How could I know if a database table is exists in ScalaQuery

问题

I'm trying ScalaQuery, it is really amazing. I could defined the database table using Scala class, and query it easily.

But I would like to know, in the following code, how could I check if a table is exists, so I won't call 'Table.ddl.create' twice and get a exception when I run this program twice?

object Users extends Table[(Int, String, String)]("Users") {
    def id = column[Int]("id")
    def first = column[String]("first")
    def last = column[String]("last")
    def * = id ~ first ~ last
}

object Main
{
    val database = Database.forURL("jdbc:sqlite:sample.db", driver = "org.sqlite.JDBC")

    def main(args: Array[String]) {
        database withSession {
            // How could I know table Users is alrady in the DB?
            if ( ??? )  {
                Users.ddl.create
            }
        }
    }
}

回答1:

ScalaQuery version 0.9.4 includes a number of helpful SQL metadata wrapper classes in the org.scalaquery.meta package, such as MTable:

http://scalaquery.org/doc/api/scalaquery-0.9.4/#org.scalaquery.meta.MTable

In the test code for ScalaQuery, we can see examples of these classes being used. In particular, see org.scalaquery.test.MetaTest.

I wrote this little function to give me a map of all the known tables, keyed by table name.

import org.scalaquery.meta.{MTable}
def makeTableMap(dbsess: Session) : Map[String, MTable] = {
    val tableList = MTable.getTables.list()(dbsess);
    val tableMap = tableList.map{t => (t.name.name, t)}.toMap;
    tableMap;
}

So now, before I create an SQL table, I can check "if (!tableMap.contains(tableName))".

回答2:

This thread is a bit old, but maybe someone will find this useful. All my DAOs include this:

def create = db withSession {
    if (!MTable.getTables.list.exists(_.name.name == MyTable.tableName))
        MyTable.ddl.create
}

回答3:

Here's a full solution that checks on application start using a PostGreSQL DB for PlayFramework

import globals.DBGlobal
import models.UsersTable
import org.scalaquery.meta.MTable
import org.scalaquery.session.Session

import play.api.GlobalSettings
import play.api.Application

object Global extends GlobalSettings {

    override def onStart(app: Application)  {

        DBGlobal.db.withSession { session : Session =>
            import org.scalaquery.session.Database.threadLocalSession
            import org.scalaquery.ql.extended.PostgresDriver.Implicit._
            if (!makeTableMap(session).contains("tableName")) {
                UsersTable.ddl.create(session)
            }
        }
    }

    def makeTableMap(dbsess: Session): Map[String, MTable] = {
        val tableList = MTable.getTables.list()(dbsess)
        val tableMap = tableList.map {
        t => (t.name.name, t)
    }.toMap
        tableMap
    }
}

回答4:

With java.sql.DatabaseMetaData (Interface). Depending on your Database, more or less functions might be implemented.

回答5:

See also the related discussion here.I personally prefer hezamu's suggestion and extend it as follows to keep it DRY:

def createIfNotExists(tables: TableQuery[_ <: Table[_]]*)(implicit session: Session) {
  tables foreach {table => if(MTable.getTables(table.baseTableRow.tableName).list.isEmpty) table.ddl.create}
}

Then you can just create your tables with the implicit session:

db withSession {
  implicit session =>
    createIfNotExists(table1, table2, ..., tablen)
}

回答6:

You can define in your DAO impl the following method (taken from Slick MTable.getTables always fails with Unexpected exception[JdbcSQLException: Invalid value 7 for parameter columnIndex [90008-60]]) that gives you a true o false depending if there a defined table in your db:

    def checkTable() : Boolean = {
        val action = MTable.getTables
        val future = db.run(action)
        val retVal = future map {result =>
          result map {x => x}
        }

        val x = Await.result(retVal, Duration.Inf)

        if (x.length > 0) {
          true
        } else {
          false
        }
      }

Or, you can check if some "GIVENTABLENAME" or something exists with println method:

      def printTable() ={
          val q = db.run(MTable.getTables)
          println(Await.result(q, Duration.Inf).toList(0)) //prints first MTable element
          println(Await.result(q, Duration.Inf).toList(1))//prints second MTable element
          println(Await.result(q, Duration.Inf).toList.toString.contains("MTable(MQName(public.GIVENTABLENAME_pkey),INDEX,null,None,None,None)"))
      }

Don't forget to add

    import slick.jdbc.meta._

Then call the methods from anywhere with the usual @Inject(). Using play 2.4 and play-slick 1.0.0.

Cheers,

来源：https://stackoverflow.com/questions/5442034/how-could-i-know-if-a-database-table-is-exists-in-scalaquery

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