[CFX-206] Errors when specifying the same user twice when sharing a file

improve: use a more efficient database query when finding the users to share a file with (used to do two full table scans of the user table!)
(merged from 10.06.x, r16793)

SVN: 16896

dbmigration/source/java/ch/systemsx/cisd/dbmigration/DatabaseConfigurationContext.java

@@ -515,6 +515,18 @@ public class DatabaseConfigurationContext implements DisposableBean
         this.databaseKind = StringUtils.trim(databaseKind);
     }
 
+    /**
+     * Returns the {@link DatabaseEngine}.
+     */
+    public DatabaseEngine getDatabaseEngine() throws ConfigurationFailureException
+    {
+        if (databaseEngine == null)
+        {
+            throw new ConfigurationFailureException("No database engine defined.");
+        }
+        return databaseEngine;
+    }
+
     /**
      * Returns the code of the database engine.
      *