Auto Session

Why AutoSession?

Typically, ScalikeJDBC operations are encapsulated within DB.autoCommit, DB.readOnly, and other transaction blocks. For reusing methods across different transaction contexts as below,

def findById(id: Long) = DB readOnly {
  sql"select id, name from members where id = ${id}"
    .map(rs => Member(rs)).single.apply()
}

AutoSession becomes essential. In a transaction block, this method may not perform as expected because it uses a separate session and cannot access uncommitted data. The reason is that since #findById(Long) uses another session(=connection), it couldn’t access uncommitted data.

DB localTx { implicit session =>
  val id = create("Alice")
  findById(id) // Not found!
}

With this change, the method can access the current transaction context. Instead of having DB blocks inside, your method can accept an implict DBSession paramter from external code to join an existing transactional session.

def findById(id: Long)(implicit session: DBSession) =
  sql"select id, name from members where id = ${id}"
    .map(rs => Member(rs)).single.apply()

With the change, the following code works as you expect.

DB localTx { implicit session =>
  val id = create("Alice")
  findById(id) // Found!
}

But unfortunately, now that we need to pass implicit parameter to #findById every time to use the method, it could be troublesome especially for simple code snippets.

// now we cannot use this method directly
findById(id) // implicit parameter not found!

DB readOnly { implicit session => findById(id) }

AutoSession is a solution for the issue. You can have AutoSession as default value of the implicit parameter.

def findById(id: Long)(implicit session: DBSession = AutoSession) =
  sql"select id, name from members where id = ${id}"
    .map(rs => Member(rs)).single.apply()

Having the default implement value can make #findById even more flexible plus much simpler.

findById(id) // borrows a read-only session and gives it back
DB localTx { implicit session => findById(id) } // using implicit session

When you do the same with NamedDB, you can use NamedAutoSession as below:

def findById(id: Long)(implicit session: DBSession = NamedAutoSession("named")) =
  sql"select id, name from members where id = ${id}"

ReadOnlyAutoSession

Since version 1.7.4, ReadOnlyAutoSession and NamedReadOnlyAutoSession is also available, which are tailored for read-only operations, preventing any update or execute operations.