,
you can set it like so:
Post.transaction(isolation: :serializable) do
# ...
end
Valid isolation levels are:
* `:read_uncommitted`
* `:read_committed`
* `:repeatable_read`
* `:serializable`
You should consult the documentation for your database to understand the
semantics of these different levels:
* http://www.postgresql.org/docs/9.1/static/transaction-iso.html
* https://dev.mysql.com/doc/refman/5.0/en/set-transaction.html
An `ActiveRecord::TransactionIsolationError` will be raised if:
* The adapter does not support setting the isolation level
* You are joining an existing open transaction
* You are creating a nested (savepoint) transaction
The mysql, mysql2 and postgresql adapters support setting the
transaction isolation level. However, support is disabled for mysql
versions below 5, because they are affected by a bug
(http://bugs.mysql.com/bug.php?id=39170) which means the isolation level
gets persisted outside the transaction.
